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

[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

> zheq-PC sdb # time qemu-img convert -f raw -t unsafe -O vdi /run/shm/rand 
> 1.vdi
> real  0m0.489s
> user  0m0.173s
> sys   0m0.325s