RAID Performance Questions
cswiger at mac.com
Thu Jan 25 20:15:50 UTC 2007
On Jan 25, 2007, at 10:50 AM, Milo Hyson wrote:
> The write times of both RAID configurations are slower than the
> single drive (which is expected due to having to write to multiple
> drives). However, I wasn't expecting such a drastic reduction
> (about 50%). The read times, although faster, are only marginally
> so in per-char transfer. They're a bit better in block performance,
> but still not what I would expect. It would seem to me that a read
> spread across four drives should see more than a 45% performance
> increase. The highest rate recorded here is only a quarter of the
> PCI bus-speed, so I doubt that's a bottleneck. CPU load peaks at
> 50%, so I don't see that being a problem either.
Single-byte accesses are a worst-case scenario for RAID throughput;
the block rates are generally more applicable to the performance
you'll see for decently-written applications and many use-case
scenarios. If you've got a UPS or battery-backup option for the RAID
card enabled, consider turning on write-back mode rather than write-
thru mode, which ought to improve write performance pretty
Still, you also ought to consider that a 3-disk RAID-5 configuration
is very much not ideal from either an efficiency or performance
standpoint-- you want more like 5 or 6 drives being used, in which
case your performance numbers ought to increase some. This is also
somewhat true of the 4-disk RAID-10 config; using 6 or all 8 drives
would likely improve performance compared with striping against only
> I also ran some performance tests with a stock build of PostgreSQL
> 8.0 to get a different angle on things.
[ ... ]
> Any performance benefit of RAID in these tests is almost
> nonexistent. Am I doing something wrong? Am I expecting too much?
> Any advice that can be offered in this area would be much appreciated.
Most databases dislike any form of RAID except plain old RAID-1
mirroring, but absolutely hate RAID-5. Databases can do OK with big
RAID-10 combinations, too, but ask any experienced DBA what they'd
like, and they'd rather have as many RAID-1 spindles available as
possible compared with any other drive arrangement.
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