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Re: [Qemu-block] [PATCH 4/8] block/backup: improve unallocated clusters

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] [PATCH 4/8] block/backup: improve unallocated clusters skipping
Date: Fri, 9 Aug 2019 12:47:00 +0000

09.08.2019 15:25, Max Reitz wrote:
> On 09.08.19 09:50, Vladimir Sementsov-Ogievskiy wrote:
>> 07.08.2019 21:01, Max Reitz wrote:
>>> On 07.08.19 10:07, Vladimir Sementsov-Ogievskiy wrote:
>>>> Limit block_status querying to request bounds on write notifier to
>>>> avoid extra seeking.
>>> I don’t understand this reasoning.  Checking whether something is
>>> allocated for qcow2 should just mean an L2 cache lookup.  Which we have
>>> to do anyway when we try to copy data off the source.
>> But for raw it's seeking.
> (1) That’s a bug in block_status then, isn’t it?
> file-posix cannot determine the allocation status, or rather, everything
> is allocated.  bdrv_co_block_status() should probably pass @want_zero on
> to the driver’s implementation, and file-posix should just
> unconditionally return DATA if it’s false.
> (2) Why would you even use sync=top for raw nodes?

As I described in parallel letters, raw was bad example. NBD is good.
Anyway, now I'm refactoring cluster skipping more deeply for v2.

About top-mode: finally block-status should be used to improve other
modes too. In virtuozzo we skip unallocated for full mode too, for example.
Unfortunately, backup is most long-term thing to upstream for me..

Best regards,

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