[Ovirt-devel] Modeling LVM storage

Scott Seago sseago at redhat.com
Tue Sep 16 18:48:24 UTC 2008

Some additional clarification based on a conversation I just had with 

No changes from the basic model side of things, but we do want to 
explicitly exclude any LVM bits in LUNs assigned directly to VMs. So if 
a VM carves up one of its assigned LUNs into multiple LVs, oVirt doesn't 
care -- we only show the whole LUN assigned to a VM. The other thing is 
that we don't want to explicitly scan all unused LUNs for PVs/LVs -- 
rather we should do this on-demand.

If we think of it in terms of the "new VM" ui. There are several types 
of storage available, but selecting storage will basically be a two-step 
process. Initially the user will be presented with a list of NFS storage 
pools (i.e. NFS mounts), and iSCSI storage volumes (i.e. LUNs). If an 
NFS pool is chosen, the user can pick an existing unallocated NFS 
Storage Volume (image file) or create a new NFS Volume/image file with a 
specified size (within the user's quota) to be 1) added to the NFS pool; 
and 2) attached to the VM.

If an iSCSI volume (LUN) is chosen, there are a couple of options. First 
of all, the LUN will be scanned for PVs/LVs. These PVs/LVs will be 
inserted into the oVirt DB as LVM storage pools/volumes. The user will 
be able to choose an existing LV, create a new LV, or choose the whole 
LUN to attach to the VM. The latter "whole LUN" option only exists if 
there are not any LVs currently attached to other VMs. When a whole LUN 
is attached to a VM, the individual PVs/LVs are removed from the ovirt 
DB, since they can be overwritten at any time by the VM anyway.

The other aspect of this is what is seen on the Hardware Pool/Storage 
tab. Right now we see NFS/iSCSI pools in the top-level list, and 
NFS/iSSCSI storage volumes when we drill down. For NFS, we need to allow 
creation/deletion of NFS Volumes (image files). For iSCSI,  we need to 
allow the user to drill down from iSCSI Volumes (LUNs) to LVM Pools and 
Volumes (VGs and LVs). Two things here we need to work out:
1) drilling down will require refreshing the LVM Pool, as we only scan 
these on demand -- at any given time, oVirt will not have a complete 
picture of all LVM Pools for every LUN in the db
2) The UI is already fairly crowded with the Target -> LUN drilldown -- 
now we're adding another level. I'm guessing that at the moment, the 
best way to show this will be via another facebox popup, but we still 
need to work through the UI implications.

Perry -- does this cover most of what we discussed?


Chris Lalancette wrote:
> 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.

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