Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert

[Peter Lieven <pl@kamp.de>]

On 18.07.2013 16:20, Paolo Bonzini wrote:
> Il 18/07/2013 16:09, Peter Lieven ha scritto:
>> On 18.07.2013 15:52, Paolo Bonzini wrote:
>>> Il 18/07/2013 15:29, Peter Lieven ha scritto:
>>>>>> If the driver would have a better method of writing zeroes than
>>>>>> discard it simply should not set bdi->write_zeroes_w_discard = 1.
>>>>> If the driver had a better method of writing zeroes than discard, it
>>>>> simply should ignore the BDRV_MAY_UNMAP (or BDRV_MAY_DISCARD) flag in
>>>>> its bdrv_write_zeros implementation.
>>>> ok, but this would require an individual patch in every driver, wouldn't
>>>> it.  i am ok with that.
>>> Yes (making the drivers return the flag in the BDI would also require
>>> per-driver patches).
>> we still might need a hint for qemu-img convert that the driver does
>> support
>> writing zeroes by unmap because using write_zeroes in the main loop
>> might result in unaligned requests that the target is not able to unmap.
>> and to avoid writing several blocks twice by first writing all zeroes
>> to the target and then writing all data blocks again I would need to
>> keep the loop
>> at the beginning of qemu-img convert to write zeroes with correct
>> alignment and granularity if the driver supports write_zeroes_w_discard.
> (Mis)alignment and granularity can be handled later.  We can ignore them
> for now.  Later, if we decide the best way to support them is a flag,
> we'll add it.  Let's not put the cart before the horse.
>
> BTW, I expect alignment!=0 to be really, really rare.
To explain my concerns:

I know that my target has internal page size of 15MB. I will check what happens
if I deallocate this 15MB in chunks of lets say 1MB. If the page gets unprovisioned
after the last chunk is unmapped it would be fine :-)

Peter