Re: md-raid paranoia mode?

[Bart Kus <me@bartk.us>]

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.

--Bart


On 6/11/2014 2:53 AM, Roberto Spadim wrote:
> Hi
> IMHO
>
> For silent corrupt i think it's better a periodic raid check 
> instead of a paranoid mode
>
> Normally a silent corrupt occurs with an 'old disk' or with old data, 
> but it don't occurs at every disk read (must check disk studies)
>
> I think a 'paranoid' mode is nice, but i think it will reduce all 
> system performace, maybe an crond daily check is better than a 'all 
> read, check' (paranoid)
>
> Em quarta-feira, 11 de junho de 2014, Bart Kus <me@bartk.us 
> <mailto:me@bartk.us>> escreveu:
>
>     Hello,
>
>     As far as I understand, md-raid relies on the underlying devices
>     to inform it of IO errors before it'll seek redundant/parity data
>     to fulfill the read request.  I have, however, seen certain hard
>     drives report successful reads while returning garbage data.
>
>     Is it possible to set md-raid into a paranoid mode, in which it
>     reads all available data and confirms integrity?  Here's how it
>     would work:
>
>     RAID6: read data + parity 1 + parity 2.  If 1 of the 3 mismatches,
>     correct it, and write corrected data to the corrupt source.  Log
>     the event.  If all 3 disagree, alert user somehow.
>     RAID5: read data + parity.  If they mismatch, alert user somehow.
>     RAID1: read data 1 + data 2.  If they mismatch, alert user somehow.
>
>     You can see this is mostly useful for RAID6 mode, where there is a
>     chance at automated recovery.  However, it can also be used to
>     prevent silent data corruption in the other modes, by making it
>     not silent.
>
>     --Bart
>
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>
>
> -- 
> Roberto Spadim
> SPAEmpresarial
> Eng. Automação e Controle
>

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