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

[Dennis Yang]

Hi,

Thanks for replying.

In my use case, I will have couples of 50TB thin devices (less than
20) with different services running on them. Also, I will take hourly
read-only snapshot on some of these thin devices and prevent one
single thin device from having over 1024 snapshots by deleting the
oldest snapshot when it has to. During the deletion of a snapshot or a
thin device, I/O gets blocked and some of the latency-sensitive
services stop and return error code.

I am aware of that the current design is not suitable for me to put
all the thin devices on the same pool. However, It seems that this I/O
blocking problem will still exist even when I have only one thin
device and couple of read-only snapshots of it on the same pool.

Dennis

2016-01-20 19:27 GMT+08:00 Zdenek Kabelac <zkabelac at redhat.com>:
> 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
>
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