Reminder -- RAID 5 is not your friend

[Bill Bogstad]

On Mon, Mar 15, 2010 at 5:36 PM, Daniel Feenberg <feenberg-fCu/yNAGv6M at public.gmane.org> wrote:
>
>
> On Mon, 15 Mar 2010, Bill Bogstad wrote:
>

>>
>> I don't understand what you are suggesting here.  When Disk 2 was made
>> a fully functioning member of the RAID subsystem, why wasn't every
>> block relevant to using it for recovery written at least once to
>> initialize it?   Isn't that what a RAID build/rebuild guarantees?  I
>> could see in the case of mirrored drives that the block was only
>> written once and never read again (any reads fulfilled by a different
>> drive), but I don't see how the block gets away with never being
>> written at all.
>
> Sorry, my mistake - the sector would have to go bad between the original
> build and the other drive failing, but that may be years, not just the time
> between the failure and completing the rebuild.

Sure.   The answer to this is disc scrubbing (do continuous slow speed
reads of ALL drives, pro-actively looking
for blocks that have gone bad since they were written).    Which you
already know about based on

http://www.nber.org/sys-admin/linux-nas-raid.html

Bill Bogstad