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Re: [lvm-devel] dmevent plugin

Zdenek Kabelac <zkabelac redhat com> writes:

> Dne 23.4.2013 20:30, M. Mohan Kumar napsal(a):
>> Hello,
Zdenek, thanks for the response.

>> As per my understanding lvm2 does not support other applications to
>> register with dmevent daemon to handle events generated in interested
> there was at least one non-lvm plugin from dmraid project.
> dmeventd test is based on 'dmsetup status' - and the plugin is executing
> lvm code only in case something triggers this action (i.e. fill over 
> threshold) - but since lvm library is not threadsafe - only one plugin
> could execute lvm code within mutex.
Yes, the plugins execute interested commands/actions only when a
specific event is generated, in thin pool case when it reaches the
>> devices. Function monitor_dev_for_events() in lib/activate/activate.c
>> registers with the default events library (if its available).
>> When a dm-thinpool is created from SAN[1] and typically multiple hosts have
>> visibility to the same dm-thinpool. In this case there are chances that
>> more than one dm-eventd thin plugin will be registered to monitor
>> it. When dm-thinpool reaches low water mark threshold, these plugins try
>> to resize the thin-pool causing simultaneous block allocate requests and
>> dm-thin-pool module may not be capable to handle this situation.
> dmeventd plugin for lvm thin pool is essentially calling command:
> 'lvextend --usepolicies'
> when dmsetup status is report values above threashold.
When I last checked 'lvextend' did not have --usepolicy option
enabled. Only lvconvert has this option, so even with thin plugin the
pool is not resized when it reached threshold. I have to patch thin
plugin to run lvresize command to increase the pool size and it worked.
>> There could be specific applications using this dm-thinpool in a SAN
>> environment and wanting to handle the dm-thinpool specific events by
>> themselves.
> Code from thin plugin might be easily duplicated and modified.
> But since you are repeatedly mentioning  dm-thinpool - it seems like
> you do not plan to lvm2 thin support here and you want to create
> thin pool natively via dmsetup commands ?
No, lvcreate will be used to create a thin pool.

>> By using GlusterFS and BD xlator[2] we are planning to use dm-thinpool
>> to provide thin provisioned storage for hosting the VM images. This pool
>> could come from a SAN box but there will be a 1:1 mapping between
>> Glusterfs server and dm-thinpool. It provides controlled clustered
>> access to dm-thinpool when various Glusterfs clients access the same
>> dm-thinpool through single GlusterFS server. The idea is to extend the
>> dm-thinpool(when low water mark threshold reached) from respective
>> GlusterFS server so that there is only one entity controlling that
>> dm-thinpool in a clustered environment.
> There is work-in-progress for clustered usage of thin pools.
Could you give some pointers to this work, like whats the current
status, target date et?

>> Is there any way to avoid this default registration? So that only
> All LVs have monitoring feature so you could always disable monitoring
> for particular LV - is that what you mean ?
Yeah, as a prerequisite we can ask users to disable monitoring
interested thin pool. But user has to do that in all nodes that are
connected to SAN, so I expected unless explicitly asked this default
registration should not happen.

>> specific applicationcode can register itself with interested dm-thinpool
>> and take the necessary action when low watermark threshold is reached?
>> [1] Basic SAN without supporting thin provisioning
>> [2] http://review.gluster.com/#/c/4714/
>> --
>> lvm-devel mailing list
>> lvm-devel redhat com
>> https://www.redhat.com/mailman/listinfo/lvm-devel
> Zdenek

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