Re: Awful RAID5 random read performance

[Bill Davidsen <davidsen@tmr.com>]

John Robinson wrote:
> 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...

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.

All tests done on raw devices and raw arrays, and ext3 devices and 
arrays. The ratios stay about the same, tuning stripe size (stride) can 
be helpful for improving write speed.

Short summary - the numbers look close enough to mine that I would say 
they are at least useful approximations.

-- 
Bill Davidsen <davidsen@tmr.com>
  Even purely technical things can appear to be magic, if the documentation is
obscure enough. For example, PulseAudio is configured by dancing naked around a
fire at midnight, shaking a rattle with one hand and a LISP manual with the
other, while reciting the GNU manifesto in hexadecimal. The documentation fails
to note that you must circle the fire counter-clockwise in the southern
hemisphere.



--
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