Hi Ted,

On Mon, Sep 20 2010 at 9:14am -0400,
Theodore Ts'o <tytso@mit.edu> 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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On Mon, Sep 20 2010 at 1:40pm -0400,
Mike Snitzer <snitzer@redhat.com> wrote:

> Hi Ted,
>
> On Mon, Sep 20 2010 at 9:14am -0400,
> Theodore Ts'o <tytso@mit.edu> 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):

Here is a trace during mkfs.ext4 of a bio-based striped DM device (with
4 stripes) on a stock 2.6.36-rc4:

mkfs.ext4-3835 [000] 6050624.201765: scsi_dispatch_cmd_start: host_no=10 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)
mkfs.ext4-3835 [000] 6050624.201790: scsi_dispatch_cmd_start: host_no=10 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)
mkfs.ext4-3835 [000] 6050624.201806: scsi_dispatch_cmd_start: host_no=10 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)
mkfs.ext4-3835 [000] 6050624.201823: scsi_dispatch_cmd_start: host_no=10 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] 6050624.202772: scsi_dispatch_cmd_done: host_no=10 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)
<idle>-0 [000] 6050624.202774: scsi_dispatch_cmd_done: host_no=10 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)
<idle>-0 [000] 6050624.202775: scsi_dispatch_cmd_done: host_no=10 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)
<idle>-0 [000] 6050624.202775: scsi_dispatch_cmd_done: host_no=10 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)

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On Mon, 20 Sep 2010, Theodore Ts'o 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.
>
> Is this deliberate, or an oversight?
>
> Thanks, regards,

Hi Ted,

I have tested this with 2.6.35 and it really did not work for me as
well, but with 2.6.36-rc5 (latests kernel) it works just fine. Though,
not sure when it was added. But according to the log it seems it was
merged in with 8357422d4bf33bc2c35884d4016c3fc9efbbc1d2.

-Lukas

>
> - Ted
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On Tue, Sep 21, 2010 at 6:42 AM, Lukas Czerner <lczerner@redhat.com> wrote:
> On Mon, 20 Sep 2010, Theodore Ts'o 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.
>>
>> Is this deliberate, or an oversight?
>>
>> Thanks, regards,
>
> Hi Ted,
>
> I have tested this with 2.6.35 and it really did not work for me as
> well, but with 2.6.36-rc5 (latests kernel) it works just fine. Though,
> not sure when it was added. But according to the log it seems it was
> merged in with 8357422d4bf33bc2c35884d4016c3fc9efbbc1d2.

Makes sense, DM's discard support was added during the 2.6.36 merge window.

Mike

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