Re: [Qemu-devel] block: Review of .has_zero_init use

[Kevin Wolf]

Am 25.06.2013 um 15:11 hat MORITA Kazutaka geschrieben:
> At Tue, 25 Jun 2013 13:39:11 +0200,
> Kevin Wolf wrote:
> > 
> > Hi all,
> > 
> > while discussing some iscsi patches with Peter, we came to have a look
> > at which block drivers implement has_zero_init() to return 0, and which
> > don't (returning 1 is the default).
> > 
> > The meaning of this value is that if has_zero_init != 0, after
> > bdrv_create() one can assume that the whole image would read back as all
> > zero. For example, this is true for the traditional image files, but not
> > for host_device, where the block device isn't really created during
> > bdrv_create() but only checked for size.
> > 
> > The full list of protocol level block drivers is:
> > 
> > * blkdebug      - doesn't have bdrv_create
> > * blkverify     - doesn't have bdrv_create
> > * curl          - doesn't have bdrv_create
> > * gluster       - currently has_zero_init = 1 (is this correct?)
> > * iscsi         - has_zero_init = 0
> > * nbd           - doesn't have bdrv_create
> > * file          - has_zero_init = 1
> > * host_*        - has_zero_init = 0
> > * rbd           - currently has_zero_init = 1 (is this correct?)
> > * sheepdog      - currently has_zero_init = 1 (is this correct?)
> > * ssh           - currently has_zero_init = 1 (is this correct?)
> > * vvfat         - doesn't have bdrv_create
> > 
> > Can you please review for the gluster, rbd, sheepdog and ssh driver
> > whether it's safe to assume that the image reads back as zeros after
> > bdrv_create?
> 
> It's safe for Sheepdog.  Sheepdog uses ftruncate or fallocate to
> create data blocks and it is guaranteed that the space will be
> initialized to zero.

Note that ftruncate/fallocate don't zero-initialise block devices, only
regular files. Not sure if you can use block devices to back Sheepdog
images?

Kevin