summaryrefslogtreecommitdiffstats
path: root/Documentation/filesystems/ext4/journal.rst
blob: 5fad38860f17432b537bf33aa6c5df7d2c54d085 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
.. SPDX-License-Identifier: GPL-2.0

Journal (jbd2)
--------------

Introduced in ext3, the ext4 filesystem employs a journal to protect the
filesystem against metadata inconsistencies in the case of a system crash. Up
to 10,240,000 file system blocks (see man mke2fs(8) for more details on journal
size limits) can be reserved inside the filesystem as a place to land
“important” data writes on-disk as quickly as possible. Once the important
data transaction is fully written to the disk and flushed from the disk write
cache, a record of the data being committed is also written to the journal. At
some later point in time, the journal code writes the transactions to their
final locations on disk (this could involve a lot of seeking or a lot of small
read-write-erases) before erasing the commit record. Should the system
crash during the second slow write, the journal can be replayed all the
way to the latest commit record, guaranteeing the atomicity of whatever
gets written through the journal to the disk. The effect of this is to
guarantee that the filesystem does not become stuck midway through a
metadata update.

For performance reasons, ext4 by default only writes filesystem metadata
through the journal. This means that file data blocks are /not/
guaranteed to be in any consistent state after a crash. If this default
guarantee level (``data=ordered``) is not satisfactory, there is a mount
option to control journal behavior. If ``data=journal``, all data and
metadata are written to disk through the journal. This is slower but
safest. If ``data=writeback``, dirty data blocks are not flushed to the
disk before the metadata are written to disk through the journal.

In case of ``data=ordered`` mode, Ext4 also supports fast commits which
help reduce commit latency significantly. The default ``data=ordered``
mode works by logging metadata blocks to the journal. In fast commit
mode, Ext4 only stores the minimal delta needed to recreate the
affected metadata in fast commit space that is shared with JBD2.
Once the fast commit area fills in or if fast commit is not possible
or if JBD2 commit timer goes off, Ext4 performs a traditional full commit.
A full commit invalidates all the fast commits that happened before
it and thus it makes the fast commit area empty for further fast
commits. This feature needs to be enabled at mkfs time.

The journal inode is typically inode 8. The first 68 bytes of the
journal inode are replicated in the ext4 superblock. The journal itself
is normal (but hidden) file within the filesystem. The file usually
consumes an entire block group, though mke2fs tries to put it in the
middle of the disk.

All fields in jbd2 are written to disk in big-endian order. This is the
opposite of ext4.

NOTE: Both ext4 and ocfs2 use jbd2.

The maximum size of a journal embedded in an ext4 filesystem is 2^32
blocks. jbd2 itself does not seem to care.

Layout
~~~~~~

Generally speaking, the journal has this format:

.. list-table::
   :widths: 16 48 16
   :header-rows: 1

   * - Superblock
     - descriptor\_block (data\_blocks or revocation\_block) [more data or
       revocations] commmit\_block
     - [more transactions...]
   * - 
     - One transaction
     -

Notice that a transaction begins with either a descriptor and some data,
or a block revocation list. A finished transaction always ends with a
commit. If there is no commit record (or the checksums don't match), the
transaction will be discarded during replay.

External Journal
~~~~~~~~~~~~~~~~

Optionally, an ext4 filesystem can be created with an external journal
device (as opposed to an internal journal, which uses a reserved inode).
In this case, on the filesystem device, ``s_journal_inum`` should be
zero and ``s_journal_uuid`` should be set. On the journal device there
will be an ext4 super block in the usual place, with a matching UUID.
The journal superblock will be in the next full block after the
superblock.

.. list-table::
   :widths: 12 12 12 32 12
   :header-rows: 1

   * - 1024 bytes of padding
     - ext4 Superblock
     - Journal Superblock
     - descriptor\_block (data\_blocks or revocation\_block) [more data or
       revocations] commmit\_block
     - [more transactions...]
   * - 
     -
     -
     - One transaction
     -

Block Header
~~~~~~~~~~~~

Every block in the journal starts with a common 12-byte header
``struct journal_header_s``:

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Type
     - Name
     - Description
   * - 0x0
     - \_\_be32
     - h\_magic
     - jbd2 magic number, 0xC03B3998.
   * - 0x4
     - \_\_be32
     - h\_blocktype
     - Description of what this block contains. See the jbd2_blocktype_ table
       below.
   * - 0x8
     - \_\_be32
     - h\_sequence
     - The transaction ID that goes with this block.

.. _jbd2_blocktype:

The journal block type can be any one of:

.. list-table::
   :widths: 16 64
   :header-rows: 1

   * - Value
     - Description
   * - 1
     - Descriptor. This block precedes a series of data blocks that were
       written through the journal during a transaction.
   * - 2
     - Block commit record. This block signifies the completion of a
       transaction.
   * - 3
     - Journal superblock, v1.
   * - 4
     - Journal superblock, v2.
   * - 5
     - Block revocation records. This speeds up recovery by enabling the
       journal to skip writing blocks that were subsequently rewritten.

Super Block
~~~~~~~~~~~

The super block for the journal is much simpler as compared to ext4's.
The key data kept within are size of the journal, and where to find the
start of the log of transactions.

The journal superblock is recorded as ``struct journal_superblock_s``,
which is 1024 bytes long:

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Type
     - Name
     - Description
   * -
     -
     -
     - Static information describing the journal.
   * - 0x0
     - journal\_header\_t (12 bytes)
     - s\_header
     - Common header identifying this as a superblock.
   * - 0xC
     - \_\_be32
     - s\_blocksize
     - Journal device block size.
   * - 0x10
     - \_\_be32
     - s\_maxlen
     - Total number of blocks in this journal.
   * - 0x14
     - \_\_be32
     - s\_first
     - First block of log information.
   * -
     -
     -
     - Dynamic information describing the current state of the log.
   * - 0x18
     - \_\_be32
     - s\_sequence
     - First commit ID expected in log.
   * - 0x1C
     - \_\_be32
     - s\_start
     - Block number of the start of log. Contrary to the comments, this field
       being zero does not imply that the journal is clean!
   * - 0x20
     - \_\_be32
     - s\_errno
     - Error value, as set by jbd2\_journal\_abort().
   * -
     -
     -
     - The remaining fields are only valid in a v2 superblock.
   * - 0x24
     - \_\_be32
     - s\_feature\_compat;
     - Compatible feature set. See the table jbd2_compat_ below.
   * - 0x28
     - \_\_be32
     - s\_feature\_incompat
     - Incompatible feature set. See the table jbd2_incompat_ below.
   * - 0x2C
     - \_\_be32
     - s\_feature\_ro\_compat
     - Read-only compatible feature set. There aren't any of these currently.
   * - 0x30
     - \_\_u8
     - s\_uuid[16]
     - 128-bit uuid for journal. This is compared against the copy in the ext4
       super block at mount time.
   * - 0x40
     - \_\_be32
     - s\_nr\_users
     - Number of file systems sharing this journal.
   * - 0x44
     - \_\_be32
     - s\_dynsuper
     - Location of dynamic super block copy. (Not used?)
   * - 0x48
     - \_\_be32
     - s\_max\_transaction
     - Limit of journal blocks per transaction. (Not used?)
   * - 0x4C
     - \_\_be32
     - s\_max\_trans\_data
     - Limit of data blocks per transaction. (Not used?)
   * - 0x50
     - \_\_u8
     - s\_checksum\_type
     - Checksum algorithm used for the journal.  See jbd2_checksum_type_ for
       more info.
   * - 0x51
     - \_\_u8[3]
     - s\_padding2
     -
   * - 0x54
     - \_\_be32
     - s\_num\_fc\_blocks
     - Number of fast commit blocks in the journal.
   * - 0x58
     - \_\_u32
     - s\_padding[42]
     -
   * - 0xFC
     - \_\_be32
     - s\_checksum
     - Checksum of the entire superblock, with this field set to zero.
   * - 0x100
     - \_\_u8
     - s\_users[16\*48]
     - ids of all file systems sharing the log. e2fsprogs/Linux don't allow
       shared external journals, but I imagine Lustre (or ocfs2?), which use
       the jbd2 code, might.

.. _jbd2_compat:

The journal compat features are any combination of the following:

.. list-table::
   :widths: 16 64
   :header-rows: 1

   * - Value
     - Description
   * - 0x1
     - Journal maintains checksums on the data blocks.
       (JBD2\_FEATURE\_COMPAT\_CHECKSUM)

.. _jbd2_incompat:

The journal incompat features are any combination of the following:

.. list-table::
   :widths: 16 64
   :header-rows: 1

   * - Value
     - Description
   * - 0x1
     - Journal has block revocation records. (JBD2\_FEATURE\_INCOMPAT\_REVOKE)
   * - 0x2
     - Journal can deal with 64-bit block numbers.
       (JBD2\_FEATURE\_INCOMPAT\_64BIT)
   * - 0x4
     - Journal commits asynchronously. (JBD2\_FEATURE\_INCOMPAT\_ASYNC\_COMMIT)
   * - 0x8
     - This journal uses v2 of the checksum on-disk format. Each journal
       metadata block gets its own checksum, and the block tags in the
       descriptor table contain checksums for each of the data blocks in the
       journal. (JBD2\_FEATURE\_INCOMPAT\_CSUM\_V2)
   * - 0x10
     - This journal uses v3 of the checksum on-disk format. This is the same as
       v2, but the journal block tag size is fixed regardless of the size of
       block numbers. (JBD2\_FEATURE\_INCOMPAT\_CSUM\_V3)
   * - 0x20
     - Journal has fast commit blocks. (JBD2\_FEATURE\_INCOMPAT\_FAST\_COMMIT)

.. _jbd2_checksum_type:

Journal checksum type codes are one of the following.  crc32 or crc32c are the
most likely choices.

.. list-table::
   :widths: 16 64
   :header-rows: 1

   * - Value
     - Description
   * - 1
     - CRC32
   * - 2
     - MD5
   * - 3
     - SHA1
   * - 4
     - CRC32C

Descriptor Block
~~~~~~~~~~~~~~~~

The descriptor block contains an array of journal block tags that
describe the final locations of the data blocks that follow in the
journal. Descriptor blocks are open-coded instead of being completely
described by a data structure, but here is the block structure anyway.
Descriptor blocks consume at least 36 bytes, but use a full block:

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Type
     - Name
     - Descriptor
   * - 0x0
     - journal\_header\_t
     - (open coded)
     - Common block header.
   * - 0xC
     - struct journal\_block\_tag\_s
     - open coded array[]
     - Enough tags either to fill up the block or to describe all the data
       blocks that follow this descriptor block.

Journal block tags have any of the following formats, depending on which
journal feature and block tag flags are set.

If JBD2\_FEATURE\_INCOMPAT\_CSUM\_V3 is set, the journal block tag is
defined as ``struct journal_block_tag3_s``, which looks like the
following. The size is 16 or 32 bytes.

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Type
     - Name
     - Descriptor
   * - 0x0
     - \_\_be32
     - t\_blocknr
     - Lower 32-bits of the location of where the corresponding data block
       should end up on disk.
   * - 0x4
     - \_\_be32
     - t\_flags
     - Flags that go with the descriptor. See the table jbd2_tag_flags_ for
       more info.
   * - 0x8
     - \_\_be32
     - t\_blocknr\_high
     - Upper 32-bits of the location of where the corresponding data block
       should end up on disk. This is zero if JBD2\_FEATURE\_INCOMPAT\_64BIT is
       not enabled.
   * - 0xC
     - \_\_be32
     - t\_checksum
     - Checksum of the journal UUID, the sequence number, and the data block.
   * -
     -
     -
     - This field appears to be open coded. It always comes at the end of the
       tag, after t_checksum. This field is not present if the "same UUID" flag
       is set.
   * - 0x8 or 0xC
     - char
     - uuid[16]
     - A UUID to go with this tag. This field appears to be copied from the
       ``j_uuid`` field in ``struct journal_s``, but only tune2fs touches that
       field.

.. _jbd2_tag_flags:

The journal tag flags are any combination of the following:

.. list-table::
   :widths: 16 64
   :header-rows: 1

   * - Value
     - Description
   * - 0x1
     - On-disk block is escaped. The first four bytes of the data block just
       happened to match the jbd2 magic number.
   * - 0x2
     - This block has the same UUID as previous, therefore the UUID field is
       omitted.
   * - 0x4
     - The data block was deleted by the transaction. (Not used?)
   * - 0x8
     - This is the last tag in this descriptor block.

If JBD2\_FEATURE\_INCOMPAT\_CSUM\_V3 is NOT set, the journal block tag
is defined as ``struct journal_block_tag_s``, which looks like the
following. The size is 8, 12, 24, or 28 bytes:

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Type
     - Name
     - Descriptor
   * - 0x0
     - \_\_be32
     - t\_blocknr
     - Lower 32-bits of the location of where the corresponding data block
       should end up on disk.
   * - 0x4
     - \_\_be16
     - t\_checksum
     - Checksum of the journal UUID, the sequence number, and the data block.
       Note that only the lower 16 bits are stored.
   * - 0x6
     - \_\_be16
     - t\_flags
     - Flags that go with the descriptor. See the table jbd2_tag_flags_ for
       more info.
   * -
     -
     -
     - This next field is only present if the super block indicates support for
       64-bit block numbers.
   * - 0x8
     - \_\_be32
     - t\_blocknr\_high
     - Upper 32-bits of the location of where the corresponding data block
       should end up on disk.
   * -
     -
     -
     - This field appears to be open coded. It always comes at the end of the
       tag, after t_flags or t_blocknr_high. This field is not present if the
       "same UUID" flag is set.
   * - 0x8 or 0xC
     - char
     - uuid[16]
     - A UUID to go with this tag. This field appears to be copied from the
       ``j_uuid`` field in ``struct journal_s``, but only tune2fs touches that
       field.

If JBD2\_FEATURE\_INCOMPAT\_CSUM\_V2 or
JBD2\_FEATURE\_INCOMPAT\_CSUM\_V3 are set, the end of the block is a
``struct jbd2_journal_block_tail``, which looks like this:

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Type
     - Name
     - Descriptor
   * - 0x0
     - \_\_be32
     - t\_checksum
     - Checksum of the journal UUID + the descriptor block, with this field set
       to zero.

Data Block
~~~~~~~~~~

In general, the data blocks being written to disk through the journal
are written verbatim into the journal file after the descriptor block.
However, if the first four bytes of the block match the jbd2 magic
number then those four bytes are replaced with zeroes and the “escaped”
flag is set in the descriptor block tag.

Revocation Block
~~~~~~~~~~~~~~~~

A revocation block is used to prevent replay of a block in an earlier
transaction. This is used to mark blocks that were journalled at one
time but are no longer journalled. Typically this happens if a metadata
block is freed and re-allocated as a file data block; in this case, a
journal replay after the file block was written to disk will cause
corruption.

**NOTE**: This mechanism is NOT used to express “this journal block is
superseded by this other journal block”, as the author (djwong)
mistakenly thought. Any block being added to a transaction will cause
the removal of all existing revocation records for that block.

Revocation blocks are described in
``struct jbd2_journal_revoke_header_s``, are at least 16 bytes in
length, but use a full block:

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Type
     - Name
     - Description
   * - 0x0
     - journal\_header\_t
     - r\_header
     - Common block header.
   * - 0xC
     - \_\_be32
     - r\_count
     - Number of bytes used in this block.
   * - 0x10
     - \_\_be32 or \_\_be64
     - blocks[0]
     - Blocks to revoke.

After r\_count is a linear array of block numbers that are effectively
revoked by this transaction. The size of each block number is 8 bytes if
the superblock advertises 64-bit block number support, or 4 bytes
otherwise.

If JBD2\_FEATURE\_INCOMPAT\_CSUM\_V2 or
JBD2\_FEATURE\_INCOMPAT\_CSUM\_V3 are set, the end of the revocation
block is a ``struct jbd2_journal_revoke_tail``, which has this format:

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Type
     - Name
     - Description
   * - 0x0
     - \_\_be32
     - r\_checksum
     - Checksum of the journal UUID + revocation block

Commit Block
~~~~~~~~~~~~

The commit block is a sentry that indicates that a transaction has been
completely written to the journal. Once this commit block reaches the
journal, the data stored with this transaction can be written to their
final locations on disk.

The commit block is described by ``struct commit_header``, which is 32
bytes long (but uses a full block):

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Type
     - Name
     - Descriptor
   * - 0x0
     - journal\_header\_s
     - (open coded)
     - Common block header.
   * - 0xC
     - unsigned char
     - h\_chksum\_type
     - The type of checksum to use to verify the integrity of the data blocks
       in the transaction. See jbd2_checksum_type_ for more info.
   * - 0xD
     - unsigned char
     - h\_chksum\_size
     - The number of bytes used by the checksum. Most likely 4.
   * - 0xE
     - unsigned char
     - h\_padding[2]
     -
   * - 0x10
     - \_\_be32
     - h\_chksum[JBD2\_CHECKSUM\_BYTES]
     - 32 bytes of space to store checksums. If
       JBD2\_FEATURE\_INCOMPAT\_CSUM\_V2 or JBD2\_FEATURE\_INCOMPAT\_CSUM\_V3
       are set, the first ``__be32`` is the checksum of the journal UUID and
       the entire commit block, with this field zeroed. If
       JBD2\_FEATURE\_COMPAT\_CHECKSUM is set, the first ``__be32`` is the
       crc32 of all the blocks already written to the transaction.
   * - 0x30
     - \_\_be64
     - h\_commit\_sec
     - The time that the transaction was committed, in seconds since the epoch.
   * - 0x38
     - \_\_be32
     - h\_commit\_nsec
     - Nanoseconds component of the above timestamp.

Fast commits
~~~~~~~~~~~~

Fast commit area is organized as a log of tag length values. Each TLV has
a ``struct ext4_fc_tl`` in the beginning which stores the tag and the length
of the entire field. It is followed by variable length tag specific value.
Here is the list of supported tags and their meanings:

.. list-table::
   :widths: 8 20 20 32
   :header-rows: 1

   * - Tag
     - Meaning
     - Value struct
     - Description
   * - EXT4_FC_TAG_HEAD
     - Fast commit area header
     - ``struct ext4_fc_head``
     - Stores the TID of the transaction after which these fast commits should
       be applied.
   * - EXT4_FC_TAG_ADD_RANGE
     - Add extent to inode
     - ``struct ext4_fc_add_range``
     - Stores the inode number and extent to be added in this inode
   * - EXT4_FC_TAG_DEL_RANGE
     - Remove logical offsets to inode
     - ``struct ext4_fc_del_range``
     - Stores the inode number and the logical offset range that needs to be
       removed
   * - EXT4_FC_TAG_CREAT
     - Create directory entry for a newly created file
     - ``struct ext4_fc_dentry_info``
     - Stores the parent inode number, inode number and directory entry of the
       newly created file
   * - EXT4_FC_TAG_LINK
     - Link a directory entry to an inode
     - ``struct ext4_fc_dentry_info``
     - Stores the parent inode number, inode number and directory entry
   * - EXT4_FC_TAG_UNLINK
     - Unlink a directory entry of an inode
     - ``struct ext4_fc_dentry_info``
     - Stores the parent inode number, inode number and directory entry

   * - EXT4_FC_TAG_PAD
     - Padding (unused area)
     - None
     - Unused bytes in the fast commit area.

   * - EXT4_FC_TAG_TAIL
     - Mark the end of a fast commit
     - ``struct ext4_fc_tail``
     - Stores the TID of the commit, CRC of the fast commit of which this tag
       represents the end of

Fast Commit Replay Idempotence
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fast commits tags are idempotent in nature provided the recovery code follows
certain rules. The guiding principle that the commit path follows while
committing is that it stores the result of a particular operation instead of
storing the procedure.

Let's consider this rename operation: 'mv /a /b'. Let's assume dirent '/a'
was associated with inode 10. During fast commit, instead of storing this
operation as a procedure "rename a to b", we store the resulting file system
state as a "series" of outcomes:

- Link dirent b to inode 10
- Unlink dirent a
- Inode 10 with valid refcount

Now when recovery code runs, it needs "enforce" this state on the file
system. This is what guarantees idempotence of fast commit replay.

Let's take an example of a procedure that is not idempotent and see how fast
commits make it idempotent. Consider following sequence of operations:

1) rm A
2) mv B A
3) read A

If we store this sequence of operations as is then the replay is not idempotent.
Let's say while in replay, we crash after (2). During the second replay,
file A (which was actually created as a result of "mv B A" operation) would get
deleted. Thus, file named A would be absent when we try to read A. So, this
sequence of operations is not idempotent. However, as mentioned above, instead
of storing the procedure fast commits store the outcome of each procedure. Thus
the fast commit log for above procedure would be as follows:

(Let's assume dirent A was linked to inode 10 and dirent B was linked to
inode 11 before the replay)

1) Unlink A
2) Link A to inode 11
3) Unlink B
4) Inode 11

If we crash after (3) we will have file A linked to inode 11. During the second
replay, we will remove file A (inode 11). But we will create it back and make
it point to inode 11. We won't find B, so we'll just skip that step. At this
point, the refcount for inode 11 is not reliable, but that gets fixed by the
replay of last inode 11 tag. Thus, by converting a non-idempotent procedure
into a series of idempotent outcomes, fast commits ensured idempotence during
the replay.

Journal Checkpoint
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Checkpointing the journal ensures all transactions and their associated buffers
are submitted to the disk. In-progress transactions are waited upon and included
in the checkpoint. Checkpointing is used internally during critical updates to
the filesystem including journal recovery, filesystem resizing, and freeing of
the journal_t structure.

A journal checkpoint can be triggered from userspace via the ioctl
EXT4_IOC_CHECKPOINT. This ioctl takes a single, u64 argument for flags.
Currently, three flags are supported. First, EXT4_IOC_CHECKPOINT_FLAG_DRY_RUN
can be used to verify input to the ioctl. It returns error if there is any
invalid input, otherwise it returns success without performing
any checkpointing. This can be used to check whether the ioctl exists on a
system and to verify there are no issues with arguments or flags. The
other two flags are EXT4_IOC_CHECKPOINT_FLAG_DISCARD and
EXT4_IOC_CHECKPOINT_FLAG_ZEROOUT. These flags cause the journal blocks to be
discarded or zero-filled, respectively, after the journal checkpoint is
complete. EXT4_IOC_CHECKPOINT_FLAG_DISCARD and EXT4_IOC_CHECKPOINT_FLAG_ZEROOUT
cannot both be set. The ioctl may be useful when snapshotting a system or for
complying with content deletion SLOs.