Re: [Qemu-devel] [PATCH 0/3] block: blk_co_pcache

[Vladimir Sementsov-Ogievskiy]

06.06.2019 16:55, Eric Blake wrote:
> On 6/6/19 8:48 AM, Vladimir Sementsov-Ogievskiy wrote:
>> Hi all!
>>
>> Here is small new io API: blk_co_pcache, which does copy-on-read without
>> extra buffer for read data. This means that only parts that needs COR
>> will be actually read and only corresponding buffers allocated, no more.
>>
>> This allows to improve a bit block-stream and NBD_CMD_CACHE
> 
> I'd really like to see qemu-io gain access to calling this command, so
> that we can add iotests coverage of this new feature. Note that the
> in-development libnbd
> (https://github.com/libguestfs/libnbd/commits/master) is also usable as
> an NBD client that can drive NBD_CMD_CACHE, although it's still new
> enough that we probably don't want to rely on it being available yet.
> 

Hmm, don't you think that blk_co_pcache sends NBD_CMD_CACHE if called on nbd 
driver?
I didn't implement it. But may be I should..

May aim was only to avoid extra allocation and unnecessary reads. But if we 
implement
full-featured io request, what should it do?

On qcow2 with backing it should pull data from backing to top, like in 
copy-on-read.
And for nbd it will send NBD_CMD_CACHE?
These semantics seems different, but why not?

-- 
Best regards,
Vladimir