On Sun, Jun 10, 2007 at 01:56:20AM +0200, Peter Rabbitson wrote:

Hi,

This question might be better suited for the lvm mailing list, but 
raid10 being rather new, I decided to ask here first. Feel free to 
direct me elsewhere.

I want to use lvm on top of a raid10 array, as I need the snapshot 
capability for backup purposes. The tuning and creation of the array 
went fine, I am getting the read performance I am looking for. However 
as soon as I create a VG using the array as the only PV, the raw read 
performance drops to the ground. I suspect it has to do with some minima 
l tuning of LVM parameters, but I am at a loss on what to tweak (and 
Google is certainly evil to me today). Below I am including my 
configuration and test results, please let me know if you spot anything 
wrong, or have any suggestions.

[snip]

root@Arzamas:~# dd if=/dev/md1 of=/dev/null bs=512 count=4000000
4000000+0 records in
4000000+0 records out
2048000000 bytes (2.0 GB) copied, 11.4032 seconds, 180 MB/s
root@Arzamas:~#


root@Arzamas:~# dd if=/dev/raid10/space of=/dev/null bs=1M count=2000
2000+0 records in
2000+0 records out
2097152000 bytes (2.1 GB) copied, 25.7089 seconds, 81.6 MB/s
root@Arzamas:~#

Try to increase the read-ahead size of your lvm devices:

blockdev --setra 8192 /dev/raid10/space

or increase it at least to the same size as of your raid (blockdev
--getra /dev/mdX).


Hope it helps,
Bernd