Hi!
I have been reading the "Clarifications about check/repair, i.e.
RAID SCRUBBING" thread, and there were some answers which were still
slightly unclear to me, and I'd like to get a bit more clarification.
This is all in reference to the /sys/block/md0/md/sync_action setting.
Neil Brown wrote:
> 'check' just reads everything and doesn't trigger any writes unless a
> read error is detected, in which case the normally read-error handing
> kicks in. So it can be useful on a read-only array.
>
> 'repair' does that same but when it finds an inconsistency is corrects
> it by writing something.
> If any raid personality had not be taught to specifically understand
> 'check', then a 'check' run would effect a 'repair'. I think 2.6.17
> will have all personalities doing the right thing.
>
> check doesn't keep a record of problems, just a count. 'repair' will
> reprocess the whole array.
What exactly is the normal read-error handling behavior that is invoked
when 'check' detects a read error, especially in regards to raid level
5?
Also, why exactly is 'check' useful on a read-only array? Does this mean
on an array which has 'array_state' set to 'readonly'? In this case, I'm
guessing this is designed to be useful in repairing a dirty and degraded
array, correct?
Lastly, under which mounted conditions are 'check' and 'repair'
safe/useful? Can I run both on a read-write mounted partition? Should
they be read-only, or totally unmounted? I'm assuming since resyncs can
occur while the overlying fs is mounted read-write, that it is both safe
and useful to both 'check' and 'repair' while the fs is mounted
read-write as well, but I don't see this mentioned explicitly anywhere.
Please CC: me, as I'm not on the list.
Thank you very much!
TJ Harrell
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