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Re: [Qemu-block] [PATCH] block: don't probe zeroes in bs->file by defaul

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] [PATCH] block: don't probe zeroes in bs->file by default on block_status
Date: Wed, 23 Jan 2019 12:04:40 +0000

22.01.2019 21:57, Kevin Wolf wrote:
> Am 11.01.2019 um 12:40 hat Vladimir Sementsov-Ogievskiy geschrieben:
>> 11.01.2019 13:41, Kevin Wolf wrote:
>>> Am 10.01.2019 um 14:20 hat Vladimir Sementsov-Ogievskiy geschrieben:
>>>> drv_co_block_status digs bs->file for additional, more accurate search
>>>> for hole inside region, reported as DATA by bs since 5daa74a6ebc.
>>>> This accuracy is not free: assume we have qcow2 disk. Actually, qcow2
>>>> knows, where are holes and where is data. But every block_status
>>>> request calls lseek additionally. Assume a big disk, full of
>>>> data, in any iterative copying block job (or img convert) we'll call
>>>> lseek(HOLE) on every iteration, and each of these lseeks will have to
>>>> iterate through all metadata up to the end of file. It's obviously
>>>> ineffective behavior. And for many scenarios we don't need this lseek
>>>> at all.
>>>> So, let's "5daa74a6ebc" by default, leaving an option to return
>>>> previous behavior, which is needed for scenarios with preallocated
>>>> images.
>>>> Add iotest illustrating new option semantics.
>>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
>>> I still think that an option isn't a good solution and we should try use
>>> some heuristics instead.
>> Do you think that heuristics would be better than fair cache for lseek 
>> results?
> I just played a bit with this (qemu-img convert only), and how much
> caching lseek() results helps depends completely on the image. As it
> happened, my test image was the worst case where caching didn't buy us
> much. Obviously, I can just as easily construct an image where it makes
> a huge difference. I think that most real-world images should be able to
> take good advantage of it, though, and it doesn't hurt, so maybe that's
> a first thing that we can do in any case. It might not be the complete
> solution, though.

Hmm, and one more idea from Den:

We can detect preallocated image, comparing allocated size of real file with
number of non-zero qcow2 refcounts. So, real allocation is much less than
allocation in qcow2 point of view, we'll enable lseeks, otherwise - not.

Best regards,

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