topic Volume used and LUNS in ONTAP Discussions

Volume used and LUNS

jblake — Thu, 05 Jun 2025 06:45:58 GMT

I’m a bit confused with the space some volumes are reporting.

I have

710Gb Volume (Fractional Reserve 20% / Snap Reserve 0)
The volume contains a 460Gb lun of which 452GB is being used.
My snapshots report 117 Gb used

Therefore I would expect space used by the volume to be

Volume – Lun used size – FractionalReserve - SnapShot Used

710 – 452 – 142 – 117 = -1

The volume actually reports 669Gb used and 41Gb free?

So from the figures above resulting in -1, I'm assuming that I'm using some of my Fractional Reserve for snapshots, however each snapshot is only about 15gb so I would expect the volume available space to get to 0 before Fractional Reserve starts being used. I can't see how you can see how much of the Fractiona Reserve is actually being use.

I’m sure I’m doing something wrong but just can’t see it.

Thanks

Volume used and LUNS

peter_lehmann — Wed, 14 Sep 2011 11:23:17 GMT

The Fractional reserve is a setting for how important it is for the snapshot of this LUN to be able to be created. Set it to 20% and it will "reserve" 20% of the size of the LUN to "guarantee" the creation of a snapshot that needs 92G of space. If you create a snapshot that nedds more space then 92G, it will fail...

Try this math:

vol = 720G

LUN = 460G

free1 = 260G

snap used = 117G

free2 = 143G

Fractional = 92G (20% of 460G)

free3 = 51G

space used = 460 + 117 + 92 = 669

Hope this helps,

Peter

Volume used and LUNS

jblake — Wed, 14 Sep 2011 11:33:54 GMT

Brilliant I always thought that Fractional Reserve was based on volume size as its set as volume level not on Lun size.