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

[Peter Lieven <pl@kamp.de>]

On 19.07.2013 07:58, Paolo Bonzini wrote:
> Il 18/07/2013 21:28, Peter Lieven ha scritto:
>> thanks for the details. I think to have optimal performance and best
>> change for unmapping in qemu-img convert
>> it might be best to export the OPTIMAL UNMAP GRANULARITY
> Agreed about this.
>
>> as well as the write_zeroes_w_discard capability via the BDI
> But why this?!?  It is _not_ needed.  All you need is to change the
> default of the "-S" option to be the OPTIMAL UNMAP GRANULARITY if it is
> nonzero.
2 reasons:
a) Does this guarantee that the requests are aligned to multiple of the -S value?

b) If I this flag exists qemu-img convert can do the "zeroing" a priori. This
has the benefit that combined with bdrv_get_block_status requests before it might
not need to touch large areas of the volume. Speaking for iSCSI its likely that
the user sets a fresh volume as the destination, but its not guaranteed.
With the Patch 4 there are only done a few get_block_status requests to verify
this. If we just write zeroes with BDRV_MAY_UNMAP, we send hundreds or
thousands of writesame requests for possibly already unmapped data.

To give an example. If I take my storage with an 1TB volume. It takes about 10-12
get_block_status requests to verify that it is completely unmapped. After this
I am safe to set has_zero_init = 1 in qemu-img convert.

If I would convert a 1TB image to this target where lets say 50% are at leat 15MB
zero blocks (15MB is the OUG of my storage) it would take ~35000 write same
requests to achieve the same.

Peter
>
> Paolo
>
>> and than zero
>> out the whole device with bdrv_write_zeroes and the BDRV_MAY_DISCARD
>> flag.
>>
>> the logic for this is already prepared in patch4 (topic of this email). except that
>> i have to exchange bdrv_discard with bdrv_write_zeroes w/ BDRV_MAY_DISCARD.
>>
>> what do you think?
>>
>>
>>
>>>
>>> On Thu, Jul 18, 2013 at 11:43 AM, Peter Lieven <pl@kamp.de> wrote:
>>>> Am 18.07.2013 um 16:35 schrieb Paolo Bonzini <pbonzini@redhat.com>:
>>>>
>>>>> Il 18/07/2013 16:32, Peter Lieven ha scritto:
>>>>>>> (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 :-)
>>>>> You're talking of granularity here, not (mis)alignment.
>>>> you are right. for the target i am talking about this is 30720 512-byte blocks for the granularity (pagesize) and 0 for the alignment.
>>>> i will see what happens if I write same w/unmap the whole 30720 blocks in smaller blocks ;-) otherwise i will have to add support for honoring this values in qemu-img convert
>>>> as a follow up.
>>>>
>>>> Peter
>>>>
>>
>>


-- 

Mit freundlichen Grüßen

Peter Lieven

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