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Re: [Qemu-devel] [PATCH v12 09/10] qcow2: skip writing zero buffers to e

From: Anton Nefedov
Subject: Re: [Qemu-devel] [PATCH v12 09/10] qcow2: skip writing zero buffers to empty COW areas
Date: Wed, 16 Jan 2019 09:32:33 +0000

On 15/1/2019 6:27 PM, Alberto Garcia wrote:
> On Mon 14 Jan 2019 12:18:30 PM CET, Anton Nefedov wrote:
>> If COW areas of the newly allocated clusters are zeroes on the backing image,
>> efficient bdrv_write_zeroes(flags=BDRV_REQ_ALLOCATE) can be used on the whole
>> cluster instead of writing explicit zero buffers later in perform_cow().
>> iotest 060:
>> write to the discarded cluster does not trigger COW anymore.
>> Use a backing image instead.
>> Signed-off-by: Anton Nefedov <address@hidden>
>> Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> Reviewed-by: Alberto Garcia <address@hidden>
>> +        ret = handle_alloc_space(bs, l2meta);
> I insist that it would be nice to have a short comment explaining what
> this does.

Right sorry forgot your comment.
I'd go with:

+        /* Try to efficiently initialize the physical space with zeroes */
          ret = handle_alloc_space(bs, l2meta);
          if (ret < 0) {

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