Re: The chunk size paradox

[Phillip Susi <psusi@ubuntu.com>]

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Peter, please make sure to use your mail client's reply-to-all feature
and avoid any BS reply-to-list command, which breaks cross posted
threads, and delays my seeing your reply since I didn't get a copy.

On 12/30/2013 6:38 PM, Peter Grandi wrote:

> Therefore a larger chunk size increases the amount of data that can
> be fetched on each device without waiting for the other device to
> get to the desires angular position. It has of course the advnatage
> that you mention, but also the advantage that random IO might be
> improved.

Yes, and that is a good reason not to use 4k chunk size.  I believe
that 64k is plenty large enough for this purpose though.

>> and in the case of raid5 you run into problems with the stripe 
>> cache.
> 
> IIRC the stripe cache can be up to 32MB for RAID device, and that's
> a lot of stripes for any sensible-sized RAID set. But it never
> stopped people who "know better" to do very wide RAID5 or RAID6
> sets :-).

That's kind of my point: you don't want a *very* large stripe cache,
but limiting the chunk size means you get seek overhead to skip the
redundant data, so you are stuck between a rock and a hard place.  It
isn't as big of a deal on a 5 disk raid5/6 but on a 2 or 3 disk
raid10, a 512k chunk size has a hefty seek overhead.


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