One thing that might be of interest - XFS does tend to pass variable size requests down to the block layer, and this has tripped up md and other drivers in 2.4 in the distant past.
Log IO is typically 512 byte aligned (as opposed to block or page size aligned), as are IOs into several of XFS' metadata structures.
Hey, thanks for the pointer! I think we're getting somewhere now. Here's a recap of the tested combinations:
XFS on raw disk: OK XFS on LVM2 on single disk: OK XFS on LVM2 on RAID-5: fails ext2 on LVM2 on RAID-5: OK
I just tested XFS on LVM2 on RAID-5 using "-l sunit=8" while creating the filesystem to force log writes be block-sized and block-aligned; this seems to work :-) I have not been able to force a failure using my test script, although ATM the system is still running a RAID-5 resync of the array, but that should only make the problem more likely, not less.
So, this does appear to be an md/dm stacking problem, that is exposed by XFS sending non-block-sized and/or non-block-aligned IOs.
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