topic Re: Fixed LUN size and constant Volume growth in ONTAP Discussions

Fixed LUN size and constant Volume growth

areti — Wed, 04 Jun 2025 10:13:43 GMT

Hello!

I am observing a strange situation.

I created volume 6.1 Gb Thick Provision with Autogrow Mode enable and LUN fixed size 6GB Thick Provision for the VMs to guarantee space for the VMs.

The LUN size does not change, but the volume on which this LUN is located is growing in size all the time.

On the LUN, virtual machines occupy only 720 Gb out of 6 TB (see attach file LUN.jpg).

But volume already takes 6.81 TB and only 160 Gb is free on it! (see attach file Volume1.jpg)

It means that the metadata is 650 GB!

720 Gb users data and 650 GB metadata - this is more than strange!!!

How can I predict volume growth? Can you accurately calculate the maximum size the volume will have if it has a 6TB Thick Provision LUN?

Re: Fixed LUN size and constant Volume growth

Ontapforrum — Fri, 17 Sep 2021 09:49:37 GMT

By default, fractional reserve is set to 100%. Volume has to account for 'lun + fractional reserve + snapshot'. Turn off fractional reserve on the volume and check if there are any snapshots, delete them and check the volume space.

What is the proper configuration for a volume containing a LUN?
https://kb.netapp.com/Advice_and_Troubleshooting/Data_Storage_Software/ONTAP_OS/What_is_the_proper_configuration_for_a_volume_containing_a_LUN%3F

Best practice is to go THIN, you get back lot of valuable space back. From performance perspective, THICK or THIN makes no difference to NetApp.

Re: Fixed LUN size and constant Volume growth

NetApp_SR — Fri, 17 Sep 2021 10:10:36 GMT

Check the fractional reserve setting on the volume. If it is set to 100% it means that 100% of the space required for overwrites is reserved. The volume will have space reserved so that there will always be space to take a volume snapshot.

Ontap Fractional reserve is being allocated even though no snapshots exist
https://kb.netapp.com/Advice_and_Troubleshooting%2FData_Storage_Software%2FONTAP_OS%2FOntap_Fractional_reserve_is_being_allocated_even_though_no_snapshots_exist