1 <!DOCTYPE Article PUBLIC "-//Davenport//DTD DocBook V3.0//EN"> 2 3 <Article> 4 5 <ArtHeader> 6 7 <Title>The extended-2 filesystem overview</Title> 8 <AUTHOR 9 > 10 <FirstName>Gadi Oxman, tgud (a] tochnapc2.technion.ac.il</FirstName> 11 </AUTHOR 12 > 13 <PubDate>v0.1, August 3 1995</PubDate> 14 15 </ArtHeader> 16 17 <Sect1> 18 <Title>Preface</Title> 19 20 <Para> 21 This document attempts to present an overview of the internal structure of 22 the ext2 filesystem. It was written in summer 95, while I was working on the 23 <Literal remap="tt">ext2 filesystem editor project (EXT2ED)</Literal>. 24 </Para> 25 26 <Para> 27 In the process of constructing EXT2ED, I acquired knowledge of the various 28 design aspects of the the ext2 filesystem. This document is a result of an 29 effort to document this knowledge. 30 </Para> 31 32 <Para> 33 This is only the initial version of this document. It is obviously neither 34 error-prone nor complete, but at least it provides a starting point. 35 </Para> 36 37 <Para> 38 In the process of learning the subject, I have used the following sources / 39 tools: 40 41 <ItemizedList> 42 <ListItem> 43 44 <Para> 45 Experimenting with EXT2ED, as it was developed. 46 </Para> 47 </ListItem> 48 <ListItem> 49 50 <Para> 51 The ext2 kernel sources: 52 53 <ItemizedList> 54 <ListItem> 55 56 <Para> 57 The main ext2 include file, 58 <FILENAME>/usr/include/linux/ext2_fs.h</FILENAME> 59 </Para> 60 </ListItem> 61 <ListItem> 62 63 <Para> 64 The contents of the directory <FILENAME>/usr/src/linux/fs/ext2</FILENAME>. 65 </Para> 66 </ListItem> 67 <ListItem> 68 69 <Para> 70 The VFS layer sources (only a bit). 71 </Para> 72 </ListItem> 73 74 </ItemizedList> 75 76 </Para> 77 </ListItem> 78 <ListItem> 79 80 <Para> 81 The slides: The Second Extended File System, Current State, Future 82 Development, by <personname><firstname>Remy</firstname> <surname>Card</surname></personname>. 83 </Para> 84 </ListItem> 85 <ListItem> 86 87 <Para> 88 The slides: Optimisation in File Systems, by <personname><firstname>Stephen</firstname> <surname>Tweedie</surname></personname>. 89 </Para> 90 </ListItem> 91 <ListItem> 92 93 <Para> 94 The various ext2 utilities. 95 </Para> 96 </ListItem> 97 98 </ItemizedList> 99 100 </Para> 101 102 </Sect1> 103 104 <Sect1> 105 <Title>Introduction</Title> 106 107 <Para> 108 The <Literal remap="tt">Second Extended File System (Ext2fs)</Literal> is very popular among Linux 109 users. If you use Linux, chances are that you are using the ext2 filesystem. 110 </Para> 111 112 <Para> 113 Ext2fs was designed by <personname><firstname>Remy</firstname> <surname>Card</surname></personname> and <personname><firstname>Wayne</firstname> <surname>Davison</surname></personname>. It was 114 implemented by <personname><firstname>Remy</firstname> <surname>Card</surname></personname> and was further enhanced by <personname><firstname>Stephen</firstname> 115 <surname>Tweedie</surname></personname> and <personname><firstname>Theodore</firstname> <surname>Ts'o</surname></personname>. 116 </Para> 117 118 <Para> 119 The ext2 filesystem is still under development. I will document here 120 version 0.5a, which is distributed along with Linux 1.2.x. At this time of 121 writing, the most recent version of Linux is 1.3.13, and the version of the 122 ext2 kernel source is 0.5b. A lot of fancy enhancements are planned for the 123 ext2 filesystem in Linux 1.3, so stay tuned. 124 </Para> 125 126 </Sect1> 127 128 <Sect1> 129 <Title>A filesystem - Why do we need it?</Title> 130 131 <Para> 132 I thought that before we dive into the various small details, I'll reserve a 133 few minutes for the discussion of filesystems from a general point of view. 134 </Para> 135 136 <Para> 137 A <Literal remap="tt">filesystem</Literal> consists of two word - <Literal remap="tt">file</Literal> and <Literal remap="tt">system</Literal>. 138 </Para> 139 140 <Para> 141 Everyone knows the meaning of the word <Literal remap="tt">file</Literal> - A bunch of data put 142 somewhere. where? This is an important question. I, for example, usually 143 throw almost everything into a single drawer, and have difficulties finding 144 something later. 145 </Para> 146 147 <Para> 148 This is where the <Literal remap="tt">system</Literal> comes in - Instead of just throwing the data 149 to the device, we generalize and construct a <Literal remap="tt">system</Literal> which will 150 virtualize for us a nice and ordered structure in which we could arrange our 151 data in much the same way as books are arranged in a library. The purpose of 152 the filesystem, as I understand it, is to make it easy for us to update and 153 maintain our data. 154 </Para> 155 156 <Para> 157 Normally, by <Literal remap="tt">mounting</Literal> filesystems, we just use the nice and logical 158 virtual structure. However, the disk knows nothing about that - The device 159 driver views the disk as a large continuous paper in which we can write notes 160 wherever we wish. It is the task of the filesystem management code to store 161 bookkeeping information which will serve the kernel for showing us the nice 162 and ordered virtual structure. 163 </Para> 164 165 <Para> 166 In this document, we consider one particular administrative structure - The 167 Second Extended Filesystem. 168 </Para> 169 170 </Sect1> 171 172 <Sect1> 173 <Title>The Linux VFS layer</Title> 174 175 <Para> 176 When Linux was first developed, it supported only one filesystem - The 177 <Literal remap="tt">Minix</Literal> filesystem. Today, Linux has the ability to support several 178 filesystems concurrently. This was done by the introduction of another layer 179 between the kernel and the filesystem code - The Virtual File System (VFS). 180 </Para> 181 182 <Para> 183 The kernel "speaks" with the VFS layer. The VFS layer passes the kernel's 184 request to the proper filesystem management code. I haven't learned much of 185 the VFS layer as I didn't need it for the construction of EXT2ED so that I 186 can't elaborate on it. Just be aware that it exists. 187 </Para> 188 189 </Sect1> 190 191 <Sect1> 192 <Title>About blocks and block groups</Title> 193 194 <Para> 195 In order to ease management, the ext2 filesystem logically divides the disk 196 into small units called <Literal remap="tt">blocks</Literal>. A block is the smallest unit which 197 can be allocated. Each block in the filesystem can be <Literal remap="tt">allocated</Literal> or 198 <Literal remap="tt">free</Literal>. 199 <FOOTNOTE> 200 201 <Para> 202 The Ext2fs source code refers to the concept of <Literal remap="tt">fragments</Literal>, which I 203 believe are supposed to be sub-block allocations. As far as I know, 204 fragments are currently unsupported in Ext2fs. 205 </Para> 206 207 </FOOTNOTE> 208 209 The block size can be selected to be 1024, 2048 or 4096 bytes when creating 210 the filesystem. 211 </Para> 212 213 <Para> 214 Ext2fs groups together a fixed number of sequential blocks into a <Literal remap="tt">group 215 block</Literal>. The resulting situation is that the filesystem is managed as a 216 series of group blocks. This is done in order to keep related information 217 physically close on the disk and to ease the management task. As a result, 218 much of the filesystem management reduces to management of a single blocks 219 group. 220 </Para> 221 222 </Sect1> 223 224 <Sect1> 225 <Title>The view of inodes from the point of view of a blocks group</Title> 226 227 <Para> 228 Each file in the filesystem is reserved a special <Literal remap="tt">inode</Literal>. I don't want 229 to explain inodes now. Rather, I would like to treat it as another resource, 230 much like a <Literal remap="tt">block</Literal> - Each blocks group contains a limited number of 231 inode, while any specific inode can be <Literal remap="tt">allocated</Literal> or 232 <Literal remap="tt">unallocated</Literal>. 233 </Para> 234 235 </Sect1> 236 237 <Sect1> 238 <Title>The group descriptors</Title> 239 240 <Para> 241 Each blocks group is accompanied by a <Literal remap="tt">group descriptor</Literal>. The group 242 descriptor summarizes some necessary information about the specific group 243 block. Follows the definition of the group descriptor, as defined in 244 <FILENAME>/usr/include/linux/ext2_fs.h</FILENAME>: 245 </Para> 246 247 <Para> 248 249 <ProgramListing> 250 struct ext2_group_desc 251 { 252 __u32 bg_block_bitmap; /* Blocks bitmap block */ 253 __u32 bg_inode_bitmap; /* Inodes bitmap block */ 254 __u32 bg_inode_table; /* Inodes table block */ 255 __u16 bg_free_blocks_count; /* Free blocks count */ 256 __u16 bg_free_inodes_count; /* Free inodes count */ 257 __u16 bg_used_dirs_count; /* Directories count */ 258 __u16 bg_pad; 259 __u32 bg_reserved[3]; 260 }; 261 </ProgramListing> 262 263 </Para> 264 265 <Para> 266 The last three variables: <Literal remap="tt">bg_free_blocks_count, bg_free_inodes_count and bg_used_dirs_count</Literal> provide statistics about the use of the three 267 resources in a blocks group - The <Literal remap="tt">blocks</Literal>, the <Literal remap="tt">inodes</Literal> and the 268 <Literal remap="tt">directories</Literal>. I believe that they are used by the kernel for balancing 269 the load between the various blocks groups. 270 </Para> 271 272 <Para> 273 <Literal remap="tt">bg_block_bitmap</Literal> contains the block number of the <Literal remap="tt">block allocation 274 bitmap block</Literal>. This is used to allocate / deallocate each block in the 275 specific blocks group. 276 </Para> 277 278 <Para> 279 <Literal remap="tt">bg_inode_bitmap</Literal> is fully analogous to the previous variable - It 280 contains the block number of the <Literal remap="tt">inode allocation bitmap block</Literal>, which 281 is used to allocate / deallocate each specific inode in the filesystem. 282 </Para> 283 284 <Para> 285 <Literal remap="tt">bg_inode_table</Literal> contains the block number of the start of the 286 <Literal remap="tt">inode table of the current blocks group</Literal>. The <Literal remap="tt">inode table</Literal> is 287 just the actual inodes which are reserved for the current block. 288 </Para> 289 290 <Para> 291 The block bitmap block, inode bitmap block and the inode table are created 292 when the filesystem is created. 293 </Para> 294 295 <Para> 296 The group descriptors are placed one after the other. Together they make the 297 <Literal remap="tt">group descriptors table</Literal>. 298 </Para> 299 300 <Para> 301 Each blocks group contains the entire table of group descriptors in its 302 second block, right after the superblock. However, only the first copy (in 303 group 0) is actually used by the kernel. The other copies are there for 304 backup purposes and can be of use if the main copy gets corrupted. 305 </Para> 306 307 </Sect1> 308 309 <Sect1> 310 <Title>The block bitmap allocation block</Title> 311 312 <Para> 313 Each blocks group contains one special block which is actually a map of the 314 entire blocks in the group, with respect to their allocation status. Each 315 <Literal remap="tt">bit</Literal> in the block bitmap indicated whether a specific block in the 316 group is used or free. 317 </Para> 318 319 <Para> 320 The format is actually quite simple - Just view the entire block as a series 321 of bits. For example, 322 </Para> 323 324 <Para> 325 Suppose the block size is 1024 bytes. As such, there is a place for 326 1024*8=8192 blocks in a group block. This number is one of the fields in the 327 filesystem's <Literal remap="tt">superblock</Literal>, which will be explained later. 328 </Para> 329 330 <Para> 331 332 <ItemizedList> 333 <ListItem> 334 335 <Para> 336 Block 0 in the blocks group is managed by bit 0 of byte 0 in the bitmap 337 block. 338 </Para> 339 </ListItem> 340 <ListItem> 341 342 <Para> 343 Block 7 in the blocks group is managed by bit 7 of byte 0 in the bitmap 344 block. 345 </Para> 346 </ListItem> 347 <ListItem> 348 349 <Para> 350 Block 8 in the blocks group is managed by bit 0 of byte 1 in the bitmap 351 block. 352 </Para> 353 </ListItem> 354 <ListItem> 355 356 <Para> 357 Block 8191 in the blocks group is managed by bit 7 of byte 1023 in the 358 bitmap block. 359 </Para> 360 </ListItem> 361 362 </ItemizedList> 363 364 </Para> 365 366 <Para> 367 A value of "<Literal remap="tt">1</Literal>" in the appropriate bit signals that the block is 368 allocated, while a value of "<Literal remap="tt">0</Literal>" signals that the block is 369 unallocated. 370 </Para> 371 372 <Para> 373 You will probably notice that typically, all the bits in a byte contain the 374 same value, making the byte's value <Literal remap="tt">0</Literal> or <Literal remap="tt">0ffh</Literal>. This is done by 375 the kernel on purpose in order to group related data in physically close 376 blocks, since the physical device is usually optimized to handle such a close 377 relationship. 378 </Para> 379 380 </Sect1> 381 382 <Sect1> 383 <Title>The inode allocation bitmap</Title> 384 385 <Para> 386 The format of the inode allocation bitmap block is exactly like the format of 387 the block allocation bitmap block. The explanation above is valid here, with 388 the work <Literal remap="tt">block</Literal> replaced by <Literal remap="tt">inode</Literal>. Typically, there are much less 389 inodes then blocks in a blocks group and thus only part of the inode bitmap 390 block is used. The number of inodes in a blocks group is another variable 391 which is listed in the <Literal remap="tt">superblock</Literal>. 392 </Para> 393 394 </Sect1> 395 396 <Sect1> 397 <Title>On the inode and the inode tables</Title> 398 399 <Para> 400 An inode is a main resource in the ext2 filesystem. It is used for various 401 purposes, but the main two are: 402 403 <ItemizedList> 404 <ListItem> 405 406 <Para> 407 Support of files 408 </Para> 409 </ListItem> 410 <ListItem> 411 412 <Para> 413 Support of directories 414 </Para> 415 </ListItem> 416 417 </ItemizedList> 418 419 </Para> 420 421 <Para> 422 Each file, for example, will allocate one inode from the filesystem 423 resources. 424 </Para> 425 426 <Para> 427 An ext2 filesystem has a total number of available inodes which is determined 428 while creating the filesystem. When all the inodes are used, for example, you 429 will not be able to create an additional file even though there will still 430 be free blocks on the filesystem. 431 </Para> 432 433 <Para> 434 Each inode takes up 128 bytes in the filesystem. By default, <Literal remap="tt">mke2fs</Literal> 435 reserves an inode for each 4096 bytes of the filesystem space. 436 </Para> 437 438 <Para> 439 The inodes are placed in several tables, each of which contains the same 440 number of inodes and is placed at a different blocks group. The goal is to 441 place inodes and their related files in the same blocks group because of 442 locality arguments. 443 </Para> 444 445 <Para> 446 The number of inodes in a blocks group is available in the superblock variable 447 <Literal remap="tt">s_inodes_per_group</Literal>. For example, if there are 2000 inodes per group, 448 group 0 will contain the inodes 1-2000, group 2 will contain the inodes 449 2001-4000, and so on. 450 </Para> 451 452 <Para> 453 Each inode table is accessed from the group descriptor of the specific 454 blocks group which contains the table. 455 </Para> 456 457 <Para> 458 Follows the structure of an inode in Ext2fs: 459 </Para> 460 461 <Para> 462 463 <ProgramListing> 464 struct ext2_inode { 465 __u16 i_mode; /* File mode */ 466 __u16 i_uid; /* Owner Uid */ 467 __u32 i_size; /* Size in bytes */ 468 __u32 i_atime; /* Access time */ 469 __u32 i_ctime; /* Creation time */ 470 __u32 i_mtime; /* Modification time */ 471 __u32 i_dtime; /* Deletion Time */ 472 __u16 i_gid; /* Group Id */ 473 __u16 i_links_count; /* Links count */ 474 __u32 i_blocks; /* Blocks count */ 475 __u32 i_flags; /* File flags */ 476 union { 477 struct { 478 __u32 l_i_reserved1; 479 } linux1; 480 struct { 481 __u32 h_i_translator; 482 } hurd1; 483 struct { 484 __u32 m_i_reserved1; 485 } masix1; 486 } osd1; /* OS dependent 1 */ 487 __u32 i_block[EXT2_N_BLOCKS];/* Pointers to blocks */ 488 __u32 i_version; /* File version (for NFS) */ 489 __u32 i_file_acl; /* File ACL */ 490 __u32 i_dir_acl; /* Directory ACL */ 491 __u32 i_faddr; /* Fragment address */ 492 union { 493 struct { 494 __u8 l_i_frag; /* Fragment number */ 495 __u8 l_i_fsize; /* Fragment size */ 496 __u16 i_pad1; 497 __u32 l_i_reserved2[2]; 498 } linux2; 499 struct { 500 __u8 h_i_frag; /* Fragment number */ 501 __u8 h_i_fsize; /* Fragment size */ 502 __u16 h_i_mode_high; 503 __u16 h_i_uid_high; 504 __u16 h_i_gid_high; 505 __u32 h_i_author; 506 } hurd2; 507 struct { 508 __u8 m_i_frag; /* Fragment number */ 509 __u8 m_i_fsize; /* Fragment size */ 510 __u16 m_pad1; 511 __u32 m_i_reserved2[2]; 512 } masix2; 513 } osd2; /* OS dependent 2 */ 514 }; 515 </ProgramListing> 516 517 </Para> 518 519 <Sect2> 520 <Title>The allocated blocks</Title> 521 522 <Para> 523 The basic functionality of an inode is to group together a series of 524 allocated blocks. There is no limitation on the allocated blocks - Each 525 block can be allocated to each inode. Nevertheless, block allocation will 526 usually be done in series to take advantage of the locality principle. 527 </Para> 528 529 <Para> 530 The inode is not always used in that way. I will now explain the allocation 531 of blocks, assuming that the current inode type indeed refers to a list of 532 allocated blocks. 533 </Para> 534 535 <Para> 536 It was found experimently that many of the files in the filesystem are 537 actually quite small. To take advantage of this effect, the kernel provides 538 storage of up to 12 block numbers in the inode itself. Those blocks are 539 called <Literal remap="tt">direct blocks</Literal>. The advantage is that once the kernel has the 540 inode, it can directly access the file's blocks, without an additional disk 541 access. Those 12 blocks are directly specified in the variables 542 <Literal remap="tt">i_block[0] to i_block[11]</Literal>. 543 </Para> 544 545 <Para> 546 <Literal remap="tt">i_block[12]</Literal> is the <Literal remap="tt">indirect block</Literal> - The block pointed by 547 i_block[12] will <Literal remap="tt">not</Literal> be a data block. Rather, it will just contain a 548 list of direct blocks. For example, if the block size is 1024 bytes, since 549 each block number is 4 bytes long, there will be place for 256 indirect 550 blocks. That is, block 13 till block 268 in the file will be accessed by the 551 <Literal remap="tt">indirect block</Literal> method. The penalty in this case, compared to the 552 direct blocks case, is that an additional access to the device is needed - 553 We need <Literal remap="tt">two</Literal> accesses to reach the required data block. 554 </Para> 555 556 <Para> 557 In much the same way, <Literal remap="tt">i_block[13]</Literal> is the <Literal remap="tt">double indirect block</Literal> 558 and <Literal remap="tt">i_block[14]</Literal> is the <Literal remap="tt">triple indirect block</Literal>. 559 </Para> 560 561 <Para> 562 <Literal remap="tt">i_block[13]</Literal> points to a block which contains pointers to indirect 563 blocks. Each one of them is handled in the way described above. 564 </Para> 565 566 <Para> 567 In much the same way, the triple indirect block is just an additional level 568 of indirection - It will point to a list of double indirect blocks. 569 </Para> 570 571 </Sect2> 572 573 <Sect2> 574 <Title>The i_mode variable</Title> 575 576 <Para> 577 The i_mode variable is used to determine the <Literal remap="tt">inode type</Literal> and the 578 associated <Literal remap="tt">permissions</Literal>. It is best described by representing it as an 579 octal number. Since it is a 16 bit variable, there will be 6 octal digits. 580 Those are divided into two parts - The rightmost 4 digits and the leftmost 2 581 digits. 582 </Para> 583 584 <Sect3> 585 <Title>The rightmost 4 octal digits</Title> 586 587 <Para> 588 The rightmost 4 digits are <Literal remap="tt">bit options</Literal> - Each bit has its own 589 purpose. 590 </Para> 591 592 <Para> 593 The last 3 digits (Octal digits 0,1 and 2) are just the usual permissions, 594 in the known form <Literal remap="tt">rwxrwxrwx</Literal>. Digit 2 refers to the user, digit 1 to 595 the group and digit 2 to everyone else. They are used by the kernel to grant 596 or deny access to the object presented by this inode. 597 <FOOTNOTE> 598 599 <Para> 600 A <Literal remap="tt">smarter</Literal> permissions control is one of the enhancements planned for 601 Linux 1.3 - The ACL (Access Control Lists). Actually, from browsing of the 602 kernel source, some of the ACL handling is already done. 603 </Para> 604 605 </FOOTNOTE> 606 607 </Para> 608 609 <Para> 610 Bit number 9 signals that the file (I'll refer to the object presented by 611 the inode as file even though it can be a special device, for example) is 612 <Literal remap="tt">set VTX</Literal>. I still don't know what is the meaning of "VTX". 613 </Para> 614 615 <Para> 616 Bit number 10 signals that the file is <Literal remap="tt">set group id</Literal> - I don't know 617 exactly the meaning of the above either. 618 </Para> 619 620 <Para> 621 Bit number 11 signals that the file is <Literal remap="tt">set user id</Literal>, which means that 622 the file will run with an effective user id root. 623 </Para> 624 625 </Sect3> 626 627 <Sect3> 628 <Title>The leftmost two octal digits</Title> 629 630 <Para> 631 Note the the leftmost octal digit can only be 0 or 1, since the total number 632 of bits is 16. 633 </Para> 634 635 <Para> 636 Those digits, as opposed to the rightmost 4 digits, are not bit mapped 637 options. They determine the type of the "file" to which the inode belongs: 638 639 <ItemizedList> 640 <ListItem> 641 642 <Para> 643 <Literal remap="tt">01</Literal> - The file is a <Literal remap="tt">FIFO</Literal>. 644 </Para> 645 </ListItem> 646 <ListItem> 647 648 <Para> 649 <Literal remap="tt">02</Literal> - The file is a <Literal remap="tt">character device</Literal>. 650 </Para> 651 </ListItem> 652 <ListItem> 653 654 <Para> 655 <Literal remap="tt">04</Literal> - The file is a <Literal remap="tt">directory</Literal>. 656 </Para> 657 </ListItem> 658 <ListItem> 659 660 <Para> 661 <Literal remap="tt">06</Literal> - The file is a <Literal remap="tt">block device</Literal>. 662 </Para> 663 </ListItem> 664 <ListItem> 665 666 <Para> 667 <Literal remap="tt">10</Literal> - The file is a <Literal remap="tt">regular file</Literal>. 668 </Para> 669 </ListItem> 670 <ListItem> 671 672 <Para> 673 <Literal remap="tt">12</Literal> - The file is a <Literal remap="tt">symbolic link</Literal>. 674 </Para> 675 </ListItem> 676 <ListItem> 677 678 <Para> 679 <Literal remap="tt">14</Literal> - The file is a <Literal remap="tt">socket</Literal>. 680 </Para> 681 </ListItem> 682 683 </ItemizedList> 684 685 </Para> 686 687 </Sect3> 688 689 </Sect2> 690 691 <Sect2> 692 <Title>Time and date</Title> 693 694 <Para> 695 Linux records the last time in which various operations occured with the 696 file. The time and date are saved in the standard C library format - The 697 number of seconds which passed since 00:00:00 GMT, January 1, 1970. The 698 following times are recorded: 699 700 <ItemizedList> 701 <ListItem> 702 703 <Para> 704 <Literal remap="tt">i_ctime</Literal> - The time in which the inode was last allocated. In 705 other words, the time in which the file was created. 706 </Para> 707 </ListItem> 708 <ListItem> 709 710 <Para> 711 <Literal remap="tt">i_mtime</Literal> - The time in which the file was last modified. 712 </Para> 713 </ListItem> 714 <ListItem> 715 716 <Para> 717 <Literal remap="tt">i_atime</Literal> - The time in which the file was last accessed. 718 </Para> 719 </ListItem> 720 <ListItem> 721 722 <Para> 723 <Literal remap="tt">i_dtime</Literal> - The time in which the inode was deallocated. In 724 other words, the time in which the file was deleted. 725 </Para> 726 </ListItem> 727 728 </ItemizedList> 729 730 </Para> 731 732 </Sect2> 733 734 <Sect2> 735 <Title>i_size</Title> 736 737 <Para> 738 <Literal remap="tt">i_size</Literal> contains information about the size of the object presented by 739 the inode. If the inode corresponds to a regular file, this is just the size 740 of the file in bytes. In other cases, the interpretation of the variable is 741 different. 742 </Para> 743 744 </Sect2> 745 746 <Sect2> 747 <Title>User and group id</Title> 748 749 <Para> 750 The user and group id of the file are just saved in the variables 751 <Literal remap="tt">i_uid</Literal> and <Literal remap="tt">i_gid</Literal>. 752 </Para> 753 754 </Sect2> 755 756 <Sect2> 757 <Title>Hard links</Title> 758 759 <Para> 760 Later, when we'll discuss the implementation of directories, it will be 761 explained that each <Literal remap="tt">directory entry</Literal> points to an inode. It is quite 762 possible that a <Literal remap="tt">single inode</Literal> will be pointed to from <Literal remap="tt">several</Literal> 763 directories. In that case, we say that there exist <Literal remap="tt">hard links</Literal> to the 764 file - The file can be accessed from each of the directories. 765 </Para> 766 767 <Para> 768 The kernel keeps track of the number of hard links in the variable 769 <Literal remap="tt">i_links_count</Literal>. The variable is set to "1" when first allocating the 770 inode, and is incremented with each additional link. Deletion of a file will 771 delete the current directory entry and will decrement the number of links. 772 Only when this number reaches zero, the inode will be actually deallocated. 773 </Para> 774 775 <Para> 776 The name <Literal remap="tt">hard link</Literal> is used to distinguish between the alias method 777 described above, to another alias method called <Literal remap="tt">symbolic linking</Literal>, 778 which will be described later. 779 </Para> 780 781 </Sect2> 782 783 <Sect2> 784 <Title>The Ext2fs extended flags</Title> 785 786 <Para> 787 The ext2 filesystem associates additional flags with an inode. The extended 788 attributes are stored in the variable <Literal remap="tt">i_flags</Literal>. <Literal remap="tt">i_flags</Literal> is a 32 789 bit variable. Only the 7 rightmost bits are defined. Of them, only 5 bits 790 are used in version 0.5a of the filesystem. Specifically, the 791 <Literal remap="tt">undelete</Literal> and the <Literal remap="tt">compress</Literal> features are not implemented, and 792 are to be introduced in Linux 1.3 development. 793 </Para> 794 795 <Para> 796 The currently available flags are: 797 798 <ItemizedList> 799 <ListItem> 800 801 <Para> 802 bit 0 - Secure deletion. 803 804 When this bit is on, the file's blocks are zeroed when the file is 805 deleted. With this bit off, they will just be left with their 806 original data when the inode is deallocated. 807 </Para> 808 </ListItem> 809 <ListItem> 810 811 <Para> 812 bit 1 - Undelete. 813 814 This bit is not supported yet. It will be used to provide an 815 <Literal remap="tt">undelete</Literal> feature in future Ext2fs developments. 816 </Para> 817 </ListItem> 818 <ListItem> 819 820 <Para> 821 bit 2 - Compress file. 822 823 This bit is also not supported. The plan is to offer "compression on 824 the fly" in future releases. 825 </Para> 826 </ListItem> 827 <ListItem> 828 829 <Para> 830 bit 3 - Synchronous updates. 831 832 With this bit on, the meta-data will be written synchronously to the 833 disk, as if the filesystem was mounted with the "sync" mount option. 834 </Para> 835 </ListItem> 836 <ListItem> 837 838 <Para> 839 bit 4 - Immutable file. 840 841 When this bit is on, the file will stay as it is - Can not be 842 changed, deleted, renamed, no hard links, etc, before the bit is 843 cleared. 844 </Para> 845 </ListItem> 846 <ListItem> 847 848 <Para> 849 bit 5 - Append only file. 850 851 With this option active, data will only be appended to the file. 852 </Para> 853 </ListItem> 854 <ListItem> 855 856 <Para> 857 bit 6 - Do not dump this file. 858 859 I think that this bit is used by the port of dump to linux (ported by 860 <Literal remap="tt">Remy Card</Literal>) to check if the file should not be dumped. 861 </Para> 862 </ListItem> 863 864 </ItemizedList> 865 866 </Para> 867 868 </Sect2> 869 870 <Sect2> 871 <Title>Symbolic links</Title> 872 873 <Para> 874 The <Literal remap="tt">hard links</Literal> presented above are just another pointers to the same 875 inode. The important aspect is that the inode number is <Literal remap="tt">fixed</Literal> when 876 the link is created. This means that the implementation details of the 877 filesystem are visible to the user - In a pure abstract usage of the 878 filesystem, the user should not care about inodes. 879 </Para> 880 881 <Para> 882 The above causes several limitations: 883 884 <ItemizedList> 885 <ListItem> 886 887 <Para> 888 Hard links can be done only in the same filesystem. This is obvious, 889 since a hard link is just an inode number in some directory entry, 890 and the above elements are filesystem specific. 891 </Para> 892 </ListItem> 893 <ListItem> 894 895 <Para> 896 You can not "replace" the file which is pointed to by the hard link 897 after the link creation. "Replacing" the file in one directory will 898 still leave the original file in the other directory - The 899 "replacement" will not deallocate the original inode, but rather 900 allocate another inode for the new version, and the directory entry 901 at the other place will just point to the old inode number. 902 </Para> 903 </ListItem> 904 905 </ItemizedList> 906 907 </Para> 908 909 <Para> 910 <Literal remap="tt">Symbolic link</Literal>, on the other hand, is analyzed at <Literal remap="tt">run time</Literal>. A 911 symbolic link is just a <Literal remap="tt">pathname</Literal> which is accessible from an inode. 912 As such, it "speaks" in the language of the abstract filesystem. When the 913 kernel reaches a symbolic link, it will <Literal remap="tt">follow it in run time</Literal> using 914 its normal way of reaching directories. 915 </Para> 916 917 <Para> 918 As such, symbolic link can be made <Literal remap="tt">across different filesystems</Literal> and a 919 replacement of a file with a new version will automatically be active on all 920 its symbolic links. 921 </Para> 922 923 <Para> 924 The disadvantage is that hard link doesn't consume space except to a small 925 directory entry. Symbolic link, on the other hand, consumes at least an 926 inode, and can also consume one block. 927 </Para> 928 929 <Para> 930 When the inode is identified as a symbolic link, the kernel needs to find 931 the path to which it points. 932 </Para> 933 934 <Sect3> 935 <Title>Fast symbolic links</Title> 936 937 <Para> 938 When the pathname contains up to 64 bytes, it can be saved directly in the 939 inode, on the <Literal remap="tt">i_block[0] - i_block[15]</Literal> variables, since those are not 940 needed in that case. This is called <Literal remap="tt">fast</Literal> symbolic link. It is fast 941 because the pathname resolution can be done using the inode itself, without 942 accessing additional blocks. It is also economical, since it allocates only 943 an inode. The length of the pathname is stored in the <Literal remap="tt">i_size</Literal> 944 variable. 945 </Para> 946 947 </Sect3> 948 949 <Sect3> 950 <Title>Slow symbolic links</Title> 951 952 <Para> 953 Starting from 65 bytes, additional block is allocated (by the use of 954 <Literal remap="tt">i_block[0]</Literal>) and the pathname is stored in it. It is called slow 955 because the kernel needs to read additional block to resolve the pathname. 956 The length is again saved in <Literal remap="tt">i_size</Literal>. 957 </Para> 958 959 </Sect3> 960 961 </Sect2> 962 963 <Sect2> 964 <Title>i_version</Title> 965 966 <Para> 967 <Literal remap="tt">i_version</Literal> is used with regard to Network File System. I don't know 968 its exact use. 969 </Para> 970 971 </Sect2> 972 973 <Sect2> 974 <Title>Reserved variables</Title> 975 976 <Para> 977 As far as I know, the variables which are connected to ACL and fragments 978 are not currently used. They will be supported in future versions. 979 </Para> 980 981 <Para> 982 Ext2fs is being ported to other operating systems. As far as I know, 983 at least in linux, the os dependent variables are also not used. 984 </Para> 985 986 </Sect2> 987 988 <Sect2> 989 <Title>Special reserved inodes</Title> 990 991 <Para> 992 The first ten inodes on the filesystem are special inodes: 993 994 <ItemizedList> 995 <ListItem> 996 997 <Para> 998 Inode 1 is the <Literal remap="tt">bad blocks inode</Literal> - I believe that its data 999 blocks contain a list of the bad blocks in the filesystem, which 1000 should not be allocated. 1001 </Para> 1002 </ListItem> 1003 <ListItem> 1004 1005 <Para> 1006 Inode 2 is the <Literal remap="tt">root inode</Literal> - The inode of the root directory. 1007 It is the starting point for reaching a known path in the filesystem. 1008 </Para> 1009 </ListItem> 1010 <ListItem> 1011 1012 <Para> 1013 Inode 3 is the <Literal remap="tt">acl index inode</Literal>. Access control lists are 1014 currently not supported by the ext2 filesystem, so I believe this 1015 inode is not used. 1016 </Para> 1017 </ListItem> 1018 <ListItem> 1019 1020 <Para> 1021 Inode 4 is the <Literal remap="tt">acl data inode</Literal>. Of course, the above applies 1022 here too. 1023 </Para> 1024 </ListItem> 1025 <ListItem> 1026 1027 <Para> 1028 Inode 5 is the <Literal remap="tt">boot loader inode</Literal>. I don't know its 1029 usage. 1030 </Para> 1031 </ListItem> 1032 <ListItem> 1033 1034 <Para> 1035 Inode 6 is the <Literal remap="tt">undelete directory inode</Literal>. It is also a 1036 foundation for future enhancements, and is currently not used. 1037 </Para> 1038 </ListItem> 1039 <ListItem> 1040 1041 <Para> 1042 Inodes 7-10 are <Literal remap="tt">reserved</Literal> and currently not used. 1043 </Para> 1044 </ListItem> 1045 1046 </ItemizedList> 1047 1048 </Para> 1049 1050 </Sect2> 1051 1052 </Sect1> 1053 1054 <Sect1> 1055 <Title>Directories</Title> 1056 1057 <Para> 1058 A directory is implemented in the same way as files are implemented (with 1059 the direct blocks, indirect blocks, etc) - It is just a file which is 1060 formatted with a special format - A list of directory entries. 1061 </Para> 1062 1063 <Para> 1064 Follows the definition of a directory entry: 1065 </Para> 1066 1067 <Para> 1068 1069 <ProgramListing> 1070 struct ext2_dir_entry { 1071 __u32 inode; /* Inode number */ 1072 __u16 rec_len; /* Directory entry length */ 1073 __u16 name_len; /* Name length */ 1074 char name[EXT2_NAME_LEN]; /* File name */ 1075 }; 1076 </ProgramListing> 1077 1078 </Para> 1079 1080 <Para> 1081 Ext2fs supports file names of varying lengths, up to 255 bytes. The 1082 <Literal remap="tt">name</Literal> field above just contains the file name. Note that it is 1083 <Literal remap="tt">not zero terminated</Literal>; Instead, the variable <Literal remap="tt">name_len</Literal> contains 1084 the length of the file name. 1085 </Para> 1086 1087 <Para> 1088 The variable <Literal remap="tt">rec_len</Literal> is provided because the directory entries are 1089 padded with zeroes so that the next entry will be in an offset which is 1090 a multiplition of 4. The resulting directory entry size is stored in 1091 <Literal remap="tt">rec_len</Literal>. If the directory entry is the last in the block, it is 1092 padded with zeroes till the end of the block, and rec_len is updated 1093 accordingly. 1094 </Para> 1095 1096 <Para> 1097 The <Literal remap="tt">inode</Literal> variable points to the inode of the above file. 1098 </Para> 1099 1100 <Para> 1101 Deletion of directory entries is done by appending of the deleted entry 1102 space to the previous (or next, I am not sure) entry. 1103 </Para> 1104 1105 </Sect1> 1106 1107 <Sect1> 1108 <Title>The superblock</Title> 1109 1110 <Para> 1111 The <Literal remap="tt">superblock</Literal> is a block which contains information which describes 1112 the state of the internal filesystem. 1113 </Para> 1114 1115 <Para> 1116 The superblock is located at the <Literal remap="tt">fixed offset 1024</Literal> in the device. Its 1117 length is 1024 bytes also. 1118 </Para> 1119 1120 <Para> 1121 The superblock, like the group descriptors, is copied on each blocks group 1122 boundary for backup purposes. However, only the main copy is used by the 1123 kernel. 1124 </Para> 1125 1126 <Para> 1127 The superblock contain three types of information: 1128 1129 <ItemizedList> 1130 <ListItem> 1131 1132 <Para> 1133 Filesystem parameters which are fixed and which were determined when 1134 this specific filesystem was created. Some of those parameters can 1135 be different in different installations of the ext2 filesystem, but 1136 can not be changed once the filesystem was created. 1137 </Para> 1138 </ListItem> 1139 <ListItem> 1140 1141 <Para> 1142 Filesystem parameters which are tunable - Can always be changed. 1143 </Para> 1144 </ListItem> 1145 <ListItem> 1146 1147 <Para> 1148 Information about the current filesystem state. 1149 </Para> 1150 </ListItem> 1151 1152 </ItemizedList> 1153 1154 </Para> 1155 1156 <Para> 1157 Follows the superblock definition: 1158 </Para> 1159 1160 <Para> 1161 1162 <ProgramListing> 1163 struct ext2_super_block { 1164 __u32 s_inodes_count; /* Inodes count */ 1165 __u32 s_blocks_count; /* Blocks count */ 1166 __u32 s_r_blocks_count; /* Reserved blocks count */ 1167 __u32 s_free_blocks_count; /* Free blocks count */ 1168 __u32 s_free_inodes_count; /* Free inodes count */ 1169 __u32 s_first_data_block; /* First Data Block */ 1170 __u32 s_log_block_size; /* Block size */ 1171 __s32 s_log_frag_size; /* Fragment size */ 1172 __u32 s_blocks_per_group; /* # Blocks per group */ 1173 __u32 s_frags_per_group; /* # Fragments per group */ 1174 __u32 s_inodes_per_group; /* # Inodes per group */ 1175 __u32 s_mtime; /* Mount time */ 1176 __u32 s_wtime; /* Write time */ 1177 __u16 s_mnt_count; /* Mount count */ 1178 __s16 s_max_mnt_count; /* Maximal mount count */ 1179 __u16 s_magic; /* Magic signature */ 1180 __u16 s_state; /* File system state */ 1181 __u16 s_errors; /* Behaviour when detecting errors */ 1182 __u16 s_pad; 1183 __u32 s_lastcheck; /* time of last check */ 1184 __u32 s_checkinterval; /* max. time between checks */ 1185 __u32 s_creator_os; /* OS */ 1186 __u32 s_rev_level; /* Revision level */ 1187 __u16 s_def_resuid; /* Default uid for reserved blocks */ 1188 __u16 s_def_resgid; /* Default gid for reserved blocks */ 1189 __u32 s_reserved[235]; /* Padding to the end of the block */ 1190 }; 1191 </ProgramListing> 1192 1193 </Para> 1194 1195 <Sect2> 1196 <Title>superblock identification</Title> 1197 1198 <Para> 1199 The ext2 filesystem's superblock is identified by the <Literal remap="tt">s_magic</Literal> field. 1200 The current ext2 magic number is 0xEF53. I presume that "EF" means "Extended 1201 Filesystem". In versions of the ext2 filesystem prior to 0.2B, the magic 1202 number was 0xEF51. Those filesystems are not compatible with the current 1203 versions; Specifically, the group descriptors definition is different. I 1204 doubt if there still exists such a installation. 1205 </Para> 1206 1207 </Sect2> 1208 1209 <Sect2> 1210 <Title>Filesystem fixed parameters</Title> 1211 1212 <Para> 1213 By using the word <Literal remap="tt">fixed</Literal>, I mean fixed with respect to a particular 1214 installation. Those variables are usually not fixed with respect to 1215 different installations. 1216 </Para> 1217 1218 <Para> 1219 The <Literal remap="tt">block size</Literal> is determined by using the <Literal remap="tt">s_log_block_size</Literal> 1220 variable. The block size is 1024*pow (2,s_log_block_size) and should be 1221 between 1024 and 4096. The available options are 1024, 2048 and 4096. 1222 </Para> 1223 1224 <Para> 1225 <Literal remap="tt">s_inodes_count</Literal> contains the total number of available inodes. 1226 </Para> 1227 1228 <Para> 1229 <Literal remap="tt">s_blocks_count</Literal> contains the total number of available blocks. 1230 </Para> 1231 1232 <Para> 1233 <Literal remap="tt">s_first_data_block</Literal> specifies in which of the <Literal remap="tt">device block</Literal> the 1234 <Literal remap="tt">superblock</Literal> is present. The superblock is always present at the fixed 1235 offset 1024, but the device block numbering can differ. For example, if the 1236 block size is 1024, the superblock will be at <Literal remap="tt">block 1</Literal> with respect to 1237 the device. However, if the block size is 4096, offset 1024 is included in 1238 <Literal remap="tt">block 0</Literal> of the device, and in that case <Literal remap="tt">s_first_data_block</Literal> 1239 will contain 0. At least this is how I understood this variable. 1240 </Para> 1241 1242 <Para> 1243 <Literal remap="tt">s_blocks_per_group</Literal> contains the number of blocks which are grouped 1244 together as a blocks group. 1245 </Para> 1246 1247 <Para> 1248 <Literal remap="tt">s_inodes_per_group</Literal> contains the number of inodes available in a group 1249 block. I think that this is always the total number of inodes divided by the 1250 number of blocks groups. 1251 </Para> 1252 1253 <Para> 1254 <Literal remap="tt">s_creator_os</Literal> contains a code number which specifies the operating 1255 system which created this specific filesystem: 1256 1257 <ItemizedList> 1258 <ListItem> 1259 1260 <Para> 1261 <Literal remap="tt">Linux</Literal> :-) is specified by the value <Literal remap="tt">0</Literal>. 1262 </Para> 1263 </ListItem> 1264 <ListItem> 1265 1266 <Para> 1267 <Literal remap="tt">Hurd</Literal> is specified by the value <Literal remap="tt">1</Literal>. 1268 </Para> 1269 </ListItem> 1270 <ListItem> 1271 1272 <Para> 1273 <Literal remap="tt">Masix</Literal> is specified by the value <Literal remap="tt">2</Literal>. 1274 </Para> 1275 </ListItem> 1276 1277 </ItemizedList> 1278 1279 </Para> 1280 1281 <Para> 1282 <Literal remap="tt">s_rev_level</Literal> contains the major version of the ext2 filesystem. 1283 Currently this is always <Literal remap="tt">0</Literal>, as the most recent version is 0.5B. It 1284 will probably take some time until we reach version 1.0. 1285 </Para> 1286 1287 <Para> 1288 As far as I know, fragments (sub-block allocations) are currently not 1289 supported and hence a block is equal to a fragment. As a result, 1290 <Literal remap="tt">s_log_frag_size</Literal> and <Literal remap="tt">s_frags_per_group</Literal> are always equal to 1291 <Literal remap="tt">s_log_block_size</Literal> and <Literal remap="tt">s_blocks_per_group</Literal>, respectively. 1292 </Para> 1293 1294 </Sect2> 1295 1296 <Sect2> 1297 <Title>Ext2fs error handling</Title> 1298 1299 <Para> 1300 The ext2 filesystem error handling is based on the following philosophy: 1301 1302 <OrderedList> 1303 <ListItem> 1304 1305 <Para> 1306 Identification of problems is done by the kernel code. 1307 </Para> 1308 </ListItem> 1309 <ListItem> 1310 1311 <Para> 1312 The correction task is left to an external utility, such as 1313 <Literal remap="tt">e2fsck by Theodore Ts'o</Literal> for <Literal remap="tt">automatic</Literal> analysis and 1314 correction, or perhaps <Literal remap="tt">debugfs by Theodore Ts'o</Literal> and 1315 <Literal remap="tt">EXT2ED by myself</Literal>, for <Literal remap="tt">hand</Literal> analysis and correction. 1316 </Para> 1317 </ListItem> 1318 1319 </OrderedList> 1320 1321 </Para> 1322 1323 <Para> 1324 The <Literal remap="tt">s_state</Literal> variable is used by the kernel to pass the identification 1325 result to third party utilities: 1326 1327 <ItemizedList> 1328 <ListItem> 1329 1330 <Para> 1331 <Literal remap="tt">bit 0</Literal> of s_state is reset when the partition is mounted and 1332 set when the partition is unmounted. Thus, a value of 0 on an 1333 unmounted filesystem means that the filesystem was not unmounted 1334 properly - The filesystem is not "clean" and probably contains 1335 errors. 1336 </Para> 1337 </ListItem> 1338 <ListItem> 1339 1340 <Para> 1341 <Literal remap="tt">bit 1</Literal> of s_state is set by the kernel when it detects an 1342 error in the filesystem. A value of 0 doesn't mean that there isn't 1343 an error in the filesystem, just that the kernel didn't find any. 1344 </Para> 1345 </ListItem> 1346 1347 </ItemizedList> 1348 1349 </Para> 1350 1351 <Para> 1352 The kernel behavior when an error is found is determined by the user tunable 1353 parameter <Literal remap="tt">s_errors</Literal>: 1354 1355 <ItemizedList> 1356 <ListItem> 1357 1358 <Para> 1359 The kernel will ignore the error and continue if <Literal remap="tt">s_errors=1</Literal>. 1360 </Para> 1361 </ListItem> 1362 <ListItem> 1363 1364 <Para> 1365 The kernel will remount the filesystem in read-only mode if 1366 <Literal remap="tt">s_errors=2</Literal>. 1367 </Para> 1368 </ListItem> 1369 <ListItem> 1370 1371 <Para> 1372 A kernel panic will be issued if <Literal remap="tt">s_errors=3</Literal>. 1373 </Para> 1374 </ListItem> 1375 1376 </ItemizedList> 1377 1378 </Para> 1379 1380 <Para> 1381 The default behavior is to ignore the error. 1382 </Para> 1383 1384 </Sect2> 1385 1386 <Sect2> 1387 <Title>Additional parameters used by e2fsck</Title> 1388 1389 <Para> 1390 Of-course, <Literal remap="tt">e2fsck</Literal> will check the filesystem if errors were detected 1391 or if the filesystem is not clean. 1392 </Para> 1393 1394 <Para> 1395 In addition, each time the filesystem is mounted, <Literal remap="tt">s_mnt_count</Literal> is 1396 incremented. When s_mnt_count reaches <Literal remap="tt">s_max_mnt_count</Literal>, <Literal remap="tt">e2fsck</Literal> 1397 will force a check on the filesystem even though it may be clean. It will 1398 then zero s_mnt_count. <Literal remap="tt">s_max_mnt_count</Literal> is a tunable parameter. 1399 </Para> 1400 1401 <Para> 1402 E2fsck also records the last time in which the file system was checked in 1403 the <Literal remap="tt">s_lastcheck</Literal> variable. The user tunable parameter 1404 <Literal remap="tt">s_checkinterval</Literal> will contain the number of seconds which are allowed 1405 to pass since <Literal remap="tt">s_lastcheck</Literal> until a check is reforced. A value of 1406 <Literal remap="tt">0</Literal> disables time-based check. 1407 </Para> 1408 1409 </Sect2> 1410 1411 <Sect2> 1412 <Title>Additional user tunable parameters</Title> 1413 1414 <Para> 1415 <Literal remap="tt">s_r_blocks_count</Literal> contains the number of disk blocks which are 1416 reserved for root, the user whose id number is <Literal remap="tt">s_def_resuid</Literal> and the 1417 group whose id number is <Literal remap="tt">s_deg_resgid</Literal>. The kernel will refuse to 1418 allocate those last <Literal remap="tt">s_r_blocks_count</Literal> if the user is not one of the 1419 above. This is done so that the filesystem will usually not be 100% full, 1420 since 100% full filesystems can affect various aspects of operation. 1421 </Para> 1422 1423 <Para> 1424 <Literal remap="tt">s_def_resuid</Literal> and <Literal remap="tt">s_def_resgid</Literal> contain the id of the user and 1425 of the group who can use the reserved blocks in addition to root. 1426 </Para> 1427 1428 </Sect2> 1429 1430 <Sect2> 1431 <Title>Filesystem current state</Title> 1432 1433 <Para> 1434 <Literal remap="tt">s_free_blocks_count</Literal> contains the current number of free blocks 1435 in the filesystem. 1436 </Para> 1437 1438 <Para> 1439 <Literal remap="tt">s_free_inodes_count</Literal> contains the current number of free inodes in the 1440 filesystem. 1441 </Para> 1442 1443 <Para> 1444 <Literal remap="tt">s_mtime</Literal> contains the time at which the system was last mounted. 1445 </Para> 1446 1447 <Para> 1448 <Literal remap="tt">s_wtime</Literal> contains the last time at which something was changed in the 1449 filesystem. 1450 </Para> 1451 1452 </Sect2> 1453 1454 </Sect1> 1455 1456 <Sect1> 1457 <Title>Copyright</Title> 1458 1459 <Para> 1460 This document contains source code which was taken from the Linux ext2 1461 kernel source code, mainly from <FILENAME>/usr/include/linux/ext2_fs.h</FILENAME>. Follows 1462 the original copyright: 1463 </Para> 1464 1465 <Para> 1466 1467 <ProgramListing> 1468 /* 1469 * linux/include/linux/ext2_fs.h 1470 * 1471 * Copyright (C) 1992, 1993, 1994, 1995 1472 * Remy Card (card (a] masi.ibp.fr) 1473 * Laboratoire MASI - Institut Blaise Pascal 1474 * Universite Pierre et Marie Curie (Paris VI) 1475 * 1476 * from 1477 * 1478 * linux/include/linux/minix_fs.h 1479 * 1480 * Copyright (C) 1991, 1992 Linus Torvalds 1481 */ 1482 1483 </ProgramListing> 1484 1485 </Para> 1486 1487 </Sect1> 1488 1489 <Sect1> 1490 <Title>Acknowledgments</Title> 1491 1492 <Para> 1493 I would like to thank the following people, who were involved in the 1494 design and implementation of the ext2 filesystem kernel code and support 1495 utilities: 1496 1497 <ItemizedList> 1498 <ListItem> 1499 1500 <Para> 1501 <Literal remap="tt">Remy Card</Literal> 1502 1503 Who designed, implemented and maintains the ext2 filesystem kernel 1504 code, and some of the ext2 utilities. <Literal remap="tt">Remy Card</Literal> is also the 1505 author of several helpful slides concerning the ext2 filesystem. 1506 Specifically, he is the author of <Literal remap="tt">File Management in the Linux 1507 Kernel</Literal> and of <Literal remap="tt">The Second Extended File System - Current 1508 State, Future Development</Literal>. 1509 1510 </Para> 1511 </ListItem> 1512 <ListItem> 1513 1514 <Para> 1515 <Literal remap="tt">Wayne Davison</Literal> 1516 1517 Who designed the ext2 filesystem. 1518 </Para> 1519 </ListItem> 1520 <ListItem> 1521 1522 <Para> 1523 <Literal remap="tt">Stephen Tweedie</Literal> 1524 1525 Who helped designing the ext2 filesystem kernel code and wrote the 1526 slides <Literal remap="tt">Optimizations in File Systems</Literal>. 1527 </Para> 1528 </ListItem> 1529 <ListItem> 1530 1531 <Para> 1532 <Literal remap="tt">Theodore Ts'o</Literal> 1533 1534 Who is the author of several ext2 utilities and of the ext2 library 1535 <Literal remap="tt">libext2fs</Literal> (which I didn't use, simply because I didn't know 1536 it exists when I started to work on my project). 1537 </Para> 1538 </ListItem> 1539 1540 </ItemizedList> 1541 1542 </Para> 1543 1544 <Para> 1545 Lastly, I would like to thank, of-course, <Literal remap="tt">Linus Torvalds</Literal> and the 1546 <Literal remap="tt">Linux community</Literal> for providing all of us with such a great operating 1547 system. 1548 </Para> 1549 1550 <Para> 1551 Please contact me in a case of an error report, suggestions, or just about 1552 anything concerning this document. 1553 </Para> 1554 1555 <Para> 1556 Enjoy, 1557 </Para> 1558 1559 <Para> 1560 Gadi Oxman <tgud (a] tochnapc2.technion.ac.il> 1561 </Para> 1562 1563 <Para> 1564 Haifa, August 95 1565 </Para> 1566 1567 </Sect1> 1568 1569 </Article> 1570
