Hi there!
It was recently explained to me that a volume in shared storage (e.g. iSCSI, FC) must have a certain amount of headroom (i.e. empty disk space) so that guest snapshots can taken without any problems. I was told that this headroom was 1/3 of the usable capacity of the storage volume, so for volume with 10TB usable disk space we should treat it as actually being 6.6TB. This seems strange to me, that we should outright disregard 3.4TB of usable disk space--I would think that the overhead, if any, would be dependent on the virtual disk sizes of the guests located in the storage volume.
This is really puzzling me. Can some knowledgeable person please shed some light on this topic?