Re: Awful RAID5 random read performance

[John Robinson <john.robinson@anonymous.org.uk>]

On 03/06/2009 19:38, Bill Davidsen wrote:
> John Robinson wrote:
>> On 02/06/2009 20:47, Keld Jørn Simonsen wrote:
[...]
>>> In your case, using 3 disks, raid5 should give about 210 % of the 
>>> nominal
>>> single disk speed for big file reads, and maybe 180 % for big file
>>> writes. raid10,f2 should give about 290 % for big file reads and 140%
>>> for big file writes. Random reads should be about the same for raid5 and
>>> raid10,f2 - raid10,f2 maybe 15 % faster, while random writes should be
>>> mediocre for raid5, and good for raid10,f2.
>>
>> I'd be interested in reading about where you got these figures from 
>> and/or the rationale behind them; I'd have guessed differently...
> 
> For small values of N, 10,f2 generally comes quite close to N*Sr, where 
> N is # of disks and Sr is single drive read speed. This is assuming 
> fiarly large reads and adequate stripe buffer space. Obviously for 
> larger values of N that saturates something else in the system, like the 
> bus, before N gets too large. I don't generally see more than (N/2-1)*Sw 
> for write, at least for large writes. I came up with those numbers based 
> on testing 3-4-5 drive arrays which do large file transfers. If you want 
> to read more than large file speed into them, feel free.

Actually it was the RAID-5 figures I'd have guessed differently. I'd 
expect ~290% (rather than 210%) for big 3-disc RAID-5 reads, and ~140% 
(rather than "mediocre") for random small writes. But of course I 
haven't tested.

Cheers,

John.
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