Re: [Qemu-devel] [PATCH v4] block/vdi: Use bdrv_flush after metadata updates

[Paolo Bonzini]

On 10/05/2015 18:02, Stefan Weil wrote:
>> Since the default qemu-img convert case isn't slowed down, I
>> would think that correctness trumps performance.  Nevertheless,
>> it's a huge difference.
> 
> I doubt that the convert case isn't slowed down.

The *default* convert case isn't slowed down because "qemu-img convert"
defaults to the "unsafe" cache mode.

The *non-default* convert case with flushes was slowed down indeed: 2x
in total (if you include the final flush done by bdrv_close), and 10x if
you only consider the sequential write part of convert.

Paolo

> Writing to a tmpfs as it was obviously done for the test is not a
> typical use case.
> With real hard disks I expect a significant slowdown.
> 
> Stefan