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Re: LVM write performance

On 8/8/2011 2:00 PM, Dion Kant wrote:
> On 08/08/2011 03:33 PM, Stan Hoeppner wrote:
>> 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.
> Hmm, Interpreting your answer, this behaviour is what you expect.
> However,  I think it is a bit strange to find, with this "improper
> test", about a factor 10 difference between reading from and writing to
> a logical volume by using dd directly on the device file. Note that  dd
> if=/dev/sys/test of=/dev/null does give disk i/o limited results.

Apparently you are Google challenged as well.  Here:

5th hit:

> What is the proper way to copy a (large) raw disk image onto a logical
> volume?

See above, and do additional research into dd and "block size".  It also
wouldn't hurt for you to actually read and understand the dd man page.

> Thanks for your advise to try Google. I already found a couple of posts
> from people describing this similar issue, but no proper explanation yet.

I already knew the answer, so maybe my search criteria is what allowed
me to "find" the answer for you in 20 seconds or less.  I hate spoon
feeding people, as spoon feeding is antithetical to learning and
remembering.  Hopefully you'll learn something from this thread, and
remember it. :)


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