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[dm-devel] Re: Data corruption on software RAID

Mikulas Patocka wrote:

During source code review, I found an unprobable but possible data corruption on RAID-1 and on DM-RAID-1. (I'm not sure about RAID-4,5,6).

The RAID code was enhanced with bitmaps in 2.6.13.

The bitmap tracks regions on the device that may be possibly out-of-sync. The purpose of the bitmap is to avoid resynchronizing the whole array in the case of crash. DM-raid uses similar bitmap too.

The write sequnce is usually:
1. turn on bit in the bitmap (if it hasn't been on before).
2. update the data.
3. when writes to all devices finish, turn the bit may be turned off.

The developers assume that when all writes to the region finish, the region is in-sync.

This assumption is wrong.

Kernel writes data while they may be modified in many places. For example, the pdflush daemon writes periodically pages and buffers without locking them. Similarly, pages may be written while they are mapped for write to the processes.

Normally, there is no problem with modify-while-write. The write sequence is something like:
* turn off Dirty bit
* write the buffer or page
--- and if the buffer or page is modified while it's being written, the Dirty bit is turned on again and the correct data are written later.

But with RAID (since 2.6.13), it can produce corruption because when the buffer is modified while being written, different versions of data can be written to devices in the RAID array. For example:

1. pdflush turns off a dirty bit on Ext2 bitmap buffer and starts writing the buffer to RAID-1 2. the kernel allocates some blocks in that Ext2 bitmap. One of RAID-1 devices writes new data, the other one gets old data. 3. The kernel turns on the buffer dirty bit, so this buffer is scheduled for next write. 4. RAID-1 subsystem sees that both writes finished, it thinks that this region is in-sync, turns off its dirty bit in its region bitmap and writes the bitmap to disk.
5. before pdflush writes the Ext2 bitmap buffer again, the system CRASHES

6. after new boot, RAID-1 sees the bit for this region off, so it doesn't resynchronize it. 7. during fsck, RAID-1 reads the Ext2 bitmap from the device where the bit is on. fsck sees that the bitmap is correct and doesn't touch it. 8. some times later kernel reads the Ext2 bitmap from the other device. It sees the bit off, allocates some data there and creates cross-linked files.

The same corruption may happen with some jorunaled filesystems (probably not Ext3) or applications that do their own crash recovery (databases, etc.). The key point is that an application expects that after a crash it reads old data or new data, but it doesn't expect that subsequent reads to the same place may alternatively return old or new data --- which may happen on RAID-1.

Possibilities how to fix it:

1. lock the buffers and pages while they are being written --- this would cause performance degradation (the most severe degradation would be in case when one process does repeatedly sync() and other unrelated process repeatedly writes to some file).

Lock the buffers and pages only for RAID --- would create many special cases and possible bugs.

2. never turn the region dirty bit off until the filesystem is unmounted. --- this is the simplest fix. If the computer crashes after a long time, it resynchronizes the whole device. But there won't cause application-visible or filesystem-visible data corruption.

3. turn off the region bit if the region wasn't written in one pdflush period --- requires an interaction with pdflush, rather complex. The problem here is that pdflush makes its best effort to write data in dirty_writeback_centisecs interval, but it is not guaranteed to do it.

4. make more region states: Region has in-memory states CLEAN, DIRTY, MAYBE_DIRTY, CLEAN_CANDIDATE.

When you start writing to the region, it is always moved to DIRTY state (and on-disk bit is turned on).

When you finish all writes to the region, move it to MAYBE_DIRTY state, but leave bit on disk on. We now don't know if the region is dirty or no.

Run a helper thread that does periodically:
Issue sync()
Change CLEAN_CANDIDATE regions to CLEAN state and clear their on-disk bit.

The rationale is that if the above write-while-modify scenario happens, the page is always dirty. Thus, sync() will write the page, kick the region back from CLEAN_CANDIDATE to MAYBE_DIRTY state and we won't mark the region as clean on disk.

I'd like to know you ideas on this, before we start coding a solution.

I looked at just this problem a while ago, and came to the conclusion that what was needed was a COW bit, to show that there was i/o in flight, and that before modification it needed to be copied. Since you don't want to let that recurse, you don't start writing the copy until the original is written and freed. Ideally you wouldn't bother to finish writing the original, but that doesn't seem possible. That allows at most two copies of a chunk to take up memory space at once, although it's still ugly and can be a bottleneck.

For reliable operation I would want all copies (and/or CRCs) to be written on an fsync, by the time I bother to fsync I really, really, want the data on the disk.

Bill Davidsen <davidsen tmr com>
 "Woe unto the statesman who makes war without a reason that will still
be valid when the war is over..." Otto von Bismark

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