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: http://lmgtfy.com/?q=lvm+block+size 5th hit: http://blog.famzah.net/2010/02/05/dd-sequential-write-performance-tests-on-a-raw-block-device-may-be-incorrect/ > 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. :) -- Stan -- 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/4e40b1ae.5080...@hardwarefreak.com
