topic clarification needed about usable/total size/fractional reserve in ONTAP Discussions

clarification needed about usable/total size/fractional reserve

allblacknz — Thu, 05 Jun 2025 07:16:55 GMT

Hi there,

Can someone please clarify this for us?

When I create a 1Tb volume, set for total size, and specify 20% for snapshot I am left with 800 Gb capacity.
My fractional reserve is set to one hundred percent. Does this mean that my reserve will kick in when files are created

and the 800Gb will be shared between data and fractional reserve data and that, in theory at least, only have 400Gb available for data.

Is this a correct assumption?

On the other hand, when I create a usable volume of 1Tb and specify 20% for snapshots I have a total size of 1.2Tb.
Where does my fractional reserve fit in here? It is said to 100 % as well.
I assume that I will always have 1Tb for data and that I need to ensure that my aggregate has another 1Tb available to grow for fractional reserve.
Or am I wrong and the fractional reserve is part of my 1Tb?
And this would mean that the difference between total and usable volumes comes down whether the snapshot is included in the specified size or gets added to specified size

Hope it all makes sense.

Re: clarification needed about usable/total size/fractional reserve

allblacknz — Mon, 22 Mar 2010 06:02:45 GMT

As I am going through some reading it seems that fractional reserve is only used when making use of LUNs.

I was not aware of that when writing the above post 🙂

I guess this answers part of my question above and to me it says it will the fractional reserve always comes from the capacity that is availble to you, correct?

Also, if it is only used when creating LUNs and my volumes don't make use of LUNs, should I just leave the fractional reserve at its default or change it?
My understanding is that it does not do anything when no LUNs are in use

Re: clarification needed about usable/total size/fractional reserve

radek_kubka — Mon, 22 Mar 2010 12:25:24 GMT

My understanding is that it does not do anything when no LUNs are in use

That is correct. You need LUNs in the volume AND at least one snapshot to see fractional reserve in action.

Have a look at Chris's blog post for a really thorough explanation:

http://communities.netapp.com/groups/chris-kranz-hardware-pro/blog/2009/03/05/fractional-reservation--lun-overwrite

Regards,

Radek