[CentOS] Re: question about software Raid 1
kay.diederichs at uni-konstanz.de
Sun Sep 21 19:01:15 UTC 2008
> Does software raid 1 compare checksums or otherwise verify that the same
> bits are coming from both disks during reads? What I'm interested in,
> is whether bit errors that were somehow undetected by the hardware would
> be detected by the raid 1 software.
I've been thinking about this as well.
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
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.
BTW, there is - even with current kernels - no speed gain in using RAID1
- see http://kernelnewbies.org/KernelProjects/Raid1ReadBalancing .
HTH a bit,
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