[linux-lvm] lvremove does not pass discards if volume is part of thin pool

Mike Snitzer snitzer at redhat.com
Tue Aug 11 14:35:27 UTC 2015

On Tue, Aug 11 2015 at  4:07am -0400,
vaLentin chernoZemski <valentin at siteground.com> wrote:

> >Plese verify your kernel has this commit:
> >19fa1a6756e ("dm thin: fix discard support to a previously shared block")
> >
> >But it doesn't look like you're using snapshots so this may not matter.
> The kernel we are using includes the changes listed in that commit.
> >If you do have the patch I referenced above then something else is going
> >on.  You should probably run with: lvremove -vvvv to see if lvm is
> >actually issuing a discard.  Or you could use blktrace to see if the
> >thin device you're removing is actually receiving a discard.
> lsblk -D shows DISC-ZERO as 1 for loop dev, thingroup_tmeta and
> thingroup_tdata
> However it shows DISC-ZERO as 0 for thingroup-tpool in both tmeta
> and tdata sections and all its child devices.

DISC-ZERO is discard_zeroes_data.  DM thinp disables that.  It doesn't
mean discard aren't enabled.  DISC-MAX and DISC-GRAN would need to be 0
for discards to be disabled.
> It appears to me that for some reason device mapper or kernel (not
> sure which should do that) are not advertising _tpool_ tmeta and
> tdata devices to support discards (as confirmed by lsblk). That's
> why during lvremove lvm skips issuing discards on those devices.

Nope, that isn't it.  The pool and thin device are advertising
discards.  You should verify that the pool is configured to passdown
discards to the underlying loop device.  Run: dmsetup table
You should see 'discard_passdown' -- which gets enabled by default if
the thin-pool's underlying data device supports discards.

> The only references in lvremove -f -vvvv that are stating discards are those
> #libdm-deptree.c:2681     Suppressed testgroup-thingroup (253:36)
> identical table reload.
> #ioctl/libdm-iface.c:1795         dm status   (253:35) ON   [16384] (*1)
> #libdm-deptree.c:1444         Thin pool transaction id: 3 status: 3
> 32/2560 1679/160928 - rw discard_passdown.
> #ioctl/libdm-iface.c:1795         dm message   (253:35) OF  delete 1
> [16384] (*1)
> #ioctl/libdm-iface.c:1795         dm message   (253:35) OF
> set_transaction_id 3 4 [16384] (*1)
> #ioctl/libdm-iface.c:1795         dm status   (253:35) ON   [16384] (*1)
> #libdm-deptree.c:1444         Thin pool transaction id: 4 status: 4
> 18/2560 0/160928 - rw discard_passdown.
> #activate/dev_manager.c:3127         Creating CLEAN tree for thingroup.
> #activate/dev_manager.c:1789         Getting device info for
> testgroup-thingroup [LVM-qg1G3n02Kkjm0KKnhGhzP7JfoeGiiemlrsYfP0Ti5MCUiiPOWhTxoyRlvclhd3EH-pool]
> #ioctl/libdm-iface.c:1795         dm info LVM-qg1G3n02Kkjm0KKnhGhzP7JfoeGiiemlrsYfP0Ti5MCUiiPOWhTxoyRlvclhd3EH-pool
> OF   [16384] (*1)
> #ioctl/libdm-iface.c:1795         dm deps   (253:36) OF   [16384] (*1)
> #ioctl/libdm-iface.c:1795         dm deps   (253:35) OF   [16384] (*1)
> #ioctl/libdm-iface.c:1795         dm deps   (253:34) OF   [16384] (*1)
> #ioctl/libdm-iface.c:1795         dm deps   (253:33) OF   [16384] (*1)

Looking above it is clear that discard_passdown _is_ enabled.

I'll have to defer to the lvm2 developers, I thought we added explicit
logging when lvm2 issues discards (as part of lvremove, etc) -- Peter,
and/or Alasdair?

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