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Diffstat (limited to 'doc/kernel/log-writes.txt')
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diff --git a/doc/kernel/log-writes.txt b/doc/kernel/log-writes.txt new file mode 100644 index 0000000..f4ebcba --- /dev/null +++ b/doc/kernel/log-writes.txt @@ -0,0 +1,140 @@ +dm-log-writes +============= + +This target takes 2 devices, one to pass all IO to normally, and one to log all +of the write operations to. This is intended for file system developers wishing +to verify the integrity of metadata or data as the file system is written to. +There is a log_write_entry written for every WRITE request and the target is +able to take arbitrary data from userspace to insert into the log. The data +that is in the WRITE requests is copied into the log to make the replay happen +exactly as it happened originally. + +Log Ordering +============ + +We log things in order of completion once we are sure the write is no longer in +cache. This means that normal WRITE requests are not actually logged until the +next REQ_PREFLUSH request. This is to make it easier for userspace to replay +the log in a way that correlates to what is on disk and not what is in cache, +to make it easier to detect improper waiting/flushing. + +This works by attaching all WRITE requests to a list once the write completes. +Once we see a REQ_PREFLUSH request we splice this list onto the request and once +the FLUSH request completes we log all of the WRITEs and then the FLUSH. Only +completed WRITEs, at the time the REQ_PREFLUSH is issued, are added in order to +simulate the worst case scenario with regard to power failures. Consider the +following example (W means write, C means complete): + +W1,W2,W3,C3,C2,Wflush,C1,Cflush + +The log would show the following + +W3,W2,flush,W1.... + +Again this is to simulate what is actually on disk, this allows us to detect +cases where a power failure at a particular point in time would create an +inconsistent file system. + +Any REQ_FUA requests bypass this flushing mechanism and are logged as soon as +they complete as those requests will obviously bypass the device cache. + +Any REQ_DISCARD requests are treated like WRITE requests. Otherwise we would +have all the DISCARD requests, and then the WRITE requests and then the FLUSH +request. Consider the following example: + +WRITE block 1, DISCARD block 1, FLUSH + +If we logged DISCARD when it completed, the replay would look like this + +DISCARD 1, WRITE 1, FLUSH + +which isn't quite what happened and wouldn't be caught during the log replay. + +Target interface +================ + +i) Constructor + + log-writes <dev_path> <log_dev_path> + + dev_path : Device that all of the IO will go to normally. + log_dev_path : Device where the log entries are written to. + +ii) Status + + <#logged entries> <highest allocated sector> + + #logged entries : Number of logged entries + highest allocated sector : Highest allocated sector + +iii) Messages + + mark <description> + + You can use a dmsetup message to set an arbitrary mark in a log. + For example say you want to fsck a file system after every + write, but first you need to replay up to the mkfs to make sure + we're fsck'ing something reasonable, you would do something like + this: + + mkfs.btrfs -f /dev/mapper/log + dmsetup message log 0 mark mkfs + <run test> + + This would allow you to replay the log up to the mkfs mark and + then replay from that point on doing the fsck check in the + interval that you want. + + Every log has a mark at the end labeled "dm-log-writes-end". + +Userspace component +=================== + +There is a userspace tool that will replay the log for you in various ways. +It can be found here: https://github.com/josefbacik/log-writes + +Example usage +============= + +Say you want to test fsync on your file system. You would do something like +this: + +TABLE="0 $(blockdev --getsz /dev/sdb) log-writes /dev/sdb /dev/sdc" +dmsetup create log --table "$TABLE" +mkfs.btrfs -f /dev/mapper/log +dmsetup message log 0 mark mkfs + +mount /dev/mapper/log /mnt/btrfs-test +<some test that does fsync at the end> +dmsetup message log 0 mark fsync +md5sum /mnt/btrfs-test/foo +umount /mnt/btrfs-test + +dmsetup remove log +replay-log --log /dev/sdc --replay /dev/sdb --end-mark fsync +mount /dev/sdb /mnt/btrfs-test +md5sum /mnt/btrfs-test/foo +<verify md5sum's are correct> + +Another option is to do a complicated file system operation and verify the file +system is consistent during the entire operation. You could do this with: + +TABLE="0 $(blockdev --getsz /dev/sdb) log-writes /dev/sdb /dev/sdc" +dmsetup create log --table "$TABLE" +mkfs.btrfs -f /dev/mapper/log +dmsetup message log 0 mark mkfs + +mount /dev/mapper/log /mnt/btrfs-test +<fsstress to dirty the fs> +btrfs filesystem balance /mnt/btrfs-test +umount /mnt/btrfs-test +dmsetup remove log + +replay-log --log /dev/sdc --replay /dev/sdb --end-mark mkfs +btrfsck /dev/sdb +replay-log --log /dev/sdc --replay /dev/sdb --start-mark mkfs \ + --fsck "btrfsck /dev/sdb" --check fua + +And that will replay the log until it sees a FUA request, run the fsck command +and if the fsck passes it will replay to the next FUA, until it is completed or +the fsck command exists abnormally. |