[wplug] RAID performance

[Patrick Wagstrom]

Michael Skowvron wrote:
> Patrick,
> 
> On software vs. hardware RAID:
> Software RAID benchmarks higher than hardware RAID because you tend to 
> have all CPU resources available for the RAID calculations during the 
> test. On a heavily loaded system, software RAID could end up having a 
> big impact on system performance because the software RAID calculations 
> compete with other tasks for CPU resources. So, hardware RAID may be 
> slower, but it will be consistent performance regardless of system load.
> 
> For comparison, my 3Ware 7500 using 4x250GB PATA drives in a RAID5:
> 
> Write:
> bfs:~/lmbench2/bin/i686-pc-linux-gnu # ./lmdd of=/safe/lmddtest move=1g 
> sync=1
> 1073.7418 MB in 26.7281 secs, 40.1728 MB/sec
> 
> Read:
> bfs:~/lmbench2/bin/i686-pc-linux-gnu # ./lmdd if=/safe/lmddtest
> 1073.7418 MB in 12.0338 secs, 89.2270 MB/sec

Maybe it was somewhat the artificial nature of the tests where I was 
seeing some of the issues with the hardware RAID getting pummeled.  In 
particular, after running lmbench I saw that the hardware raid was much 
faster for reads:

Hardware Write: ./lmdd of=dummy move=1g sync=1
1073.7418 MB in 25.6633 secs, 41.8396 MB/sec

Hardware Read: ./lmdd if=dummy
1073.7418 MB in 4.1905 secs, 256.2301 MB/sec

Software Write: ./lmdd of=/mnt/store/dummy move=1g sync=1
1073.7418 MB in 18.0682 secs, 59.4273 MB/sec

Software Read: ./lmdd if=/mnt/store/dummy
1073.7418 MB in 6.3530 secs, 169.0131 MB/sec

Of course, these are completely synthetic benchmarks but they're making 
me feel a little better about the performance of the RAID 5 on the 
system.  Some of the write difference is most likely because of the 
change from SATA I to SATA II.  I'm not positive, but I don't believe 
that the SATA I drives even have NCQ -- the hardware RAID machine is 
about two years old.

Anyway, it's been a fun little adventure,  I guess there probably isn't 
anything seriously wrong with the hardware raid on the machine.  Just 
slow disks for writes.  For reads I'm happy with the performance.  A 
PCI-X 133MHz card maxes out around 400MB/s, so getting 256MB/s over four 
disks is pretty darn nice.

Incidentally, the stripe size for both sets of devices is identical at 256k.

--Patrick