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[Ovirt-devel] Modeling LVM storage



sseago and I (and variously, other folks) had a somewhat longish conversation on
IRC today about carving up storage with LVM.  This is the second time we've
beaten this horse, so hopefully we are somewhat OK now.  The basic idea is that,
given an iSCSI LUN (and SCSI and FC LUNs in the future), we want to either:

1)  Assign the entire LUN to a guest (this is the way that ovirt works right now)
2)  Carve up the LUN using LVM, and then hand out individual logical volumes to
guests.

Libvirt handles this case sort of implicitly; that is, you first build a storage
pool for iscsi, and find all of the volumes on it.  Then you build an LVM
storage pool out of the iSCSI LUN, and then you can create volumes on top of that.

We can follow the same sort of model for ovirt.  That is, we currently have a
StoragePool defined in the model, which contains 0 or more StorageVolumes.  So
the idea is that we now add a new type of StoragePool, LVM, which consists of
one or more iSCSI StorageVolumes, and on top of that, you have a new type of
StorageVolume, LVM, which is what you eventually assign to guests.

Note that the above model should eventually support binding multiple iSCSI LUNs
into a single LVMPool, although we won't expose that functionality to the user
for the moment.

Once those model changes are in place, then we just need the backend taskomatic
code to handle this (I've worked on that somewhat today, so I have a pretty good
handle on what it will require), and the frontend WUI pieces.  This latter can
get somewhat complex, so for the time being, we will have pretty rudimentary
support.  That is, during VM creation time, we'll allow a user to either choose
an existing whole LUN for the guest, choose an already existing (but not in use)
LVM volume for the guest, or carve out a new LVM volume for the guest (subject
to physical disk space and user quota, naturally).  Later on we'll add more
complicated handling of LVM, such as deletion, resizing, etc. etc.

-- 
Chris Lalancette


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