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

[Paolo Bonzini <pbonzini@redhat.com>]

Il 13/11/2013 15:14, Peter Lieven ha scritto:
>>> >> 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.

What we have is:

* bdi->unallocated_blocks_are_zero returns true

* bdrv_create doesn't ensure that every block starts unallocated

* hence bdrv_has_zero_init returns false

Blocks that (for any reason) are unallocated after bdrv_create *will* be
zero if BLKDISCARDZEROES returns true.

Paolo