[CentOS] Re: question about software Raid 1

Tue Sep 30 19:43:57 UTC 2008
Les Mikesell <lesmikesell at gmail.com>

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 at gmail.com