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

[MORITA Kazutaka]

At Tue, 25 Jun 2013 15:20:18 +0200,
Kevin Wolf wrote:
> 
> 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?

We cannot do that.  Sheepdog heavily relies on filesystem features and
we will not support block devices for the backend of Sheepdog.

Thanks,

Kazutaka