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

[Stefan Hajnoczi <stefanha@gmail.com>]

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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.

> 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).
 * qcow2_co_flush_to_os() holds the lock around metadata updates

Stefan

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