[Ocfs2-devel] Read IOPS storm in case of reflinking running VM disk

[Eugene Istomin <E.Istomin@edss.ee>]

Hello,

after deploying reflink-based VM snapshots to production servers we discovered 
a performace degradation:

OS: Opensuse 13.1, 13.2
Hypervisors: Xen 4.4, 4.5
Dom0 kernels: 3.12, 3.16, 3.18
DomU kernels: 3.12, 3.16, 3.18
Tested DomU disk backends: tapdisk2, qdisk


1) on DomU (VM) 
#dd if=/dev/zero of=test2 bs=1M count=6000

2) atop on Dom0:
sdb - busy:92% - read:375 - write:130902
Reads are from others VMs, seems OK

3) DomU dd finished:
6291456000 bytes (6.3 GB) copied, 16.6265 s, 378 MB/s

4) Lets start dd again & do a snapshot:
#dd if=/dev/zero of=test2 bs=1M count=6000
#reflink test.raw ref/

5) atop on Dom0:
sdb - busy:97% - read:112740 - write:28037
So, Read IOPS = 112740, why?

6) DomU dd finished:
6291456000 bytes (6.3 GB) copied, 175.45 s, 35.9 MB/s

7) Second & further reflinks do not change the atop stat & dd time
#dd if=/dev/zero of=test2 bs=1M count=6000
#reflink --backup=t test.raw ref/    \\ * n times
~ 6291456000 bytes (6.3 GB) copied, 162.959 s, 38.6 MB/s

The question is why reflinking a running VM disk leads to read IOPS storm?


Thanks!

-- 
Best regards,
Eugene Istomin