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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: Fri, 11 Jan 2019 11:40:33 +0000

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 don't see good heuristics, neither want to implement lseek optimizations.
In our cases we don't need lseek under qcow2 at all, and it's obviously better
just don't lseek in these cases.

Moreover, as I understand, the cases when we need lseek are those when qcow2 
that the cluster is data, but actually it's a hole on fs... Isn't it better to
support this thing in qcow2? A kind of allocated-zeroes cluster? We have enough
reserved bits in cluster descriptor to add this feature. And than we don't need
this kind of cheating when user "better knows" where are holes than format layer
which owns the data..

Best regards,

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