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

[Zdenek Kabelac]

Dne 20.1.2016 v 11:05 Dennis Yang napsal(a):
> 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.

Hi

What is your use case.

You may possibly split the load between several thin-pools ?

Current design is not targeted to simultaneously maintain very large number
of active thin-volumes within a single thin-pool.


Zdenek