Linux LVM performace measurement

Modern Linux LVM offers great abilities to maintain snapshots of existing logical volumes. Unlike NetApp “Write Anywhere File Layout” (WAFL), Linux LVM uses “Copy-on-Write” (COW) to allow snapshots. The process, in general, can be described in this pdf document.

I have issues several small tests, just to get real-life estimations of what is the actual performance impact such COW method can cause.

Server details:

1. CPU: 2x Xion 2.8GHz

2. Disks: /dev/sda – system disk. Did not touch it; /dev/sdb – used for the LVM; /dev/sdc – used for the LVM

3. Mount: LV is mounted (and remains mounted) on /vmware

Results:

1. No snapshot, Using VG on /dev/sdb only:

# time dd if=/dev/zero of=/vmware/test.2GB bs=1M count=2048
2048+0 records in
2048+0 records out

real 0m16.088s
user 0m0.009s
sys 0m8.756s

2. With snapshot on the same disk (/dev/sdb):

# time dd if=/dev/zero of=/vmware/test.2GB bs=1M count=2048
2048+0 records in
2048+0 records out

real 6m5.185s
user 0m0.008s
sys 0m11.754s

3. With snapshot on 2nd disk (/dev/sdc):

# time dd if=/dev/zero of=/vmware/test.2GB bs=1M count=2048
2048+0 records in
2048+0 records out

real 5m17.604s
user 0m0.004s
sys 0m11.265s

4. Same as before, creating a new empty file on the disk:

# time dd if=/dev/zero of=/vmware/test2.2GB bs=1M count=2048
2048+0 records in
2048+0 records out

real 3m24.804s
user 0m0.006s
sys 0m11.907s

5. Removed the snapshot. Created a 3rd file: