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

[Itamar Heim]

On 11/05/2013 09:49 AM, Nicolas Ecarnot wrote:
> [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.
>

other than the LV for the masterfs, there is a disk for each "volume" 
(disk or snapshot).
a snapshot is copy-on-write and usually thinly-provisioned.
a disk can be pre-allocated or thinly provisioned.
a thinly provisioned is done by creating a small LV, then extending it 
as-needed while the VM is running.