Hi Ted,
On Mon, Sep 20 2010 at 9:14am -0400,
Theodore Ts'o <tytso@xxxxxxx> wrote:
> Hi there,
>
> I was just testing discard support in mke2fs, and I was surprised that
> although the dm layer appears to pass discard requests through to the
> underlying block device driver when submitted through the bio layer
> (i.e., from kernel file systems), apparently the discard ioctls (i.e.,
> BLKDISCARD, BLKDISCARDSEC, BLKDISCARDZEROES) are not currently wired up
> in the dm layer.
After enabling scsi tracing:
# mount -t debugfs none /sys/kernel/debug
# cd /sys/kernel/debug/tracing/
# echo "scsi:*" > set_event
# cat trace_pipe
I'm not seeing a problem with BLKDISCARD (via mkfs.ext4) for either
request-based DM (this is on a 2.6.36-rc4 kernel w/ the FLUSH+FUA patchset):
kworker/0:1-21 [000] 6046573.148416: scsi_dispatch_cmd_start: host_no=3 channel=0 id=0 lun=0 data_sgl=1 prot_sgl=0 cmnd=(UNMAP regions=1 raw=42 00 00 00 00 00
00 00 18 00)
<idle>-0 [000] 6046573.152001: scsi_dispatch_cmd_done: host_no=3 channel=0 id=0 lun=0 data_sgl=1 prot_sgl=0 cmnd=(UNMAP regions=1 raw=42 00 00 00 00 00 00 00 18 00) result=(driver=DRIVER_OK host=DID_OK message=COMMAND_COMPLETE status=SAM_STAT_GOOD)
...
or bio-based DM:
kworker/0:1-21 [000] 6046158.384097: scsi_dispatch_cmd_start: host_no=2 channel=0 id=0 lun=0 data_sgl=1 prot_sgl=0 cmnd=(UNMAP regions=1 raw=42 00 00 00 00 00
00 00 18 00)
<idle>-0 [000] 6046158.405034: scsi_dispatch_cmd_done: host_no=2 channel=0 id=0 lun=0 data_sgl=1 prot_sgl=0 cmnd=(UNMAP regions=1 raw=42 00 00 00 00 00 00 00 18 00) result=(driver=DRIVER_OK host=DID_OK message=COMMAND_COMPLETE status=SAM_STAT_GOOD)
...
> Is this deliberate, or an oversight?
DM should implicitly supports the BLKDISCARD* ioctls -- just like any
other block device ioctl.
Mike
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