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

[Stefan Weil <sw@weilnetz.de>]

Am 10.05.2015 um 17:01 schrieb Paolo Bonzini:
>
> On 09/05/2015 05:54, phoeagon wrote:
>> zheq-PC sdb # time ~/qemu-sync-test/bin/qemu-img convert -f raw -t writeback -O vdi /run/shm/rand 1.vdi
>>
>> real0m8.678s
>> user0m0.169s
>> sys0m0.500s
>>
>> zheq-PC sdb # time qemu-img convert -f raw -t writeback -O vdi /run/shm/rand 1.vdi
>> real0m4.320s
>> user0m0.148s
>> sys0m0.471s
> This means that 3.83 seconds are spent when bdrv_close() calls
> bdrv_flush().  That's the only difference between writeback
> and unsafe in qemu-img convert.
>
> The remaining part of the time (4.85 seconds instead of 0.49
> seconds) means that, at least on your hardware, sequential writes
> to unallocated space become 10 times slower with your patch.
>
> Since the default qemu-img convert case isn't slowed down, I
> would think that correctness trumps performance.  Nevertheless,
> it's a huge difference.
>
> Paolo

I doubt that the convert case isn't slowed down.

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