[Users] *SAN LUNs* thin-provisioning : Reclaiming block device storage

[Nicolas Ecarnot]

[This post is *not* about *VM* thin-provisioning]

Hi,

Our setup is made of an Equalogic SAN (PS6100) connected via a dedicated 
iSCSI network, an oVirt 3.3.0-4.el6 manager and CentOS nodes.
This is working fine (thank you all).

I usually create a LUN for the master storage domain, configure in 
thin-provisioned mode on the SAN side, and fill it with VMs until it 
reaches some 90%.
Then I create another LUN the same way, add it as an additional storage 
domain, move some VM on it from the first domain and it's OK.

The issue there is I have no mean to reclaim any freed storage on the 
master storage LUN. So the question (not *master* domain specific - any 
storage domain is concerned) is how to reclaim space on these domains.

I began to have a closer look at how a storage domain is setup (PV, VG, 
LVs). I also found some hints about some mount option (unmap) that could 
be passed when mounting ext4 filesystems.
But I guess the issue is way more complex, as LVM may add a layer 
between ext4 and the block device.
How is the LVM setup working? I saw there was one LV by disk (and some 
metadata LVs), in one big VG in one big PV. Is the entire disk space 
allocated for the PV, or is only a partial of it is used first, then an 
extension (growing) is made each time some more space is needed by 
adding VMs?

I have the opportunity to test and play with a test LUN in a test oVirt, 
so I fear no crash. If the only answer is "Unavailable feature", it's 
ok, but I just have to know.

-- 
Nicolas Ecarnot