HP-UX 11i v3 Using LVM Logical Volume Snapshots (September 2010)

Write performance

Performance of the first write I/O on a shared region of a logical volume with a snapshot is low

compared to that of a logical volume without snapshots or a logical volume with a single mirror copy.

This is because the write I/O requires data to be unshared on the predecessor of the original logical

volume. Over a period of time, as the snapshot data is unshared, write I/O performance on the

logical volume with a snapshot reaches that of a logical volume with one mirror and finally nears that

of a logical volume with no mirrors or snapshots.

Note

A logical volume with no mirror copies has better write performance when

compared to that of a logical volume with one or more mirror copies.

The following table shows the results of a performance test of write I/Os on a 1000 GB logical

volume when it has no mirrors, when it has one mirror copy, and when it has a snapshot associated.

Scenario

Raw random write

I/O response time

(msec)

Raw sequential

write I/O response

time (msec)

Block random

write I/O response

time (msec)

Block sequential

write I/O response

time (msec)

LV with no mirror 0.207 0.192 0.012 0.166

LV with 1 mirror 0.834 0.228 0.012 0.171

LV with snapshot: write attempt 1 63.752 0.249 0.012 0.167

LV with snapshot: write attempt 2 31.79 0.2 0.012 0.166

LV with snapshot: write attempt 3 21.115 0.203 0.012 0.164

LV with snapshot: write attempt 4 15.832 0.205 0.012 0.162

LV with snapshot: write attempt 5 2.715 0.204 0.012 0.165

LV with snapshot: write attempt 6 0.215 0.204 0.012 0.166

Note

The tests were run on a volume group with a maximum volume

group capacity of 128 TB, 4 MB extent size, 1 MB unshare unit,

and 8 KB chunks of write I/Os were issued.

In case of raw random and raw sequential write I/O, the write performance on the first attempt is low

compared to that of a non-mirrored or mirrored logical volume with no snapshots. As more data is

unshared over a period of time, the response time of the write I/O on the logical volume with

snapshot associated with it improves and nears the performance on the mirrored logical volume and

then equals the performance on the non-mirrored logical volume. In case of block random and

sequential write I/O, the performance degradation is very low and almost negligible.

The following graphs show the performance (in terms of response time) of 8 KB and 2048 KB random

raw write I/Os on a logical volume with no mirrors, with a single mirror, and with a single snapshot.

The volume group has a 4 MB extent size and 1 MB unshare unit.