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Hello,
after deploying OCFS2 reflink-based VM snapshots to production servers we discovered a performace degradation:
OS: Opensuse 13.1, 13.2
Hypervisors: Xen 4.4, 4.5, 4.5.1
Dom0 kernels: 3.12, 3.16, 3.18, 4.1
DomU kernels: 3.12, 3.16, 3.18, 4.1
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
Working perfectly If reflink is done as fully Dom0 operations (dd & reflink are in Dom0) - so, this is not (or not only) OCFS2 problem.
The question is why reflinking a running Xen VM disk leads to read IOPS storm?
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Best regards,
Eugene Istomin
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