Re: [Qemu-devel] I/O parallelism on QCOW2

[Xingbo Wu <wuxb45@gmail.com>]

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On Fri, Sep 5, 2014 at 6:02 AM, Stefan Hajnoczi <stefanha@gmail.com> wrote:

> On Thu, Sep 04, 2014 at 12:32:12PM -0400, Xingbo Wu wrote:
> >   After running a 16-thread sync-random-write test against qcow2, It is
> > observed that QCOW2 seems to be serializing all its metadata-related
> writes.
> >  If qcow2 is designed to do this,* then what is the concern?* What would
> go
> > wrong if this ordering is relaxed?
>
> How do you know that serializing part of the write request is a
> significant bottleneck?
>
> Please post your benchmark results with raw, qed, and qcow2 handling 1-,
> 8-, and 16-threads of I/O (or whatever similar benchmarks you have run).
>
> The bottleneck may actually be something else, so please share your
> benchmark configuration and results.
>
>
Here is the fio job file:
----j1.fio:
[j1]
direct=1
ioengine=psync
thread
fdatasync=1
runtime=300
numjobs=$NJ
# filename=/dev/sd? for raw disk
filename=/dev/nbd0
rw=write
bs=64k
offset_increment=1G
----EOF

qcow2 image is created on the raw disk with -o lazy_refcounts.
The raw disk is a D3200AAJS

As you can see, the test is to measure the performance on synchronize
writes.
So practically the result does not mean qcow2 has this bottleneck with real
workload.


> By providing less features, raw-file and QED scales well on parallel I/O
> > workload. I believe qcow2 does this with clear reasons. Thanks!
>
> QED serializes allocating writes, see qed_aio_write_alloc().
>
> In qcow2 the BdrvQcowState->lock is held across metadata updates.  The
> important pieces here are:
>  * qcow2_co_writev() only releases the lock around data writes
>    (including COW).
>

Thanks. This is what I want to confirm.
So the lock is held during the metadata write and related I/O acticity?
That's why I saw serialized metadata updates in the trace.
Could the lock be released during metadata I/O?

 * qcow2_co_flush_to_os() holds the lock around metadata updates
>
>
*flush_to_os moves the data down to the image file, but not necessarily
flush them to disk.

This function should usually returns fast with no actual disk I/O. The
later calls to flush the image file would incur the FLUSH to disk. Is that
correct?
If so, the locking here does not matter.


> Stefan
>



-- 

Cheers!
       吴兴博  Wu, Xingbo <wuxb45@gmail.com>

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