| blob | 5f81dd87e0b93b5d61f32ff68d57176437019aa1 |
1 .. SPDX-License-Identifier: GPL-2.0
3 Super Block
4 -----------
6 The superblock records various information about the enclosing
7 filesystem, such as block counts, inode counts, supported features,
8 maintenance information, and more.
10 If the sparse\_super feature flag is set, redundant copies of the
11 superblock and group descriptors are kept only in the groups whose group
12 number is either 0 or a power of 3, 5, or 7. If the flag is not set,
13 redundant copies are kept in all groups.
15 The superblock checksum is calculated against the superblock structure,
16 which includes the FS UUID.
18 The ext4 superblock is laid out as follows in
19 ``struct ext4_super_block``:
21 .. list-table::
22 :widths: 1 1 1 77
23 :header-rows: 1
25 * - Offset
26 - Size
27 - Name
28 - Description
29 * - 0x0
30 - \_\_le32
31 - s\_inodes\_count
32 - Total inode count.
33 * - 0x4
34 - \_\_le32
35 - s\_blocks\_count\_lo
36 - Total block count.
37 * - 0x8
38 - \_\_le32
39 - s\_r\_blocks\_count\_lo
40 - This number of blocks can only be allocated by the super-user.
41 * - 0xC
42 - \_\_le32
43 - s\_free\_blocks\_count\_lo
44 - Free block count.
45 * - 0x10
46 - \_\_le32
47 - s\_free\_inodes\_count
48 - Free inode count.
49 * - 0x14
50 - \_\_le32
51 - s\_first\_data\_block
52 - First data block. This must be at least 1 for 1k-block filesystems and
53 is typically 0 for all other block sizes.
54 * - 0x18
55 - \_\_le32
56 - s\_log\_block\_size
57 - Block size is 2 ^ (10 + s\_log\_block\_size).
58 * - 0x1C
59 - \_\_le32
60 - s\_log\_cluster\_size
61 - Cluster size is (2 ^ s\_log\_cluster\_size) blocks if bigalloc is
62 enabled. Otherwise s\_log\_cluster\_size must equal s\_log\_block\_size.
63 * - 0x20
64 - \_\_le32
65 - s\_blocks\_per\_group
66 - Blocks per group.
67 * - 0x24
68 - \_\_le32
69 - s\_clusters\_per\_group
70 - Clusters per group, if bigalloc is enabled. Otherwise
71 s\_clusters\_per\_group must equal s\_blocks\_per\_group.
72 * - 0x28
73 - \_\_le32
74 - s\_inodes\_per\_group
75 - Inodes per group.
76 * - 0x2C
77 - \_\_le32
78 - s\_mtime
79 - Mount time, in seconds since the epoch.
80 * - 0x30
81 - \_\_le32
82 - s\_wtime
83 - Write time, in seconds since the epoch.
84 * - 0x34
85 - \_\_le16
86 - s\_mnt\_count
87 - Number of mounts since the last fsck.
88 * - 0x36
89 - \_\_le16
90 - s\_max\_mnt\_count
91 - Number of mounts beyond which a fsck is needed.
92 * - 0x38
93 - \_\_le16
94 - s\_magic
95 - Magic signature, 0xEF53
96 * - 0x3A
97 - \_\_le16
98 - s\_state
99 - File system state. See super_state_ for more info.
100 * - 0x3C
101 - \_\_le16
102 - s\_errors
103 - Behaviour when detecting errors. See super_errors_ for more info.
104 * - 0x3E
105 - \_\_le16
106 - s\_minor\_rev\_level
107 - Minor revision level.
108 * - 0x40
109 - \_\_le32
110 - s\_lastcheck
111 - Time of last check, in seconds since the epoch.
112 * - 0x44
113 - \_\_le32
114 - s\_checkinterval
115 - Maximum time between checks, in seconds.
116 * - 0x48
117 - \_\_le32
118 - s\_creator\_os
119 - Creator OS. See the table super_creator_ for more info.
120 * - 0x4C
121 - \_\_le32
122 - s\_rev\_level
123 - Revision level. See the table super_revision_ for more info.
124 * - 0x50
125 - \_\_le16
126 - s\_def\_resuid
127 - Default uid for reserved blocks.
128 * - 0x52
129 - \_\_le16
130 - s\_def\_resgid
131 - Default gid for reserved blocks.
132 * -
133 -
134 -
135 - These fields are for EXT4_DYNAMIC_REV superblocks only.
137 Note: the difference between the compatible feature set and the
138 incompatible feature set is that if there is a bit set in the
139 incompatible feature set that the kernel doesn't know about, it should
140 refuse to mount the filesystem.
142 e2fsck's requirements are more strict; if it doesn't know
143 about a feature in either the compatible or incompatible feature set, it
144 must abort and not try to meddle with things it doesn't understand...
145 * - 0x54
146 - \_\_le32
147 - s\_first\_ino
148 - First non-reserved inode.
149 * - 0x58
150 - \_\_le16
151 - s\_inode\_size
152 - Size of inode structure, in bytes.
153 * - 0x5A
154 - \_\_le16
155 - s\_block\_group\_nr
156 - Block group # of this superblock.
157 * - 0x5C
158 - \_\_le32
159 - s\_feature\_compat
160 - Compatible feature set flags. Kernel can still read/write this fs even
161 if it doesn't understand a flag; fsck should not do that. See the
162 super_compat_ table for more info.
163 * - 0x60
164 - \_\_le32
165 - s\_feature\_incompat
166 - Incompatible feature set. If the kernel or fsck doesn't understand one
167 of these bits, it should stop. See the super_incompat_ table for more
168 info.
169 * - 0x64
170 - \_\_le32
171 - s\_feature\_ro\_compat
172 - Readonly-compatible feature set. If the kernel doesn't understand one of
173 these bits, it can still mount read-only. See the super_rocompat_ table
174 for more info.
175 * - 0x68
176 - \_\_u8
177 - s\_uuid[16]
178 - 128-bit UUID for volume.
179 * - 0x78
180 - char
181 - s\_volume\_name[16]
182 - Volume label.
183 * - 0x88
184 - char
185 - s\_last\_mounted[64]
186 - Directory where filesystem was last mounted.
187 * - 0xC8
188 - \_\_le32
189 - s\_algorithm\_usage\_bitmap
190 - For compression (Not used in e2fsprogs/Linux)
191 * -
192 -
193 -
194 - Performance hints. Directory preallocation should only happen if the
195 EXT4_FEATURE_COMPAT_DIR_PREALLOC flag is on.
196 * - 0xCC
197 - \_\_u8
198 - s\_prealloc\_blocks
199 - #. of blocks to try to preallocate for ... files? (Not used in
200 e2fsprogs/Linux)
201 * - 0xCD
202 - \_\_u8
203 - s\_prealloc\_dir\_blocks
204 - #. of blocks to preallocate for directories. (Not used in
205 e2fsprogs/Linux)
206 * - 0xCE
207 - \_\_le16
208 - s\_reserved\_gdt\_blocks
209 - Number of reserved GDT entries for future filesystem expansion.
210 * -
211 -
212 -
213 - Journalling support is valid only if EXT4_FEATURE_COMPAT_HAS_JOURNAL is
214 set.
215 * - 0xD0
216 - \_\_u8
217 - s\_journal\_uuid[16]
218 - UUID of journal superblock
219 * - 0xE0
220 - \_\_le32
221 - s\_journal\_inum
222 - inode number of journal file.
223 * - 0xE4
224 - \_\_le32
225 - s\_journal\_dev
226 - Device number of journal file, if the external journal feature flag is
227 set.
228 * - 0xE8
229 - \_\_le32
230 - s\_last\_orphan
231 - Start of list of orphaned inodes to delete.
232 * - 0xEC
233 - \_\_le32
234 - s\_hash\_seed[4]
235 - HTREE hash seed.
236 * - 0xFC
237 - \_\_u8
238 - s\_def\_hash\_version
239 - Default hash algorithm to use for directory hashes. See super_def_hash_
240 for more info.
241 * - 0xFD
242 - \_\_u8
243 - s\_jnl\_backup\_type
244 - If this value is 0 or EXT3\_JNL\_BACKUP\_BLOCKS (1), then the
245 ``s_jnl_blocks`` field contains a duplicate copy of the inode's
246 ``i_block[]`` array and ``i_size``.
247 * - 0xFE
248 - \_\_le16
249 - s\_desc\_size
250 - Size of group descriptors, in bytes, if the 64bit incompat feature flag
251 is set.
252 * - 0x100
253 - \_\_le32
254 - s\_default\_mount\_opts
255 - Default mount options. See the super_mountopts_ table for more info.
256 * - 0x104
257 - \_\_le32
258 - s\_first\_meta\_bg
259 - First metablock block group, if the meta\_bg feature is enabled.
260 * - 0x108
261 - \_\_le32
262 - s\_mkfs\_time
263 - When the filesystem was created, in seconds since the epoch.
264 * - 0x10C
265 - \_\_le32
266 - s\_jnl\_blocks[17]
267 - Backup copy of the journal inode's ``i_block[]`` array in the first 15
268 elements and i\_size\_high and i\_size in the 16th and 17th elements,
269 respectively.
270 * -
271 -
272 -
273 - 64bit support is valid only if EXT4_FEATURE_COMPAT_64BIT is set.
274 * - 0x150
275 - \_\_le32
276 - s\_blocks\_count\_hi
277 - High 32-bits of the block count.
278 * - 0x154
279 - \_\_le32
280 - s\_r\_blocks\_count\_hi
281 - High 32-bits of the reserved block count.
282 * - 0x158
283 - \_\_le32
284 - s\_free\_blocks\_count\_hi
285 - High 32-bits of the free block count.
286 * - 0x15C
287 - \_\_le16
288 - s\_min\_extra\_isize
289 - All inodes have at least # bytes.
290 * - 0x15E
291 - \_\_le16
292 - s\_want\_extra\_isize
293 - New inodes should reserve # bytes.
294 * - 0x160
295 - \_\_le32
296 - s\_flags
297 - Miscellaneous flags. See the super_flags_ table for more info.
298 * - 0x164
299 - \_\_le16
300 - s\_raid\_stride
301 - RAID stride. This is the number of logical blocks read from or written
302 to the disk before moving to the next disk. This affects the placement
303 of filesystem metadata, which will hopefully make RAID storage faster.
304 * - 0x166
305 - \_\_le16
306 - s\_mmp\_interval
307 - #. seconds to wait in multi-mount prevention (MMP) checking. In theory,
308 MMP is a mechanism to record in the superblock which host and device
309 have mounted the filesystem, in order to prevent multiple mounts. This
310 feature does not seem to be implemented...
311 * - 0x168
312 - \_\_le64
313 - s\_mmp\_block
314 - Block # for multi-mount protection data.
315 * - 0x170
316 - \_\_le32
317 - s\_raid\_stripe\_width
318 - RAID stripe width. This is the number of logical blocks read from or
319 written to the disk before coming back to the current disk. This is used
320 by the block allocator to try to reduce the number of read-modify-write
321 operations in a RAID5/6.
322 * - 0x174
323 - \_\_u8
324 - s\_log\_groups\_per\_flex
325 - Size of a flexible block group is 2 ^ ``s_log_groups_per_flex``.
326 * - 0x175
327 - \_\_u8
328 - s\_checksum\_type
329 - Metadata checksum algorithm type. The only valid value is 1 (crc32c).
330 * - 0x176
331 - \_\_le16
332 - s\_reserved\_pad
333 -
334 * - 0x178
335 - \_\_le64
336 - s\_kbytes\_written
337 - Number of KiB written to this filesystem over its lifetime.
338 * - 0x180
339 - \_\_le32
340 - s\_snapshot\_inum
341 - inode number of active snapshot. (Not used in e2fsprogs/Linux.)
342 * - 0x184
343 - \_\_le32
344 - s\_snapshot\_id
345 - Sequential ID of active snapshot. (Not used in e2fsprogs/Linux.)
346 * - 0x188
347 - \_\_le64
348 - s\_snapshot\_r\_blocks\_count
349 - Number of blocks reserved for active snapshot's future use. (Not used in
350 e2fsprogs/Linux.)
351 * - 0x190
352 - \_\_le32
353 - s\_snapshot\_list
354 - inode number of the head of the on-disk snapshot list. (Not used in
355 e2fsprogs/Linux.)
356 * - 0x194
357 - \_\_le32
358 - s\_error\_count
359 - Number of errors seen.
360 * - 0x198
361 - \_\_le32
362 - s\_first\_error\_time
363 - First time an error happened, in seconds since the epoch.
364 * - 0x19C
365 - \_\_le32
366 - s\_first\_error\_ino
367 - inode involved in first error.
368 * - 0x1A0
369 - \_\_le64
370 - s\_first\_error\_block
371 - Number of block involved of first error.
372 * - 0x1A8
373 - \_\_u8
374 - s\_first\_error\_func[32]
375 - Name of function where the error happened.
376 * - 0x1C8
377 - \_\_le32
378 - s\_first\_error\_line
379 - Line number where error happened.
380 * - 0x1CC
381 - \_\_le32
382 - s\_last\_error\_time
383 - Time of most recent error, in seconds since the epoch.
384 * - 0x1D0
385 - \_\_le32
386 - s\_last\_error\_ino
387 - inode involved in most recent error.
388 * - 0x1D4
389 - \_\_le32
390 - s\_last\_error\_line
391 - Line number where most recent error happened.
392 * - 0x1D8
393 - \_\_le64
394 - s\_last\_error\_block
395 - Number of block involved in most recent error.
396 * - 0x1E0
397 - \_\_u8
398 - s\_last\_error\_func[32]
399 - Name of function where the most recent error happened.
400 * - 0x200
401 - \_\_u8
402 - s\_mount\_opts[64]
403 - ASCIIZ string of mount options.
404 * - 0x240
405 - \_\_le32
406 - s\_usr\_quota\_inum
407 - Inode number of user `quota <quota>`__ file.
408 * - 0x244
409 - \_\_le32
410 - s\_grp\_quota\_inum
411 - Inode number of group `quota <quota>`__ file.
412 * - 0x248
413 - \_\_le32
414 - s\_overhead\_blocks
415 - Overhead blocks/clusters in fs. (Huh? This field is always zero, which
416 means that the kernel calculates it dynamically.)
417 * - 0x24C
418 - \_\_le32
419 - s\_backup\_bgs[2]
420 - Block groups containing superblock backups (if sparse\_super2)
421 * - 0x254
422 - \_\_u8
423 - s\_encrypt\_algos[4]
424 - Encryption algorithms in use. There can be up to four algorithms in use
425 at any time; valid algorithm codes are given in the super_encrypt_ table
426 below.
427 * - 0x258
428 - \_\_u8
429 - s\_encrypt\_pw\_salt[16]
430 - Salt for the string2key algorithm for encryption.
431 * - 0x268
432 - \_\_le32
433 - s\_lpf\_ino
434 - Inode number of lost+found
435 * - 0x26C
436 - \_\_le32
437 - s\_prj\_quota\_inum
438 - Inode that tracks project quotas.
439 * - 0x270
440 - \_\_le32
441 - s\_checksum\_seed
442 - Checksum seed used for metadata\_csum calculations. This value is
443 crc32c(~0, $orig\_fs\_uuid).
444 * - 0x274
445 - \_\_u8
446 - s\_wtime_hi
447 - Upper 8 bits of the s_wtime field.
448 * - 0x275
449 - \_\_u8
450 - s\_wtime_hi
451 - Upper 8 bits of the s_mtime field.
452 * - 0x276
453 - \_\_u8
454 - s\_mkfs_time_hi
455 - Upper 8 bits of the s_mkfs_time field.
456 * - 0x277
457 - \_\_u8
458 - s\_lastcheck_hi
459 - Upper 8 bits of the s_lastcheck_hi field.
460 * - 0x278
461 - \_\_u8
462 - s\_first_error_time_hi
463 - Upper 8 bits of the s_first_error_time_hi field.
464 * - 0x279
465 - \_\_u8
466 - s\_last_error_time_hi
467 - Upper 8 bits of the s_last_error_time_hi field.
468 * - 0x27A
469 - \_\_u8[2]
470 - s\_pad
471 - Zero padding.
472 * - 0x27C
473 - \_\_le32
474 - s\_reserved[96]
475 - Padding to the end of the block.
476 * - 0x3FC
477 - \_\_le32
478 - s\_checksum
479 - Superblock checksum.
481 .. _super_state:
483 The superblock state is some combination of the following:
485 .. list-table::
486 :widths: 1 79
487 :header-rows: 1
489 * - Value
490 - Description
491 * - 0x0001
492 - Cleanly umounted
493 * - 0x0002
494 - Errors detected
495 * - 0x0004
496 - Orphans being recovered
498 .. _super_errors:
500 The superblock error policy is one of the following:
502 .. list-table::
503 :widths: 1 79
504 :header-rows: 1
506 * - Value
507 - Description
508 * - 1
509 - Continue
510 * - 2
511 - Remount read-only
512 * - 3
513 - Panic
515 .. _super_creator:
517 The filesystem creator is one of the following:
519 .. list-table::
520 :widths: 1 79
521 :header-rows: 1
523 * - Value
524 - Description
525 * - 0
526 - Linux
527 * - 1
528 - Hurd
529 * - 2
530 - Masix
531 * - 3
532 - FreeBSD
533 * - 4
534 - Lites
536 .. _super_revision:
538 The superblock revision is one of the following:
540 .. list-table::
541 :widths: 1 79
542 :header-rows: 1
544 * - Value
545 - Description
546 * - 0
547 - Original format
548 * - 1
549 - v2 format w/ dynamic inode sizes
551 Note that ``EXT4_DYNAMIC_REV`` refers to a revision 1 or newer filesystem.
553 .. _super_compat:
555 The superblock compatible features field is a combination of any of the
556 following:
558 .. list-table::
559 :widths: 1 79
560 :header-rows: 1
562 * - Value
563 - Description
564 * - 0x1
565 - Directory preallocation (COMPAT\_DIR\_PREALLOC).
566 * - 0x2
567 - “imagic inodes”. Not clear from the code what this does
568 (COMPAT\_IMAGIC\_INODES).
569 * - 0x4
570 - Has a journal (COMPAT\_HAS\_JOURNAL).
571 * - 0x8
572 - Supports extended attributes (COMPAT\_EXT\_ATTR).
573 * - 0x10
574 - Has reserved GDT blocks for filesystem expansion
575 (COMPAT\_RESIZE\_INODE). Requires RO\_COMPAT\_SPARSE\_SUPER.
576 * - 0x20
577 - Has directory indices (COMPAT\_DIR\_INDEX).
578 * - 0x40
579 - “Lazy BG”. Not in Linux kernel, seems to have been for uninitialized
580 block groups? (COMPAT\_LAZY\_BG)
581 * - 0x80
582 - “Exclude inode”. Not used. (COMPAT\_EXCLUDE\_INODE).
583 * - 0x100
584 - “Exclude bitmap”. Seems to be used to indicate the presence of
585 snapshot-related exclude bitmaps? Not defined in kernel or used in
586 e2fsprogs (COMPAT\_EXCLUDE\_BITMAP).
587 * - 0x200
588 - Sparse Super Block, v2. If this flag is set, the SB field s\_backup\_bgs
589 points to the two block groups that contain backup superblocks
590 (COMPAT\_SPARSE\_SUPER2).
592 .. _super_incompat:
594 The superblock incompatible features field is a combination of any of the
595 following:
597 .. list-table::
598 :widths: 1 79
599 :header-rows: 1
601 * - Value
602 - Description
603 * - 0x1
604 - Compression (INCOMPAT\_COMPRESSION).
605 * - 0x2
606 - Directory entries record the file type. See ext4\_dir\_entry\_2 below
607 (INCOMPAT\_FILETYPE).
608 * - 0x4
609 - Filesystem needs recovery (INCOMPAT\_RECOVER).
610 * - 0x8
611 - Filesystem has a separate journal device (INCOMPAT\_JOURNAL\_DEV).
612 * - 0x10
613 - Meta block groups. See the earlier discussion of this feature
614 (INCOMPAT\_META\_BG).
615 * - 0x40
616 - Files in this filesystem use extents (INCOMPAT\_EXTENTS).
617 * - 0x80
618 - Enable a filesystem size of 2^64 blocks (INCOMPAT\_64BIT).
619 * - 0x100
620 - Multiple mount protection. Not implemented (INCOMPAT\_MMP).
621 * - 0x200
622 - Flexible block groups. See the earlier discussion of this feature
623 (INCOMPAT\_FLEX\_BG).
624 * - 0x400
625 - Inodes can be used to store large extended attribute values
626 (INCOMPAT\_EA\_INODE).
627 * - 0x1000
628 - Data in directory entry (INCOMPAT\_DIRDATA). (Not implemented?)
629 * - 0x2000
630 - Metadata checksum seed is stored in the superblock. This feature enables
631 the administrator to change the UUID of a metadata\_csum filesystem
632 while the filesystem is mounted; without it, the checksum definition
633 requires all metadata blocks to be rewritten (INCOMPAT\_CSUM\_SEED).
634 * - 0x4000
635 - Large directory >2GB or 3-level htree (INCOMPAT\_LARGEDIR). Prior to
636 this feature, directories could not be larger than 4GiB and could not
637 have an htree more than 2 levels deep. If this feature is enabled,
638 directories can be larger than 4GiB and have a maximum htree depth of 3.
639 * - 0x8000
640 - Data in inode (INCOMPAT\_INLINE\_DATA).
641 * - 0x10000
642 - Encrypted inodes are present on the filesystem. (INCOMPAT\_ENCRYPT).
644 .. _super_rocompat:
646 The superblock read-only compatible features field is a combination of any of
647 the following:
649 .. list-table::
650 :widths: 1 79
651 :header-rows: 1
653 * - Value
654 - Description
655 * - 0x1
656 - Sparse superblocks. See the earlier discussion of this feature
657 (RO\_COMPAT\_SPARSE\_SUPER).
658 * - 0x2
659 - This filesystem has been used to store a file greater than 2GiB
660 (RO\_COMPAT\_LARGE\_FILE).
661 * - 0x4
662 - Not used in kernel or e2fsprogs (RO\_COMPAT\_BTREE\_DIR).
663 * - 0x8
664 - This filesystem has files whose sizes are represented in units of
665 logical blocks, not 512-byte sectors. This implies a very large file
666 indeed! (RO\_COMPAT\_HUGE\_FILE)
667 * - 0x10
668 - Group descriptors have checksums. In addition to detecting corruption,
669 this is useful for lazy formatting with uninitialized groups
670 (RO\_COMPAT\_GDT\_CSUM).
671 * - 0x20
672 - Indicates that the old ext3 32,000 subdirectory limit no longer applies
673 (RO\_COMPAT\_DIR\_NLINK). A directory's i\_links\_count will be set to 1
674 if it is incremented past 64,999.
675 * - 0x40
676 - Indicates that large inodes exist on this filesystem
677 (RO\_COMPAT\_EXTRA\_ISIZE).
678 * - 0x80
679 - This filesystem has a snapshot (RO\_COMPAT\_HAS\_SNAPSHOT).
680 * - 0x100
681 - `Quota <Quota>`__ (RO\_COMPAT\_QUOTA).
682 * - 0x200
683 - This filesystem supports “bigalloc”, which means that file extents are
684 tracked in units of clusters (of blocks) instead of blocks
685 (RO\_COMPAT\_BIGALLOC).
686 * - 0x400
687 - This filesystem supports metadata checksumming.
688 (RO\_COMPAT\_METADATA\_CSUM; implies RO\_COMPAT\_GDT\_CSUM, though
689 GDT\_CSUM must not be set)
690 * - 0x800
691 - Filesystem supports replicas. This feature is neither in the kernel nor
692 e2fsprogs. (RO\_COMPAT\_REPLICA)
693 * - 0x1000
694 - Read-only filesystem image; the kernel will not mount this image
695 read-write and most tools will refuse to write to the image.
696 (RO\_COMPAT\_READONLY)
697 * - 0x2000
698 - Filesystem tracks project quotas. (RO\_COMPAT\_PROJECT)
700 .. _super_def_hash:
702 The ``s_def_hash_version`` field is one of the following:
704 .. list-table::
705 :widths: 1 79
706 :header-rows: 1
708 * - Value
709 - Description
710 * - 0x0
711 - Legacy.
712 * - 0x1
713 - Half MD4.
714 * - 0x2
715 - Tea.
716 * - 0x3
717 - Legacy, unsigned.
718 * - 0x4
719 - Half MD4, unsigned.
720 * - 0x5
721 - Tea, unsigned.
723 .. _super_mountopts:
725 The ``s_default_mount_opts`` field is any combination of the following:
727 .. list-table::
728 :widths: 1 79
729 :header-rows: 1
731 * - Value
732 - Description
733 * - 0x0001
734 - Print debugging info upon (re)mount. (EXT4\_DEFM\_DEBUG)
735 * - 0x0002
736 - New files take the gid of the containing directory (instead of the fsgid
737 of the current process). (EXT4\_DEFM\_BSDGROUPS)
738 * - 0x0004
739 - Support userspace-provided extended attributes. (EXT4\_DEFM\_XATTR\_USER)
740 * - 0x0008
741 - Support POSIX access control lists (ACLs). (EXT4\_DEFM\_ACL)
742 * - 0x0010
743 - Do not support 32-bit UIDs. (EXT4\_DEFM\_UID16)
744 * - 0x0020
745 - All data and metadata are commited to the journal.
746 (EXT4\_DEFM\_JMODE\_DATA)
747 * - 0x0040
748 - All data are flushed to the disk before metadata are committed to the
749 journal. (EXT4\_DEFM\_JMODE\_ORDERED)
750 * - 0x0060
751 - Data ordering is not preserved; data may be written after the metadata
752 has been written. (EXT4\_DEFM\_JMODE\_WBACK)
753 * - 0x0100
754 - Disable write flushes. (EXT4\_DEFM\_NOBARRIER)
755 * - 0x0200
756 - Track which blocks in a filesystem are metadata and therefore should not
757 be used as data blocks. This option will be enabled by default on 3.18,
758 hopefully. (EXT4\_DEFM\_BLOCK\_VALIDITY)
759 * - 0x0400
760 - Enable DISCARD support, where the storage device is told about blocks
761 becoming unused. (EXT4\_DEFM\_DISCARD)
762 * - 0x0800
763 - Disable delayed allocation. (EXT4\_DEFM\_NODELALLOC)
765 .. _super_flags:
767 The ``s_flags`` field is any combination of the following:
769 .. list-table::
770 :widths: 1 79
771 :header-rows: 1
773 * - Value
774 - Description
775 * - 0x0001
776 - Signed directory hash in use.
777 * - 0x0002
778 - Unsigned directory hash in use.
779 * - 0x0004
780 - To test development code.
782 .. _super_encrypt:
784 The ``s_encrypt_algos`` list can contain any of the following:
786 .. list-table::
787 :widths: 1 79
788 :header-rows: 1
790 * - Value
791 - Description
792 * - 0
793 - Invalid algorithm (ENCRYPTION\_MODE\_INVALID).
794 * - 1
795 - 256-bit AES in XTS mode (ENCRYPTION\_MODE\_AES\_256\_XTS).
796 * - 2
797 - 256-bit AES in GCM mode (ENCRYPTION\_MODE\_AES\_256\_GCM).
798 * - 3
799 - 256-bit AES in CBC mode (ENCRYPTION\_MODE\_AES\_256\_CBC).
801 Total size of the superblock is 1024 bytes.