Re: [Qemu-block] [PATCH 4/4] block: Cater to iscsi with non-power-of-2 discard

[Paolo Bonzini]

On 25/10/2016 14:03, Peter Lieven wrote:
> Am 01.08.2016 um 11:22 schrieb Paolo Bonzini:
>>
>> On 28/07/2016 04:39, Eric Blake wrote:
>>> On 07/27/2016 01:25 AM, Fam Zheng wrote:
>>>> On Thu, 07/21 13:34, Eric Blake wrote:
>>>>> +    max_write_zeroes = max_write_zeroes / alignment * alignment;
>>>> Not using QEMU_ALIGN_DOWN despite patch 3?
>>> Looks like I missed that on the rebase. Can fix if there is a reason for
>>> a respin.
>> Since Stefan acked this, I'm applying the patch and fixing it to use
>> QEMU_ALIGN_DOWN.
>>
>> Paolo
> 
> Hi,
> 
> I came across a sort of regression we introduced with the dropping of
> head and tail
> of an unaligned discard.
> 
> The discard alignment that we use to trim the discard request is just a
> hint.
> 
> I learned on the equallogics that a page (which is this unusal 15MB
> large) is
> unallocated even if the discard happens in pieces. E.g. in slices of 1MB
> requests.
> 
> From my point of view I would like to restore the old behaviour. What do
> you think?

The right logic should be the one in Linux: if splitting a request, and
the next starting sector would be misaligned, stop the discard at the
previous aligned sector.  Otherwise leave everything alone.

Paolo