Re: Very long raid5 init/rebuild times

[Phillip Susi <psusi@ubuntu.com>]

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On 1/28/2014 2:46 AM, Stan Hoeppner wrote:
> You usually must submit asynchronously or in parallel to reach 
> maximum throughput. Being limited by a PMP it may not matter. But 
> with your direct connected drives of your production array you
> should see a substantial increase in throughput with parallel
> submission.

Not for streaming IO; you just need to make sure your cache is big
enough so the drive is never waiting for the app.

> To significantly increase single streaming throughput you need AIO.
> A faster CPU won't make any difference.  Neither will a better SATA
> card, unless your current one is defective, or limits port
> throughput will more than one port active--I've heard of couple
> that do so.

What AIO gets you is the ability to use O_DIRECT to avoid a memory
copy to/from the kernel page cache.  That saves you some cpu time, but
doesn't make *that* much difference unless you have a crazy fast
storage array, or crazy slow ram.  And since almost nobody uses it,
it's a bit of an unrealistic benchmark.


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