Re: Awful RAID5 random read performance

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

On 02/06/2009 20:47, Keld Jørn Simonsen wrote:
[...]
> My perception is that raid10,f2 is probably the fastest also for small random
> reads because of the lower latency, and faster transfer times due to only
> using the outer disk sectors. For writes the elevator evens out the
> ramdom access. Benchmarks may not show this effect as they are often
> done on clean file systems, where the files are allocated in the
> beginning of the fs.
> 
> For cases where you need cheap disk space, and have big files like
> .iso's then raid5 could be a good choice because it has the most space
> while maintaining fair to good performance for big files. 
> 
> 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...

Cheers,

John.
--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html