[libvirt] [PATCH 0/1] Multipath pool support

[Dave Allan]

Daniel P. Berrange wrote:
> On Tue, Jul 21, 2009 at 05:25:36PM -0400, David Allan wrote:
>> The following patch implements multipath pool support.  It's very 
>> basic functionality, consisting of creating a pool that contains 
>> all the multipath devices on the host.  That will cover the common 
>> case of users who just want to discover all the available multipath
>> devices and assign them to guests.
>> 
>> It doesn't currently allow configuration of multipathing, so for 
>> now setting the multipath configuration will have to continue to be
>>  done as part of the host system build.
>> 
>> Example XML to create the pool is:
>> 
>> <pool type="mpath"> <name>mpath</name> <target> 
>> <path>/dev/mapper</path> </target> </pool>
> 
> So this is in essence a 'singleton' pool, since there's only really 
> one of them per host. There is also no quanity of storage associated 
> with a mpath pool - it is simply dealing with volumes from other 
> pools. This falls into the same conceptual bucket as things like 
> DM-RAID, MD-RAID and even loopback device management.

It is a singleton pool, in that there is only one dm instance per host. 
  With regard to capacity, the dm devices have capacity, and their 
constituent devices could be members of other pools.  Can you elaborate 
on what you see as the implications of those points?

> The question I've never been able to satisfactorily answer myself is
>  whether these things(mpath,raid,loopback) should be living in the 
> storage pool APIs, or in the host device APIs.
> 
> I also wonder people determine the assoication between the volumes in
>  the mpath pool, and the volumes for each corresponding path. eg, how
>  do they determine that /dev/mapper/dm-4 multipath device is 
> associated with devices from the SCSI storage pool 'xyz'. The storage
>  volume APIs & XML format don't really have a way to express this 
> relationship.

It's not difficult to query to find out what devices are parents of a 
given device, but what is the use case for finding out the pools of the 
parent devices?

> The host device APIs have a much more limited set of operations 
> (list, create, delete) but this may well be all that's needed for 
> things like raid/mpath/loopback devices, and with its XML format 
> being capability based we could add a multipath capability under 
> which we list the constituent paths of each device.

If we decide to implement creation and destruction of multipath devices, 
I would think the node device APIs would be the place to do it.

> Now, if my understanding is correct, then if multipath is active it 
> should automatically create multipath devices for each unique LUN on 
> a storage array. DM does SCSI queries to determine which block 
> devices are paths to the same underlying LUN.

That's basically correct, and the administrator can configure which 
devices have multipath devices created.

> Taking a simple iSCSI storage pool
> 
> <pool type='iscsi'> <name>virtimages</name> <source> <host 
> name="iscsi.example.com"/> <device path="demo-target"/> </source> 
> <target> <path>/dev/disk/by-path</path> </target> </pool>
> 
> this example would show you each individual block device, generating
>  paths under /dev/disk/by-path.
> 
> Now, we decide we want to make use of multipath for this particular 
> pool. We should be able to just change the target path, to point to 
> /dev/mpath,
> 
> <pool type='iscsi'> <name>virtimages</name> <source> <host 
> name="iscsi.example.com"/> <device path="demo-target"/> </source> 
> <target> <path>/dev/mpath</path> </target> </pool>
> 
> and have it give us back the unique multipath enabled LUNs, instead 
> of each individual block device.

The problem with this approach is that dm devices are not SCSI devices, 
so putting them in a SCSI pool seems wrong.  iSCSI pools have always 
contained volumes which are iSCSI block devices, directory pools have 
always had volumes which are files.  We shouldn't break that assumption 
unless we have a good reason.  It's not impossible to do what you 
describe, but I don't understand why it's a benefit.

>> The target element is ignored, as it is by the disk pool, but the 
>> config code rejects the XML if it does not exist.  That behavior 
>> should obviously be cleaned up, but I think that should be done in 
>> a separate patch, as it's really a bug in the config code, not 
>> related to the addition of the new pool type.
> 
> The target element is not ignored by the disk pool. This is used to 
> form the stable device paths via virStorageBackendStablePath() for 
> all block device based pools.

Hmm--on my system the path I specify shows up in the pool XML, but is
unused as far as I can tell.  I can hand it something totally bogus and
it doesn't complain.  I think your next point is very good, though, so
I'll make the target element meaningful in the multipath case and we can
investigate the disk behavior separately.

> Even for multipath, there are 3 possible directories under which you 
> can see LUNs, with varying plus & minuses for naming 
> stability/uniqueness across hosts.

That's a good point.  I'll make the target element meaningful.

Dave