Re: Slow write performance on Compaq Smart Array 64xx (ciss0)

> On Sun, Jan 28, 2007 at 12:29:21AM -0800, Joe wrote:
>> Why is the write performance of my RAID controller so slow?
> ...
>> (write test running bsd kernel)
>> # dd if=/dev/zero of=/data/testfile count=20000 bs=128k
>> 20000+0 records in
>> 20000+0 records out
>> 2621440000 bytes transferred in 113.978 secs (22999380 bytes/sec)
>
> That's about 22MB per second. Sounds plausible to me, given that RAID5
> arrays intrinsically have very poor write performance. What value were you
> expecting to see?
>
> For comparison, what do you get for the same test but for read speed? i.e.
>
>     # dd if=/data/testfile of=/dev/null bs=128k
>
> RAID5 writes are slow because a single block write involves 4 disk
> operations (read old data block, read old parity, write new data block,
> write new parity). A non-volatile write-through cache can speed things up to
> a degree though.
>
> If you want high write performace, try setting up your six-disk array as
> three separate mirrored pairs, or as a single RAID-01 (strip/mirror) and see
> what you get. Of course your available storage size will be reduced to
> 3/5ths of what it was.
>
> Regards,
>
> Brian.
>