Kay Diederichs wrote:
Fact is that with CentOS-5 kernels (but not with CentOS-4, as this
functionality became available in kernel 2.6.17) you could (or rather
_should_ regularly)
echo check > /sys/block/mdX/md/sync_action
to check agreement between the two (or more) copies. When this finishes,
/sys/block/mdX/md/mismatch_cnt shows you the number of mismatches. You
can fix these with
echo repair > /sys/block/mdX/md/sync_action
This applies to at least RAID1 and RAID5.
At this point the question arises: how does the "repair job" know which
copy is the correct one? I have no answer to this question.
Thanks for posting this. I have a machine that periodically had
filesystem errors on a RAID1 volume that I eventually found were caused
by bad RAM but even after replacing it I'd still see filesystem problems
reappear every few weeks. It turned out that there were quite a few
mismatched blocks between the mirrors and the fsck passes must have
sometimes seen the good copy but subsequently the still-bad alternate
would be used. Now I've done a repair and fsck and so far everything
seems stable. It's hard to tell with problems that only happen once or
twice a month, though. I suppose I have some files with corrupt
contents on there but they are backups that will expire as more current
ones are saved anyway.
BTW, there is - even with current kernels - no speed gain in using RAID1
- see http://kernelnewbies.org/KernelProjects/Raid1ReadBalancing .
I don't think I believe that - you can see the reads alternating drives
by watching the lights.
--
Les Mikesell
lesmikesell@xxxxxxxxx
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