RAID Performance Questions

[Chuck Swiger]

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  
significantly.

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  
two disks.

> 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.

-- 
-Chuck