So I was looking through a datastore and was Storage vMotioning a few VMs and it was taking ages. After digging around I noticed that the datastore was mapped across 2 extents.
There should be no reason for this, esp when using VMFS5. After digging around in Compellent by looking at the LUN ID and the naa ID, I found out that both extents were on different Compellent SANs! I have no idea how this came about, but the only way I know of to fix it is to actually, Storage vMotion everything off it to a new volume/datastore, unmount the old datastore and detach the extents.
Somebody must have done that in error when trying to extend datastore space. The second extent was labelled as belonging to a totally different area/volume. So how it exactly came about I have no idea.
But it is my job to fix it!
Also VMware did a good blog post on extents and the many misconceptions on then:
Misconception 2 is interesting as I was discussing with someone recently too, I have heard it a few times.
Nice to get it cleared up.