Fractional reserve behaviour on iSCSI LUN

I feel I've been through this a 100 times - but I can't answer my technicians question clearly so I need some help:

- My tech created a simple iSCSI LUN from his Windows 2008 server to on our filer (using SnapDrive). The LUN is 20Gb and only has 8Gb used. The volume however is 40Gb and complaining it's 100% full. Snapshot reserve is 0% as we dont need snapshots of this volume. Basically this disk is appearing as a 2nd hard-disk to our server & we use it for storing IT installation files etc (nothing important).

My understanding is that by default FR is set to 100% - so if he wants a 20Gb disk/LUN the volume needs to be 40Gb?

But what I don't understand is:

- if the data on the LUN is not changing, or nothing weird is (like disk defrags etc) has taken place, then how can the volume be full?

- and if it's not a critical LUN running a system (like Exchange or SQL), is it possible to simply set the FR on the volume to 0% and size it exactly the same size as the LUN? There is no danger if we fill the disk/LUN - we'll simply manually increase it if this occurs. Or am I going to cause another problem if I do this?

Basically for LUNs such as this I don't want to over-allocate unnecessary agg' space. If we need a 20Gb iSCSI disk for storing un-important IT data, I only wish to use 20Gb on my filer (not 40Gb).



Re: Fractional reserve behaviour on iSCSI LUN

Hi Marc,

First of all, please have a look at Chris' post which discusses the topic really in-depth & explains what fractional reserve is and what is not:

The bottom line in your particular case would probably be: set FR to 0% and have the volume slightly bigger than your LUN.