[Libguestfs] nbdkit, VDDK, extents, readahead, etc

[Martin Kletzander]

On Fri, Apr 12, 2019 at 04:30:02PM +0100, Richard W.M. Jones wrote:
>On Fri, Apr 12, 2019 at 03:52:58PM +0200, Martin Kletzander wrote:
>> On Thu, Apr 11, 2019 at 06:55:55PM +0100, Richard W.M. Jones wrote:
>> >This seems to be a bug in VDDK, possibly connected with the fact that
>> >we fork after initializing VDDK but before doing the
>> >VixDiskLib_ConnectEx.  I suspect it's something to do with the PID
>> >changing.
>> >
>> >It would be fair to deduct 2 minutes from all timings below.
>> >
>>
>> Is the PID changed because you want to exec from the parent (where
>> the init is done), but all the other calls are done in the child? Is
>> that the case so that nbdkit is part of the process that someone
>> spawned? I'm asking just to know if something can be done about it.
>
>This hang only applies when using the --run option and I guess you
>wouldn't be using that option so you wouldn't see the hang.
>
>The reason why nbdkit forks itself when using this option is so we end
>up with a situation like this:
>
>  +-- nbdkit monitoring process
>    |
>    +-- first child = nbdkit
>    |
>    +-- second child = ‘--run’ command
>
>so when the second child exits, the monitoring process (which is doing
>nothing except waiting for the second child to exit) can kill nbdkit.
>

Oh, I thought the "monitoring process" would just be a signal handler.  If the
monitoring process is just checking those two underlying ones, how come the PID
changes for the APIs?  Is the Init called before the first child forks off?

>If VDDK cannot handle this situation (and I'm just guessing that this
>is the bug) then VDDK has a bug.
>

Sure, but having a workaround could be nice, if it's not too much work.

>> >(3) Using nbdkit-noextents-filter and nbdkit-stats-filter we can
>> >nicely measure the benefits of extents:
>> >
>> >With noextents (ie. force full copy):
>> >
>> > elapsed time: 323.815 s
>> > read: 8194 ops, 17179869696 bytes, 4.24437e+08 bits/s
>> >
>> >Without noextents (ie. rely on qemu-img skipping sparse bits):
>> >
>> > elapsed time: 237.41 s
>> > read: 833 ops, 1734345216 bytes, 5.84423e+07 bits/s
>> > extents: 70 ops, 135654246400 bytes, 4.57114e+09 bits/s
>> >
>> >Note if you deduct 120 seconds (see point (1) above) from these times
>> >then it goes from 203s -> 117s, about a 40% saving.  We can likely do
>> >better by having > 32 bit requests and qemu not using
>> >NBD_CMD_FLAG_REQ_ONE.
>> >
>> How did you run qemu-img?
>
>The full command was:
>
>LD_LIBRARY_PATH=vmware-vix-disklib-distrib/lib64 \
>./nbdkit -r -U - vddk file="[datastore1] Fedora 28/Fedora 28.vmdk" \
>                      libdir=vmware-vix-disklib-distrib \
>                      server=vmware user=root password=+/tmp/passwd \
>                      thumbprint=xyz \
>                      vm=moref=3 \
>                      --filter=stats statsfile=/dev/stderr \
>                      --run '
>        unset LD_LIBRARY_PATH
>        /home/rjones/d/qemu/qemu-img convert -p $nbd /var/tmp/out
>    '
>
>(with extra filters added to the command line as appropriate for each
>test).
>
>> I think on slow CPU and fast disk this might be even bigger
>> difference if qemu-img can write whatever it gets and not searching
>> for zeros.
>
>This is RHEL 8 so /var/tmp is XFS.  The hardware is relatively new and
>the disk is an SSD.
>

Why I'm asking is because what you are measuring above still includes QEMU
looking for zero blocks in the data.  I haven't found a way to make qemu write
the sparse data it reads without explicitly sparsifying even more by checking
for zeros and not creating a fully allocated image.

I have to look into that as that would be very useful for my use case.

>Rich.
>
>-- 
>Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
>Read my programming and virtualization blog: http://rwmj.wordpress.com
>virt-df lists disk usage of guests without needing to install any
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>http://people.redhat.com/~rjones/virt-df/
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