Am 25.06.2013 um 15:42 hat MORITA Kazutaka geschrieben: > 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.
Okay, so Sheepdog is indeed safe. Thanks, Kazutaka! Kevin
