[Ovirt-devel] iscsi storage creation

[Justin Clacherty]

Scott Seago wrote:
> Ian may be able to provide a more complete answer, but there are a 
> couple things  that are probably going on with the 39GB vs. 40GB size. 
> First of all, if you've got a 40GB iSCSI LUN and you want to create an 
> oVirt storage volume under it, we're creating a Volume Group on the 
> LUN and creating Logical Volumes as storage volumes via libvirt, so 
> the size available for a single LVM volume on the lun will be slightly 
> smaller due to the LVM overhead.
>
> The second issue is that while libvirt (and the oVirt DB) deal with 
> storage sizes internally in KB, we're using GB in the UI at the moment 
> and converting on the back end. This means for now you can only deal 
> with an integral number of gigabytes. We need to do some re-thinking 
> on how we handle storage and memory units -- in the above case you may 
> have been able to create a 39.6GB volume (or some other value between 
> 39 and 40 GB).
I figured it was something like that, I was just checking before I asked 
if gigabytes was the most appropriate value for the UI.  Perhaps there 
should be a way to say "use the remaining free space".

>   If you really want to use the whole 40GB lun for a single VM you 
> don't need to create an LVM volume -- just use the iscsi lun (shown in 
> the UI as an iscsi storage volume).
hmmm...when I tried this the first time it didn't work for me so I just 
assumed you had to create the LVM volume.  I'll try it again without 
doing that.

My plan was to create a LUN per VM because as I was unsure if multiple 
nodes could use the same LUN.  My assumption was that the node mounted 
the iSCSI LUN and exported it as a disk to the VM.  In the case of 
volumes under the LUN, the node would mount the LUN and export the 
volume as a disk to the VM.  If you have two volumes on the LUN used for 
two different VMs it's conceivable they would be running on seperate 
nodes so two nodes would be mounting the same iSCSI LUN which is 
generally a bad thing.  I guess it should be ok as each node is only 
writing to it's own volume on the LUN and not touching the partition 
table on the LUN itself.

Hopefully that wasn't too confusing :)  Am I justified in worrying about 
two nodes mounting the same LUN?

Cheers,
Justin.