1 =============
2 dm-log-writes
3 =============
4
5 This target takes 2 devices, one to pass all I
6 of the write operations to. This is intended
7 to verify the integrity of metadata or data as
8 There is a log_write_entry written for every W
9 able to take arbitrary data from userspace to
10 that is in the WRITE requests is copied into t
11 exactly as it happened originally.
12
13 Log Ordering
14 ============
15
16 We log things in order of completion once we a
17 cache. This means that normal WRITE requests
18 next REQ_PREFLUSH request. This is to make it
19 the log in a way that correlates to what is on
20 to make it easier to detect improper waiting/f
21
22 This works by attaching all WRITE requests to
23 Once we see a REQ_PREFLUSH request we splice t
24 the FLUSH request completes we log all of the
25 completed WRITEs, at the time the REQ_PREFLUSH
26 simulate the worst case scenario with regard t
27 following example (W means write, C means comp
28
29 W1,W2,W3,C3,C2,Wflush,C1,Cflush
30
31 The log would show the following:
32
33 W3,W2,flush,W1....
34
35 Again this is to simulate what is actually on
36 cases where a power failure at a particular po
37 inconsistent file system.
38
39 Any REQ_FUA requests bypass this flushing mech
40 they complete as those requests will obviously
41
42 Any REQ_OP_DISCARD requests are treated like W
43 have all the DISCARD requests, and then the WR
44 request. Consider the following example:
45
46 WRITE block 1, DISCARD block 1, FLUSH
47
48 If we logged DISCARD when it completed, the re
49
50 DISCARD 1, WRITE 1, FLUSH
51
52 which isn't quite what happened and wouldn't b
53
54 Target interface
55 ================
56
57 i) Constructor
58
59 log-writes <dev_path> <log_dev_path>
60
61 ============= =============================
62 dev_path Device that all of the IO wil
63 log_dev_path Device where the log entries
64 ============= =============================
65
66 ii) Status
67
68 <#logged entries> <highest allocated secto
69
70 =========================== ==============
71 #logged entries Number of logg
72 highest allocated sector Highest alloca
73 =========================== ==============
74
75 iii) Messages
76
77 mark <description>
78
79 You can use a dmsetup message to set a
80 For example say you want to fsck a fil
81 write, but first you need to replay up
82 we're fsck'ing something reasonable, y
83 this::
84
85 mkfs.btrfs -f /dev/mapper/log
86 dmsetup message log 0 mark mkfs
87 <run test>
88
89 This would allow you to replay the log
90 then replay from that point on doing t
91 interval that you want.
92
93 Every log has a mark at the end labele
94
95 Userspace component
96 ===================
97
98 There is a userspace tool that will replay the
99 It can be found here: https://github.com/josef
100
101 Example usage
102 =============
103
104 Say you want to test fsync on your file system
105 this::
106
107 TABLE="0 $(blockdev --getsz /dev/sdb) log-wr
108 dmsetup create log --table "$TABLE"
109 mkfs.btrfs -f /dev/mapper/log
110 dmsetup message log 0 mark mkfs
111
112 mount /dev/mapper/log /mnt/btrfs-test
113 <some test that does fsync at the end>
114 dmsetup message log 0 mark fsync
115 md5sum /mnt/btrfs-test/foo
116 umount /mnt/btrfs-test
117
118 dmsetup remove log
119 replay-log --log /dev/sdc --replay /dev/sdb
120 mount /dev/sdb /mnt/btrfs-test
121 md5sum /mnt/btrfs-test/foo
122 <verify md5sum's are correct>
123
124 Another option is to do a complicated file s
125 system is consistent during the entire opera
126
127 TABLE="0 $(blockdev --getsz /dev/sdb) log-wr
128 dmsetup create log --table "$TABLE"
129 mkfs.btrfs -f /dev/mapper/log
130 dmsetup message log 0 mark mkfs
131
132 mount /dev/mapper/log /mnt/btrfs-test
133 <fsstress to dirty the fs>
134 btrfs filesystem balance /mnt/btrfs-test
135 umount /mnt/btrfs-test
136 dmsetup remove log
137
138 replay-log --log /dev/sdc --replay /dev/sdb
139 btrfsck /dev/sdb
140 replay-log --log /dev/sdc --replay /dev/sdb
141 --fsck "btrfsck /dev/sdb" --check fua
142
143 And that will replay the log until it sees a F
144 and if the fsck passes it will replay to the n
145 the fsck command exists abnormally.
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.