[dm-devel] I/O block when removing thin device on the same pool

[Dennis Yang]

Hi,

I had noticed that I/O requests to one thin device will be blocked
when the other thin device is being deleting. The root cause of this
is that to delete a thin device will eventually call dm_btree_del()
which is a slow function and can block. This means that the device
deleting process will need to hold the pool lock for a very long time
to wait for this function to delete the whole data mapping subtree.
Since I/O to the devices on the same pool needs to held the same pool
lock to lookup/insert/delete data mapping, all I/O will be blocked
until the delete process finish.

For now, I have to discard all the mappings of a thin device before
deleting it to prevent I/O from being blocked. Since these discard
requests not only take lots of time to finish but hurt the pool I/O
throughput, I am still looking for other better solutions to fix this
issue.

I think the main problem is still the big pool lock in dm-thin which
hurts both the scalability and performance of. I am wondering if there
is any plan on improving this or any better fix for the I/O block
problem.

Any help would be grateful.
Dennis