[dm-devel] BLKZEROOUT on dm devices

[Bharata B Rao]

Hi,

Is BLKZEROOUT ioctl supposed to work on dm devices ? When I try this
ioctl on an LV that sits on a WRITE SAME capable SCSI device, I see
that conventional zeroing is done since the WRITE SAME support isn't
advertised by the dm device.

# lsblk
NAME                             MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
sda                                8:0    0    50G  0 disk
└─mpathb (dm-12)                 252:12   0    50G  0 mpath
  └─ws_vg-lvol0 (dm-11)          252:11   0     1G  0 lvm
sde                                8:64   0    50G  0 disk
└─mpathb (dm-12)                 252:12   0    50G  0 mpath
  └─ws_vg-lvol0 (dm-11)          252:11   0     1G  0 lvm

# cat /sys/block/sda/queue/write_same_max_bytes
268435456
# cat /sys/block/dm-12/queue/write_same_max_bytes
0
# cat /sys/block/dm-11/queue/write_same_max_bytes
0

[root at llmvm02 ~]# cat /sys/dev/block/8\:0/queue/write_same_max_bytes
268435456
[root at llmvm02 ~]# cat /sys/dev/block/252\:12/queue/write_same_max_bytes
0
[root at llmvm02 ~]# cat /sys/dev/block/252\:11/queue/write_same_max_bytes
0

>From commit d54eaa5a0fde0a202e4e91f200f818edcef15bee, I gather that
WRITE SAME is advertised by a dm device if all of its target and
underlying devices support WRITE SAME. Also as per
4f0b70b0479101522b8645ddc1f5ee7137821db3, WRITE SAME is supported for
linear dm devices. Both of these seem to be true in my case, but still
BLKZEROOUT ioctl resorts to conventional zeroing. What am I missing
here ?

Regards,
Bharata.
--
http://raobharata.wordpress.com/