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 the Debian kernel behaves differently with respect to this "issue" from for example an openSUSE kernel, which does give symmetric (near disk i/o limited) results. What is the proper way to copy a (large) raw disk image onto a logical volume? 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. Dion. -- To UNSUBSCRIBE, email to debian-user-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org Archive: http://lists.debian.org/4e403233.5070...@concero.nl
