On Sun, Jan 08, 2012 at 06:59:57AM +0400, Jim Klimov wrote:
> 2012-01-08 5:37, Richard Elling ??????????:
>> The big question is whether they are worth the effort. Spares solve a 
>> serviceability
>> problem and only impact availability in an indirect manner. For single-parity
>> solutions, spares can make a big difference in MTTDL, but have almost no 
>> impact
>> on MTTDL for double-parity solutions (eg. raidz2).
> Well, regarding this part: in the presentation linked in my OP,
> the IBM presenter suggests that for a 6-disk raid10 (3 mirrors)
> with one spare drive, overall a 7-disk set, there are such
> options for "critical" hits to data redundancy when one of
> drives dies:
> 1) Traditional RAID - one full disk is a mirror of another
>    full disk; 100% of a disk's size is "critical" and has to
>    be prelicated into a spare drive ASAP;
> 2) Declustered RAID - all 7 disks are used for 2 unique data
>    blocks from "original" setup and one spare block (I am not
>    sure I described it well in words, his diagram shows it
>    better); if a single disk dies, only 1/7 worth of disk
>    size is critical (not redundant) and can be fixed faster.
>    For their typical 47-disk sets of RAID-7-like redundancy,
>    under 1% of data becomes critical when 3 disks die at once,
>    which is (deemed) unlikely as is.
> Apparently, in the GPFS layout, MTTDL is much higher than
> in raid10+spare with all other stats being similar.
> I am not sure I'm ready (or qualified) to sit down and present
> the math right now - I just heard some ideas that I considered
> worth sharing and discussing ;)

Thanks for the video link (http://www.youtube.com/watch?v=2g5rx4gP6yU). 
It's very interesting!

GPFS Native RAID seems to be more advanced than current ZFS,
and it even has rebalancing implemented (the infamous missing zfs bp-rewrite).

It'd definitely be interesting to have something like this implemented in ZFS.

-- Pasi

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