On Wed, 11 Jun 2014, Bart Kus wrote:

Doing the periodic check does not prevent corruption of read() data though 
(RAID6 case).  Copied files may be corrupted, even though the RAID would 
eventually fix itself after a repair is done.

Yes, there is a performance penalty, but data integrity is also improved. 
Paranoid mode should probably not be the default, but I would like the choice 
to improve data integrity at the expense of some small speed penalty.  ZFS 
implements this anti-corruption checking by using checksums on their data. 
We don't have a simple checksumming mechanism in md-raid, but we do have the 
full stripe data available ready for verification.

BTW, the idea of a daily repair operation doesn't work when it takes 14 hours 
to repair a large RAID.  That would only leave 10 hours of each day for 
normal speed access.  I schedule repairs weekly, though.

Since bytes read and written is the dominating factor behind disk failures 
these days, I certainly wouldn't want to do repairs daily. Weekly might 
even be pushing it.

/Mattias Wadenstein