Re: Help with low raid5 performance

[Peter Kerwien <peter%kerwien.homeip.net@localhost>]

I did the following test:

# dd if=/mnt/vol1/pub/foo.mkv of=/dev/null bs=100m count=1k
111+1 records in
111+1 records out
11741590082 bytes transferred in 96.724 secs (121392726 bytes/sec)
=> ~116MB/s. So yes, >100MB/s when reading.

# dd if=/dev/zero of=/mnt/vol1/pub/zero bs=100m count=1k
1024+0 records in
1024+0 records out
107374182400 bytes transferred in 4056.655 secs (26468650 bytes/sec)
=> 25MB/s

I still not understand why I cannot reach higher write performance. I
saw that the [system] process consumed like 10% (have a C2D @ 2.4GHz)
CPU. Is the write speed limited due to CPU bound? Is there a limit how
much CPU the process that calculates the xor checksums is allowed to use?

/Peter


On 11/09/2010 08:41 PM, Michael van Elst wrote:
> peter%kerwien.homeip.net@localhost (Peter Kerwien) writes:
>
>> I'm now seeing speeds between 20-25MB/s per disk, i.e. approx 40-50MB/s
>> to the raid. Still much less than I expected. I was hoping for
>> ~50-60MB/s per disk => >100MB/s to the raid.
> You probably don't have large enough writes to keep the disks busy.
> Do you get >100MB/s when _reading_ from the raid?
>