Re: [Qemu-block] Disconecting /dev/nbdX leaves stale partitions and device

[Richard W.M. Jones]

On Sat, Jul 28, 2018 at 12:34:29AM +0300, Nir Soffer wrote:
> > $ time bash -c '(qemu-img convert -p -n fedora-27.img
> > nbd:unix:/var/tmp/sock; sync)'
> >     (100.00/100%)
> >
> > real    0m9.555s
> > user    0m0.143s
> > sys     0m0.578s
> >
> >
> > $ time bash -c '( cp fedora-27.img output2; sync )'
> >
> > real   0m9.254s
> > user   0m0.005s
> > sys    0m0.846s
> >
> >
> > Maybe I'm making a mistake in my test somewhere?
> >
> 
> Looks like you test kernl page cache.

Even though I'm running sync as part of the test?

> I was testing with qemu-nbd -n
> since we always disable the cache when writing or reading from shared
> storage.

And the other significant difference is you're using block devices,
which is of course important for oVirt.

I retested on my machine using an LV backed by an SSD.  Not sure if it
supports efficient trimming however, I suspect it might not.


nbdkit (with your patch - thanks!):

$ sudo ./nbdkit file file=/dev/vg_trick_ssd/tmp_nbd_test -f -U /var/tmp/sock &
$ time sudo bash -c "qemu-img convert -n -p fedora-27.img 
nbd:unix:/var/tmp/sock ; sync"
    (100.00/100%)

real    0m54.713s
user    0m0.189s
sys     0m1.832s


qemu-nbd, but not using -n option:

$ sudo qemu-nbd /dev/vg_trick_ssd/tmp_nbd_test -k /var/tmp/sock &
$ time sudo bash -c "qemu-img convert -n -p fedora-27.img 
nbd:unix:/var/tmp/sock ; sync"
    (100.00/100%)

real    1m23.873s
user    0m0.218s
sys     0m2.114s

(This surprising result is repeatable)


qemu-nbd with -n:

$ sudo qemu-nbd /dev/vg_trick_ssd/tmp_nbd_test -k /var/tmp/sock -n &
$ time sudo bash -c "qemu-img convert -n -p fedora-27.img 
nbd:unix:/var/tmp/sock ; sync"
    (100.00/100%)

real    0m30.481s
user    0m0.203s
sys     0m2.080s


dd with 1MB block size:

$ time sudo bash -c "dd if=fedora-27.img of=/dev/vg_trick_ssd/tmp_nbd_test 
bs=1M ; sync"
6144+0 records in
6144+0 records out
6442450944 bytes (6.4 GB, 6.0 GiB) copied, 47.2462 s, 136 MB/s

real       0m47.626s
user       0m0.036s
sys        0m39.739s


I don't really know what to make of this.

Rich.

-- 
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