Dne 22.1.2016 v 14:38 Lars Ellenberg napsal(a):
On Thu, Jan 21, 2016 at 02:44:06PM -0500, Mike Snitzer wrote:
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.
Just so I'm aware: which kernel are you using?
dm_pool_delete_thin_device() takes pmd->root_lock so yes it is very
coarse-grained; especially when you consider concurrent IO to another
thin device from the same pool will call interfaces, like
dm_thin_find_block(), which also take the same pmd->root_lock.
We have seen lvremove of thin snapshots sometimes minutes,
even ~20 minutes before.
So that means blocking IO to other devices in that pool
(e.g. the typically currently in-use "origin") for minutes.
That was, iirc, with ~10 TB origin, mostly allocated,
tens of "rotating" snapshots, 64k chunk size,
and considerable random write change rate on the origin.
I'd like to propose a different approach for lvremove of thin devices
(using "made up terms" instead of the correct device mapper vocabulary,
because I'm lazy):
on lvremove of a thin device, take all the locks you need,
even if that implies blocking IO to other devices,
BUT
then don't do all the "delete" right there while holding those
locks, but convert the device into a "i-am-currently-removing-myself"
target, and release all the locks. That should be fast (enough).
Then this "i-am-currently-removing-myself" target would have its .open()
return some error, so it cannot even be opened anymore (or something
with similar effect), start some kernel thread that does the actual
"wipe" and "unref/unmap" from the tree and all that stuff "in the
background", using much finer granular temporary locking for each
processed region.
If that then takes 20 minutes, someone may still care, but at least it
does not block IO to the other active devices in the pool.
Or is something like this already going on?
Hi
Please always specify kernel in-use.
Eventually retry with last officially released one (e.g. 4.4)
There were number of improvements in speed of discard.
Also - you may try to use thin-pool with '--discards nopassdown'
(or even ignore) in case TRIM is very limiting factor
(with impacting free space in thin-pool for 'ignore' one)
Zdenek
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