Thanks for getting back to me again Mike.
On 08/11/15 17:35, Mike Snitzer wrote:
On Tue, Aug 11 2015 at 4:07am -0400,
vaLentin chernoZemski <valentin@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.
Understood.
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.
passdown was already set in lvm.conf. I forgot to mention that:
Here is a snip:
grep passdown /etc/lvm/lvm.conf
# Select one of "ignore", "nopassdown", "passdown"
thin_pool_discards = "passdown"
dmsetup table yields the following for testgroup:
testgroup-thingroup: 0 20598784 linear 253:35 0
testgroup-thingroup-tpool: 0 20598784 thin-pool 253:33 253:34 128 0 1
skip_block_zeroing
testgroup-thingroup_tdata: 0 20598784 linear 7:2 24576
testgroup-thingroup_tmeta: 0 20480 linear 7:2 20623360
testgroup-testvol: 0 409600 thin 253:35 1
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.
Got that.
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?
Once again I want to mention that discard issued by deleting file in a
mount point reaches blockdev and sparse file shrinks in size just as
expected.
However the problem is that if I am using lvremove even there are no
parent snapshots, size of sparse file remains unchanged.
Let me know if there is anything else I can try or if I can pass you
additional information.
Still did not have the chance to test all latest versions of kernel dm,
dm libs and lvm.
vaLentin
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