Hi Steve,
On 3/8/07, Steven Mandrake srmanager@gmail.com wrote:
If I reduce fractional space reservations to a volume that contain luns, what happens to lun space reservations? Is there a way to throttle lun space reservations -- as far as I can tell it's just a check box in Filerview....
You can finetune space reservations in the CLI, your keyword is fractional_reserve. It is expressed as a percentage of the LUN-space.
The main question I have is --- does setting fractional space reservation affect my ability to mount snapshots. I assume that mounting snapshots does not take any space since it's all read-only.... or am i wrong in assuming this?
This depends (as does most in IT world ;) ). When you use a snapshot-backed LUN (which is a read/write copy of the LUN, based on a snapshot) then you need enough free, non-reserved space in your volume for this action. When you really just want to mount the snapshot, you do not need this (however some free space is always nice to have).
My assumption right now is that fractional space reserve is there if I ever need to mount a snapshot to be read/write --- am I dead wrong?
Fractional space reserve is actually an option that can be used if you do *NOT* want to reserve 100% of the space of a LUN, but only a fraction of this. Upside is that you can use less space in a volume where a LUN + snapshots reside. Downside is that there is a chance that you run out of writable space within your LUN, since all your original blocks are locked inside a snapshot, and you want to write more space than is available (reserved) inside the volume. Obviously, a LUN without writable space will cause minor havoc on the hosts that use the LUN, since they get SCSI-errors when they do not expect them, and there is a potential of dataloss.
This potential dataloss can however be avoided, using tools such as SnapDrive, that can attempt to recover free space inside your volume, and gracefully disconnect the LUN if things get too critical.
This is the short version of the story. For the complete version, contact me privately :)
Gr,
Nils