On Mon 2008-06-30 05:20:18 -0400, Gavin McCullagh wrote: > -- Consistency -- > > The trouble with RAID is what happens when you get a sudden system > shutdown (eg due to a power failure) while you're writing to the > disk array. Suppose the data is written to one drive correctly, but > not yet to the other. When the system resumes, it's very difficult > for the system to determine which disk (if either) is correct. So, > you end up with some ambiguous blocks where depending on which disk > is read, you can get a different answer. An FSCK may not fix or > even detect the issue. > > Most software RAID (ZFS excepted) suffers from the above issue and > some cheaper hardware RAID cards do too.
Is this still true with modern mdadm? I thought that the RAID
superblock on modern systems contained information about
"last-written" times, to enable recovery from exactly this problem.
I'm not a RAID guru, though: if anyone could point me toward the
relevant documentation, i'd be grateful.
At any rate, RAID or not, for important machines you probably want to
have a UPS prevent sudden power failures, and you should also have a
real backup strategy. RAID (protection against hardware failures) is
not a substitute for backups (protection against human error).
--dkg
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