Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.
1 dm-log-writes 2 ============= 3 4 This target takes 2 devices, one to pass all IO to normally, and one to log all 5 of the write operations to. This is intended for file system developers wishing 6 to verify the integrity of metadata or data as the file system is written to. 7 There is a log_write_entry written for every WRITE request and the target is 8 able to take arbitrary data from userspace to insert into the log. The data 9 that is in the WRITE requests is copied into the log to make the replay happen 10 exactly as it happened originally. 11 12 Log Ordering 13 ============ 14 15 We log things in order of completion once we are sure the write is no longer in 16 cache. This means that normal WRITE requests are not actually logged until the 17 next REQ_PREFLUSH request. This is to make it easier for userspace to replay 18 the log in a way that correlates to what is on disk and not what is in cache, 19 to make it easier to detect improper waiting/flushing. 20 21 This works by attaching all WRITE requests to a list once the write completes. 22 Once we see a REQ_PREFLUSH request we splice this list onto the request and once 23 the FLUSH request completes we log all of the WRITEs and then the FLUSH. Only 24 completed WRITEs, at the time the REQ_PREFLUSH is issued, are added in order to 25 simulate the worst case scenario with regard to power failures. Consider the 26 following example (W means write, C means complete): 27 28 W1,W2,W3,C3,C2,Wflush,C1,Cflush 29 30 The log would show the following 31 32 W3,W2,flush,W1.... 33 34 Again this is to simulate what is actually on disk, this allows us to detect 35 cases where a power failure at a particular point in time would create an 36 inconsistent file system. 37 38 Any REQ_FUA requests bypass this flushing mechanism and are logged as soon as 39 they complete as those requests will obviously bypass the device cache. 40 41 Any REQ_DISCARD requests are treated like WRITE requests. Otherwise we would 42 have all the DISCARD requests, and then the WRITE requests and then the FLUSH 43 request. Consider the following example: 44 45 WRITE block 1, DISCARD block 1, FLUSH 46 47 If we logged DISCARD when it completed, the replay would look like this 48 49 DISCARD 1, WRITE 1, FLUSH 50 51 which isn't quite what happened and wouldn't be caught during the log replay. 52 53 Target interface 54 ================ 55 56 i) Constructor 57 58 log-writes <dev_path> <log_dev_path> 59 60 dev_path : Device that all of the IO will go to normally. 61 log_dev_path : Device where the log entries are written to. 62 63 ii) Status 64 65 <#logged entries> <highest allocated sector> 66 67 #logged entries : Number of logged entries 68 highest allocated sector : Highest allocated sector 69 70 iii) Messages 71 72 mark <description> 73 74 You can use a dmsetup message to set an arbitrary mark in a log. 75 For example say you want to fsck a file system after every 76 write, but first you need to replay up to the mkfs to make sure 77 we're fsck'ing something reasonable, you would do something like 78 this: 79 80 mkfs.btrfs -f /dev/mapper/log 81 dmsetup message log 0 mark mkfs 82 <run test> 83 84 This would allow you to replay the log up to the mkfs mark and 85 then replay from that point on doing the fsck check in the 86 interval that you want. 87 88 Every log has a mark at the end labeled "dm-log-writes-end". 89 90 Userspace component 91 =================== 92 93 There is a userspace tool that will replay the log for you in various ways. 94 It can be found here: https://github.com/josefbacik/log-writes 95 96 Example usage 97 ============= 98 99 Say you want to test fsync on your file system. You would do something like 100 this: 101 102 TABLE="0 $(blockdev --getsz /dev/sdb) log-writes /dev/sdb /dev/sdc" 103 dmsetup create log --table "$TABLE" 104 mkfs.btrfs -f /dev/mapper/log 105 dmsetup message log 0 mark mkfs 106 107 mount /dev/mapper/log /mnt/btrfs-test 108 <some test that does fsync at the end> 109 dmsetup message log 0 mark fsync 110 md5sum /mnt/btrfs-test/foo 111 umount /mnt/btrfs-test 112 113 dmsetup remove log 114 replay-log --log /dev/sdc --replay /dev/sdb --end-mark fsync 115 mount /dev/sdb /mnt/btrfs-test 116 md5sum /mnt/btrfs-test/foo 117 <verify md5sum's are correct> 118 119 Another option is to do a complicated file system operation and verify the file 120 system is consistent during the entire operation. You could do this with: 121 122 TABLE="0 $(blockdev --getsz /dev/sdb) log-writes /dev/sdb /dev/sdc" 123 dmsetup create log --table "$TABLE" 124 mkfs.btrfs -f /dev/mapper/log 125 dmsetup message log 0 mark mkfs 126 127 mount /dev/mapper/log /mnt/btrfs-test 128 <fsstress to dirty the fs> 129 btrfs filesystem balance /mnt/btrfs-test 130 umount /mnt/btrfs-test 131 dmsetup remove log 132 133 replay-log --log /dev/sdc --replay /dev/sdb --end-mark mkfs 134 btrfsck /dev/sdb 135 replay-log --log /dev/sdc --replay /dev/sdb --start-mark mkfs \ 136 --fsck "btrfsck /dev/sdb" --check fua 137 138 And that will replay the log until it sees a FUA request, run the fsck command 139 and if the fsck passes it will replay to the next FUA, until it is completed or 140 the fsck command exists abnormally.