On Fri, Sep 26, 2008 at 9:50 AM, Chris Picton <chris AT ecntelecoms DOT com> 
wrote:
> On Fri, 2008-09-26 at 09:43 -0400, John Drescher wrote:
>> > Just a quick test I ran using
>> > dd if=/dev/zero of=/mnt/store/00/bigfile bs=1M count=8192
>> > just to get a basic idea of speed:
>> >
>> > Raid 5: 8589934592 bytes (8.6 GB) copied, 302.948 seconds, 28.4 MB/s
>> > Raid 0: 8589934592 bytes (8.6 GB) copied, 27.2641 seconds, 315 MB/s
>> >
>> > The Raid 5 will not be able to keep up with my network cards...
>> >
>> Something is very wrong with your setup. Are you using ext3?
>
> Yes - ext3
>
> I had not changed any of the raid stripe size or sys parameters - I just
> wanted to get an idea of the speeds.
>
>
I can help you with this.

Here is what I get on a 5 drive software raid5

 # dd if=/dev/zero of=/bigfile bs=1M count=8192
8192+0 records in
8192+0 records out
8589934592 bytes (8.6 GB) copied, 44.8402 s, 192 MB/s

 # cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid6] [raid5] [raid4] [multipath]
md0 : active raid1 sde1[4] sdd1[3] sdc1[2] sdb1[1] sda1[0]
      256896 blocks [5/5] [UUUUU]
md1 : active raid5 sde3[4] sdd3[3] sdc3[2] sdb3[1] sda3[0]
      46909440 blocks level 5, 64k chunk, algorithm 2 [5/5] [UUUUU]

 # mount | grep /dev/md1
/dev/md1 on / type xfs (rw,noatime,sunit=128,swidth=512,ikeep,noquota)


And the tuning param that I use:

echo 1024 > /sys/block/md1/md/stripe_cache_size

The reason for this and why I asked you for the filesystem is that
raid5 is very slow if you write less than 1 stripe size if the stripe
is not cached. The reason for the slowness is that if the block is not
cached the os will need to read the block then calculate parity then
write. This process is very slow. So above I extend the stripe cache.
One of the problems with using ext3 without tuning is that by default
it writes too frequently in small chunks. If you keep that I would
look into the commit mount param and extend this to 10 or more
seconds.

John

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