Re: LVM write performance

[Stan Hoeppner <stan@hardwarefreak.com>]

On 8/8/2011 1:25 AM, Dion Kant wrote:
> 
> Dear list,
> 
> When writing to a logical volume (/dev/sys/test) directly through the
> device, I obtain a slow performance:
> 
> root@dom0-2:/dev/mapper# dd  of=/dev/sys/test if=/dev/zero
> 4580305+0 records in
> 4580305+0 records out
> 2345116160 bytes (2.3 GB) copied, 119.327 s, 19.7 MB/s
> 
> Making a file system on top of the LV, mounting it and write into a file
> is ok:
> 
> root@dom0-2:/dev/mapper# mkfs.xfs /dev/sys/test
> root@dom0-2:/mnt# mount /dev/sys/test /mnt/lv
> root@dom0-2:/mnt# dd  of=/mnt/lv/out if=/dev/zero
> 2647510+0 records in
> 2647510+0 records out
> 1355525120 bytes (1.4 GB) copied, 11.3235 s, 120 MB/s
> 
> Furthermore, by accident I noticed that writing directly to the block
> device is oke when the LV is mounted (of course destroying the file
> system on it):
> 
> root@dom0-2:/mnt# dd  of=/dev/sys/test if=/dev/zero
> 3703375+0 records in
> 3703374+0 records out
> 1896127488 bytes (1.9 GB) copied, 15.4927 s, 122 MB/s
> 
> Does anyone know what is going on?
> 
> The configuration is as follows:

Yes.  You lack knowledge of the Linux storage stack and of the dd
utility.  Your system is fine.  You are simply running an improper test,
and interpreting the results from that test incorrectly.

Google for more information on the "slow" results you are seeing.

-- 
Stan