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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 14:37:14 +0000

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?

Best regards,

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