On Jun 11, 2012, at 6:05 AM, Jim Klimov wrote:

> 2012-06-11 5:37, Edward Ned Harvey wrote:
>>> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss-
>>> boun...@opensolaris.org] On Behalf Of Kalle Anka
>>> Assume we have 100 disks in one zpool. Assume it takes 5 hours to scrub
>> one
>>> disk. If I scrub the zpool, how long time will it take?
>>> Will it scrub one disk at a time, so it will take 500 hours, i.e. in
>> sequence, just
>>> serial? Or is it possible to run the scrub in parallel, so it takes 5h no
>> matter
>>> how many disks?
>> It will be approximately parallel, because it's actually scrubbing only the
>> used blocks, and the order it scrubs in will be approximately the order they
>> were written, which was intentionally parallel.
> What the other posters said, plus: 100 disks is quite a lot
> of contention on the bus(es), so even if it is all parallel,
> the bus and CPU bottlenecks would raise the scrubbing time
> somewhat above the single-disk scrub time.

In general, this is not true for HDDs or modern CPUs. Modern systems
are overprovisioned for bandwidth. In fact, bandwidth has been a poor
design point for storage for a long time. Dave Patterson has some 
interesting observations on this, now 8 years dated.

SSDs tend to be a different story, and there is some interesting work being
done in this area, both on the systems side as well as the SSD side. This is
where the fun work is progressing :-)
 -- richard


ZFS and performance consulting

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