Gvinum RAID5 performance

[Greg 'groggy' Lehey]

[Format recovered--see http://www.lemis.com/email/email-format.html]

Long/short syndrome

On Monday,  1 November 2004 at 18:15:45 -0800, Julian Elischer wrote:
> Brad Knowles wrote:
>
>> At 4:22 PM +1100 2004-11-01, Alastair D'Silva wrote:
>>
>>> The offshoot of this is that to ensure data integrity, a
>>> background process is run periodically to verify the parity.
>>
>> That's not the way that RAID-5 is supposed to work, at least not
>> the way I understand it. I would be very unhappy if I was using a
>> disk storage subsystem that was configured for RAID-5 and then
>> found out it was working in this manner. At the very least, I don't
>> believe that we could/should do this by default, and adding code to
>> perform in this manner seems to me to be unnecessary complexity.
>
> You need the background task to read the blocks on the parity stripe
> because if you only ever get good reads from teh data stripes and
> never read the parity stripe you may grow bad sectors there which
> you will not detect until you need then and by then its too late. It
> need sto be run at a very low priority however so as to not
> interfere with production.

That's a possibility, but it's not implemented in old Vinum, and I
think Lukas has other issues to address right now.

Greg
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