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

From: phoeagon
Subject: Re: [Qemu-devel] [PATCH v4] block/vdi: Use bdrv_flush after metadata updates
Date: Sun, 10 May 2015 16:05:33 +0000

Just for clarity, I was not writing to tmpfs. I was READING from tmpfs. I was writing to a path named 'sdb' (as you see in the prompt) which is a btrfs on an SSD Drive. I don't have an HDD to test on though.

On Mon, May 11, 2015 at 12:02 AM Stefan Weil <address@hidden> wrote:
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.


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