linux-ext4 - Re: Is it deliberate that the device mapper is not passing the DISCARD ioctls

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Date:	Mon, 20 Sep 2010 14:55:41 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	"Theodore Ts'o" <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org, dm-devel@...hat.com
Subject: Re: Is it deliberate that the device mapper is not passing the
 DISCARD ioctls

On Mon, Sep 20 2010 at  1:40pm -0400,
Mike Snitzer <snitzer@...hat.com> wrote:

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