qcow2 overlay performance

[Yoonho Park]

I have been measuring the performance of qcow2 overlays, and I am hoping to get some help in understanding the data I collected. In my experiments, I created a VM and attached a 16G qcow2 disk to it using "qemu-img create" and "virsh attach-disk". I use fio to fill it. I create some number of snapshots (overlays) using "virsh snapshot-create-as". To mimic user activity between taking snapshots, I use fio to randomly write to 10% of each overlay right after I create it. After creating the overlays, I use fio to measure random read performance and random write performance with 2 different blocks sizes, 4K and 64K. 64K is the qcow2 cluster size used by the 16G qcow2 disk and the overlays (verified with "qemu-img info"). fio is using the attached disk as a block device to avoid as much file system overhead as possible. The VM, 16G disk, and snapshots (overlays) all reside on local disk. Below are the measurements I collected for up to 5 overlays.

          4K blocks                64K blocks

olays rd bw rd iops wr bw  wr iops rd bw rd iops wr bw  wr iops

0     4510  1127    438028 109507  67854 1060    521808 8153

1     4692  1173    2924   731     66801 1043    104297 1629

2     4524  1131    2781   695     66801 1043    104297 1629

3     4573  1143    3034   758     65500 1023    95627  1494

4     4556  1139    2971   742     67973 1062    108099 1689

5     4471  1117    2937   734     66615 1040    98472  1538

Read performance is not affected by overlays. However, write performance drops even with a single overlay. My understanding is that writing 4K blocks requires a read-modify-write because you must fetch a complete cluster from deeper in the overlay chain before writing to the active overlay. However, this does not explain the drop in performance when writing 64K blocks. The performance drop is not as significant, but if the write block size matches the cluster size then it seems that there should not be any performance drop because the write can go directly to the active overlay.

Another issue I hit is that I cannot set or change the cluster size of overlays. Is this possible with "virsh snapshot-create-as"?

I am using qemu-system-x86_64 version 4.2.0 and virsh version 6.0.0.

Thank you for any insights or advice you have.