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[linux-lvm] Performance tunning on LVM2




Hello,
My configuration:
Server DELL 2860 Intel(R) Xeon(R) CPU  X3230 @ 2.66GHz (Quad Core)
8GB of  memory
2 x SAS 15000 300G0 RAID 0 hardware
SLES 10 SP2
Kernel 2.6.16.60-0.21-xen

i have one volume group vg0 ( whith one PV, the two disks in raid0) whith many lvm
I am very surprise about LVM2 performance when a snapshot is done.
Write speed on the Original volume is very bad when a snaphot is active...

For exemple:

Speed on /dev/vg0/test when there is NO snapshot :

suse2:~ # dd if=/dev/zero of=/dev/vg0/test bs=2M count=400
400+0 records in
400+0 records out
838860800 bytes (839 MB) copied, 6.42741 seconds, 131 MB/s

Speed on /dev/vg0/test when there is one snapshot of this original volume :

suse2:~ # lvremove --force /dev/vg0/test3.snap
  Logical volume "test3.snap" successfully removed
suse2:~ # dd if=/dev/zero of=/dev/vg0/test bs=2M count=400
400+0 records in
400+0 records out
838860800 bytes (839 MB) copied, 6.42741 seconds, 131 MB/s
suse2:~ # lvcreate -s -L1G -ntest.snap /dev/vg0/test
  Logical volume "test.snap" created
suse2:~ # dd if=/dev/zero of=/dev/vg0/test bs=2M count=400
400+0 records in
400+0 records out
838860800 bytes (839 MB) copied, 204.862 seconds, 4.1 MB/s


Speed on /dev/vg0/test when there is 2 snapshots of this original volume :

suse2:~ # lvcreate -s -L1G -ntest1.snap /dev/vg0/test
  Logical volume "test1.snap" created
suse2:~ # lvcreate -s -L1G -ntest2.snap /dev/vg0/test
  Logical volume "test2.snap" created
suse2:~ # lvremove /dev/vg0/test2.snap
Do you really want to remove active logical volume "test2.snap"? [y/n]: y
  Logical volume "test2.snap" successfully removed
suse2:~ # dd if=/dev/zero of=/dev/vg0/test bs=2M count=400
400+0 records in
400+0 records out
838860800 bytes (839 MB) copied, 270.928 seconds, 3.1 MB/s


Do you know  some elements about tunning performance?,?

Performances are disastrous when a snaphot is active
Could you give your speed result? and your amelioration??

ps:Results are the same whithout Kernel Xen and whith a kernel more recent (2.6.24.2)
Cordialement,

MARTINEAU
Antony

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