Re: Raid Checks

[Jonathan Tripathy <jonnyt@abpni.co.uk>]

> On 03/04/2011 12:30, Jonathan Tripathy wrote:
>>>> This is what state my server is currently in:
>>>>
>>>> State : clean, resyncing
>>> With mdadm 3.2.1, this will show "checking" when it is checking,
>>> rather than "resyncing".
>> Interesting. So during a normal "resync", I'm guessing redundancy will
>> be lost?
>
> You only need a resync when you've already lost redundancy. The only 
> normal resync is at array creation time, when mdadm will instantly 
> create the array with no redundancy and then proceed to generate the 
> mirror/parity information. Any other resync is only required after 
> something else caused a loss of redundancy e.g. a disc dying or a 
> system crash, or perhaps a drive threw up a bad sector. The check 
> process will attempt to discover any such problems.
Understood. If a problem is detected, would I have to involk a re-sync 
manually (assuming that CHECK is enabled in the script)
>
> As Doug Ledford noted in the Red Hat Bugzilla there are times when a 
> md RAID 1 can harmlessly end up out of sync.
>
>> Also, if the "checking" process finds an error, where will I
>> find it? /var/log/messages?
>
> In the email the raid-check process sends you. Any errors found are 
> not reported in detail on RHEL/CentOS 5 - I think more recent 
> kernel/mdadm can be asked to be more verbose about the location of 
> errors. Requesting a repair rather than a check would get the check 
> process to automatically resync any stripes which had bad mirrors/parity.
How do I set up emails? Is it just the monitor deamon?
>
> In short, it is safe to ignore a modest mismatch_cnt on RAID 1 as long 
> as you aren't seeing disc errors, and that is what Doug Ledford's 
> patch to raid-check does.
>