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

[Eugene Istomin]

Hello Goldwyn,

Do you know something about such behavior?
The question is why a reflink operation on VM disk leads to plenty of read ops?
Is this related to CoW specific structures? 

We can provide others details & ssh to testbed.

-- 
Best regards,
Eugene Istomin


On Friday, May 08, 2015 08:56:57 AM Eugene Istomin wrote:
> 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!