[linux-lvm] [BUG?] OOM executing blkdiscard on a thin lv

[Mike Snitzer]

On Wed, Nov 26 2014 at  2:54am -0500,
Qu Wenruo <quwenruo at cn.fujitsu.com> wrote:

> Hi,
> 
> [Bug]
> When I try to discard the whole thin lv using blkdiscard, an OOM
> occurs and I lost my ssh connection.
> TTY login also fails to start, although kernel and network still
> looks alive but no way to login into the system.
> Have to reboot.
> 
> Not sure if this is a bug in blkdiscard or lvm.

It is a bug in the kernel's blkdev_issue_discard() for allowing too many
inflight bios at once.  We probably should add an upper bound for how
many inflight discard bios blkdev_issue_discard() will allow at any one
time.

Reason why so many discard bios are being issued is that the DM thin
device currently only allows a discard as large as the DM thin-pool's
configured blocksize.  So if you're using something like a 64K or
whatever for the thin-pool blocksize then divide your 1TB thin lv by
that value and you can quickly see why it takes so may discard bios to
cover the entire logical address space of the 1TB thin device.

Some good news is that Joe has been working on range (multi block)
discard support to allow for very large discards to be issued to DM thin
devices.  He is focusing on getting this working on dm-cache at the
moment but once that is complete bringing it to dm-thin is next.  It is
a stretch goal to have that land in time for 3.19 merge window (but it
is quickly approaching so it may need to wait for 3.20).

> [lvs output]
>   LV       VG           Attr       LSize  Pool     Origin Data% Move
> Log Cpy%Sync Convert
>   disk1    test_vg      Vwi-a-tz--  1.00t thinpool 0.77
>   thinpool test_vg      twi-a-tz-- 30.00g 26.28
> 
> [Reproducer]
> 1. Make a thin pool with 30G space.
> 2. Create 1T thin lv from the thin pool created in 1.
> 3. Do some operation to occupy some space in thin lv
> 4. Exec blkdiscard on the thin lv
> 
> [Related package version]
> kernel-3.17.3-200.fc20.x86_64
> lvm2-2.02.106-1.fc20.x86_64
> util-linux-2.24.2-1.fc20.x86_64
> 
> [How to discard thin lv]
> BTW, what's the proper method to discard all allocated space for a thin lv?
> Lvremove and lvcreate?!

Moving forward it'll be a new tool that Joe wrote called thin_trim:
https://github.com/jthornber/thin-provisioning-tools/commit/8e921580554ed91e84bb68ea32a8c2ea47ad6ff3

But that code hasn't been fully tested and included in the
device-mapper-persistent-data package yet.