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| author | Linus Torvalds <torvalds@linux-foundation.org> | 2026-08-19 10:09:22 -0700 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2026-08-19 10:09:22 -0700 |
| commit | f4cdf7ca9a1fdcca413157df19753f388a5a224e (patch) | |
| tree | 60a31895b2446cd7fa27514e8c2843534a0de0da /Documentation/admin-guide/device-mapper/log-writes.rst | |
| download | linux-stable-f4cdf7ca9a1fdcca413157df19753f388a5a224e.tar.gz linux-stable-f4cdf7ca9a1fdcca413157df19753f388a5a224e.zip | |
Merge tag 'media/v7.3-1' of git://git.kernel.org/pub/scm/linux/kernel/git/mchehab/linux-mediagrafted
Pull media updates from Mauro Carvalho Chehab:
- v4l2-core: added ISP statistics support and per-block validation
- v4l2-core: Allow unknown HDR10 white point and luminance
- New camera sensors: Sony IMX678 and IMX471m, Himax HM1092 IR sensor
- New codec: Milos: VPU v2.0 codec support
- isp driver: gained support for Dreamchip RPPX1 ISP framework
- vsp1 driver: gained support for RZ/T2H and RZ/N2H
- Novalake driver: gained CVS support for new NVL hardware
- dvb-core: fix feed leak on failed DMX_ADD_PID
- several driver fixes, cleanups and minor improvements
* tag 'media/v7.3-1' of git://git.kernel.org/pub/scm/linux/kernel/git/mchehab/linux-media: (308 commits)
media: ipu-bridge: check all DMI entries when overriding sensor rotation
media: v4l2-async: avoid deleting unlinked ASC entry on link error
media: rzg2l-cru: Align bytesperline to hardware DMA stride requirement
media: intel/ipu6: fix async notifier cleanup leak on parse error
media: staging/ipu7: fix async notifier UAF on probe error path
media: amd: isp4: fix self-deadlock in isp4sd_pwron_and_init() error path
media: amd: isp4: release partial allocations in isp4if_alloc_fw_gpumem()
media: rcar-isp: Fix VSPX reference leaks
media: rcar-isp: Release ISPCORE resources
media: i2c: imx415: Release runtime PM reference on VBLANK error
media: i2c: imx415: Return test pattern write errors
media: renesas: vsp1: Declare index variables in for loop statement
media: renesas: vsp1: Make reset control optional to support platforms without a reset line
media: dt-bindings: media: renesas,vsp1: Document RZ/T2H and RZ/N2H SoCs
media: dt-bindings: media: renesas,fcp: Document RZ/T2H and RZ/N2H SoCs
media: nxp: imx8-isi: Add additional 32-bit RGB format support
media: nxp: imx8-isi: Add 16-bit raw Bayer format support
media: nxp: imx8-isi: Implement per-stream reference counting for multiplexed streams
media: nxp: imx8-isi: Use BIT_ULL() for 64-bit stream masks
media: nxp: imx8-isi: Correct color map between V4L2 and ISI
...
Diffstat (limited to 'Documentation/admin-guide/device-mapper/log-writes.rst')
| -rw-r--r-- | Documentation/admin-guide/device-mapper/log-writes.rst | 145 |
1 files changed, 145 insertions, 0 deletions
diff --git a/Documentation/admin-guide/device-mapper/log-writes.rst b/Documentation/admin-guide/device-mapper/log-writes.rst new file mode 100644 index 000000000..23141f2ff --- /dev/null +++ b/Documentation/admin-guide/device-mapper/log-writes.rst @@ -0,0 +1,145 @@ +============= +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_OP_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. |
