TestBot news
Francois Gouget
fgouget at codeweavers.com
Wed Dec 7 02:42:32 CST 2016
On Mon, 5 Dec 2016, Jonas Maebe wrote:
> On 05/12/16 01:33, Francois Gouget wrote:
> > On Sun, 4 Dec 2016, Jonas Maebe wrote:
> >
> > > Francois Gouget wrote:
> > > > Indeed during the revert vm1 shows no read traffic (lots of RAM to cache
> > > > that), but a steady 6 MB/s stream of writes ! In contrast on vm2 writes
> > > > quickly ramp up to 80 MB/s, then stop after ~5 seconds and QEMU just
> > > > uses CPU for the last ~4 seconds.
> > >
> > > Maybe vm2 mounts its file systems with noatime?
> >
> > The VM hosts all use relatime which provides essentially all the
> > benefits of noatime.
>
> I actually meant the file system in the VM, but I guess there are Windows
> rather than Linux VMs?
Yes, Windows VMs, Vista for the one I have mentionned before.
> Additionally, what is the "revert" operation exactly?
> Is it like an "svn revert"/"git reset --hard HEAD" in the VM, or some qemu
> operation, or something else?
virsh --connect qemu:///system snapshot-revert wtbwvista up2014-wtb
This reverts the wtbwvista VM to the up2014-wtb which is a live
snapshot.
> > But in this case I expect the writes all go to the qcow2 disk image and
> > I know vm1 is capable of sustaining more than 6 MB/s writes (e.g. when
> > copying >100 GB around).
>
> One thing you could look at is the output of iostat on the host while the
> operations are going on, in partical the transactions-per-second, to check
> whether the issue is that one is using a lot of small writes (for what ever
> reason) while the other uses fewer, larger writes.
Well, the average read size seems to be the same, 32KB, but the number
of transactions sure is different.
vm1
$ iostat -d -h /dev/sda 1
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sda
2.00 0.00 16.00 0 16
3.00 0.00 52.00 0 52
83.00 0.00 3268.00 0 3268
208.00 0.00 6656.00 0 6656
204.00 0.00 6528.00 0 6528
193.00 0.00 6208.00 0 6208
198.00 0.00 6344.00 0 6344
208.00 0.00 6656.00 0 6656
200.00 0.00 6400.00 0 6400
192.00 0.00 6144.00 0 6144
210.00 0.00 6720.00 0 6720
201.00 0.00 6416.00 0 6416
199.00 0.00 6028.00 0 6028
203.00 0.00 6528.00 0 6528
200.00 0.00 6400.00 0 6400
194.00 0.00 6208.00 0 6208
201.00 0.00 6444.00 0 6444
207.00 0.00 6592.00 0 6592
202.00 0.00 6464.00 0 6464
209.00 0.00 6656.00 0 6656
208.00 0.00 6656.00 0 6656
194.00 0.00 6232.00 0 6232
202.00 0.00 6404.00 0 6404
191.00 0.00 6988.00 0 6988
203.00 0.00 6528.00 0 6528
200.00 0.00 6400.00 0 6400
205.00 0.00 6588.00 0 6588
211.00 0.00 6720.00 0 6720
202.00 0.00 6464.00 0 6464
196.00 0.00 6272.00 0 6272
203.00 0.00 6464.00 0 6464
197.00 0.00 6284.00 0 6284
190.00 0.00 5860.00 0 5860
200.00 0.00 6400.00 0 6400
204.00 0.00 6528.00 0 6528
204.00 0.00 6528.00 0 6528
196.00 0.00 6232.00 0 6232
194.00 0.00 6212.00 0 6212
200.00 0.00 6400.00 0 6400
202.00 0.00 6464.00 0 6464
196.00 0.00 6272.00 0 6272
192.00 0.00 6144.00 0 6144
192.00 0.00 6092.00 0 6092
190.00 0.00 6080.00 0 6080
192.00 0.00 6144.00 0 6144
201.00 0.00 6400.00 0 6400
191.00 0.00 6144.00 0 6144
182.00 4.00 5284.00 4 5284
3.00 0.00 0.00 0 0
6.00 0.00 72.00 0 72
$ filefrag /var/lib/libvirt/images/wtbwvista.qcow2
/var/lib/libvirt/images/wtbwvista.qcow2: 284 extents found
$ ls -lh /var/lib/libvirt/images/wtbwvista.qcow2
-rw-r--r-- 1 libvirt-qemu libvirt-qemu 31G Dec 7 01:48 /var/lib/libvirt/images/wtbwvista.qcow2
vm2
$ iostat -d -h /dev/sda 1
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sda
2514.00 0.00 81220.00 0 81220
2856.00 64.00 91048.00 64 91048
2368.00 88.00 75592.00 88 75592
1563.00 384.00 50128.00 384 50128
53.00 60.00 1844.00 60 1844
0.00 0.00 0.00 0 0
424.00 616.00 12008.00 616 12008
392.00 2652.00 11364.00 2652 11364
495.00 972.00 15872.00 972 15872
425.00 360.00 14016.00 360 14016
$ filefrag /var/lib/libvirt/images/wtbwvista.qcow2
/var/lib/libvirt/images/wtbwvista.qcow2: 79 extents found
$ ls -lh /var/lib/libvirt/images/wtbwvista.qcow2
-rw-r--r-- 1 root root 31G Dec 7 01:43 /var/lib/libvirt/images/wtbwvista.qcow2
On a spinning disk 2500+ IO/s only makes sense if they are contiguous.
200 IO/s however makes sense for random IO. But eve on vm1 the disk
image file is not that fragmented. And given that it was restored from
the same backup on both machines at a couple of days interval I see no
reason for one to cause random IO and not the other.
--
Francois Gouget <fgouget at codeweavers.com>
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