[Ovirt-devel] Modeling LVM storage

Steve Ofsthun sofsthun at virtualiron.com
Tue Sep 16 21:05:12 UTC 2008

Scott Seago wrote:
> Some additional clarification based on a conversation I just had with 
> pmyers.
> 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.

How would this "on-demand" scanning be done?

The only way to selectively scan volumes (with pvscan) is to update /etc/lvm/lvm.conf to restrict the desired list of volumes to scan.  This can be a pain to manage, particularly if the local node requires LVM access to boot.  If a newly scanned LUN contains volume information that conflicts with an existing volume group (say VolGroup00), the pvscan will error out due to conflicting volume group information (not so bad).  More importantly though, if the system is rebooted at this point, the reboot will fail to activate the real VolGroup00 possibly compromising the entire boot process (not so good).  Even if a boot volume isn't compromised, some active volume group is no longer accessible.

A safer alternative would be to modify pvscan to allow selective volume scanning without changing /etc/lvm/lvm.conf.  Is this a possible alternative?  Would a modified pvscan be acceptable in upstream lvm?

Another alternative, as Daniel mentioned, is to not allow a guest direct access to the entire LUN.


> 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?
> Scott
> 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 
>> 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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