Re: Help with low raid5 performance

[matthew sporleder <msporleder%gmail.com@localhost>]

On Thu, Nov 18, 2010 at 1:25 PM, Peter Kerwien 
<peter%kerwien.homeip.net@localhost> wrote:
> Moving the start of the partition to sector 128 did the trick!
>
> And then did the following change:
>
> START array
> 1 3 0
>
> START disks
> /dev/wd1e
> /dev/wd2e
> /dev/wd3e
>
> START layout
> 32 1 1 5
>
> START queue
> fifo 100
>
> Made a partition starting at sector 2048, created the filesystem with:
>
> # newfs -O2 -b32k -s -64m /dev/rdk0
>
> I then got the following result:
>
> $ dd if=/mnt/vol1/pub/foo.mkv of=/dev/null bs=100m count=100
> 100+0 records in
> 100+0 records out
> 10485760000 bytes transferred in 89.019 secs (117792381 bytes/sec)
>
> $ dd if=/dev/zero of=/mnt/vol1/pub/zero bs=100m count=100
> 100+0 records in
> 100+0 records out
> 10485760000 bytes transferred in 83.216 secs (126006537 bytes/sec)
>
> Which is approx 120MB/s during read and 112MB/s during write! :-)
>
> I still don't get all details or have mixed up some terms / settings
>
>>> START layout
>>> 64 1 1 5
>> this says that the stripe width is 64 blocks...  With 2 data blocks and
>> 1 parity block in each stripe, that gives you a total of 128 blocks of
>> data in a stripe.
> Isn't stripe width == the number of stripes you can write/read to/from
> at the same time? I.e. a 3 disk RAID-5 has a stripe width of 3?
>
> I thought 64 in my example, set the stripe size to 64 blocks, meaning
> that the partition should be aligned to 64, 128, ... blocks.
>


Shouldn't this be automatically determined somehow?  How many people
are unknowingly suffering from a misalignment?