Re: [Qemu-devel] [PATCH 09/11] raw-posix: implement write_zeroes with MAY_UNMAP for block devices

[Peter Lieven]

Am 13.11.2013 um 10:44 schrieb Paolo Bonzini <address@hidden>:

> Il 13/11/2013 07:29, Peter Lieven ha scritto:
>> Wouldn't it be good to add bdi->can_write_zeroes_with_unmap here as well?
> 
> We do:
> 
>> +    bdi->unallocated_blocks_are_zero = s->discard_zeroes;
>> +    bdi->can_write_zeroes_with_unmap = s->discard_zeroes;

Sorry, I missed this.

> 
>> This would automatically avoid full allocation when converting something to 
>> a host device
>> supporting BLKDISCARDZEROES.
> 
> Yes, that's (part of) the point of this patch.
> 
> Regarding the question you posed in the previous patch:
> 
>> does BLKDISCARDZEROES ioctl guarantee that a device is
>> zero initialized or does it just guarantee that a discard may not
>> fail and that it reads as zeroes afterwards?
> 
> Only the latter.  ".bdrv_has_zero_init" is only present in the bdrv_file
> BlockDriver.

Then bdi->unallocated_blocks_are_zero must stay 0. .bdrv_has_zero_init's
semantic is to reflect the zero status of all blocks of the device right after 
bdrv_create
independently of their allocation status. bdi->unallocated_blocks_are_zero
reflects the zero status of every unallocated block regardless if it was
unallocated right from the beginning or became unallocated through a discard.

Peter