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

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-devel] [PATCH] block: don't probe zeroes in bs->file by default on block_status
Date: Thu, 24 Jan 2019 15:53:54 +0000

24.01.2019 18:39, Kevin Wolf wrote:
> Am 24.01.2019 um 15:37 hat Vladimir Sementsov-Ogievskiy geschrieben:
>> 23.01.2019 15:04, Vladimir Sementsov-Ogievskiy wrote:
>>> 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.
>> Kevin, what do you think?
> I'm unsure. I think it requires scanning all refcount blocks in
> qcow2_open(), right? This could be slow on huge images. On the other
> hand, the first cluster allocation will probably do this anyway, so it
> might be reasonable enough.

Seems like better is doing it on first block_status, not on open.

> How would you communicate this? Another block_status return flag that
> says "don't bother to ask the protocol layer" and which we would only
> set in qcow2 if the probing came to the conclusion that it's not
> preallocated?

Or bool flag on BlockDriverState.

> Kevin

Best regards,

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