topic Re: Converting Thin LUN to Thick LUN in ONTAP Discussions

Converting Thin LUN to Thick LUN

arsalankhan — Wed, 04 Jun 2025 13:15:35 GMT

We’re converting thin-provisioned luns to thick provisioned and it seems that to do that, we need extra storage space assigned to the volumes containing the luns.
Example. a 450GB thin-provisioned lun requires around 620GB size volume to be able to convert it from thin to thick.

Each volume has one lun and when we convert the luns from thin to thick provisioning, it needed ~250GB added to the volume to be able to complete successfully.
Right now the extra storage is less because I had the user run space reclamation and resized the volume, but it is still more than what they need. All they need is 450GB lun and the volume needs about 620GB to host a 450GB lun.

Could someone please explain this to me.

LUN            Filer               LUN Size (GB)    Thick provisioned
UN19 nas5                460                   no
UN20 nas5                470                   no
UN24 nas5                450                   no
UN28 nas5                450                   no
UN29 nas5                450                   no
UN30 nas5                450                   no
UN31 nas5                450                   no

Thank you in advance.

Re: Converting Thin LUN to Thick LUN

DJ-Potsdam — Thu, 04 Oct 2018 13:56:37 GMT

Hi,

I think its explained well in this discussion:

https://community.netapp.com/t5/Data-ONTAP-Discussions/Converting-Thick-LUN-to-Thin-help-required/td-p/112042

Thanks

Re: Converting Thin LUN to Thick LUN

arsalankhan — Thu, 04 Oct 2018 22:16:51 GMT

Hi,

Thanks for reply.
Actually I’m looking for something which explains me other way around. I’m converting a thin Lun to thick Lun but it requires almost 250GB more in the volume then actual lun size to convert it back to thick. Why is it asking for more space in the volume then needed for Lun?

Re: Converting Thin LUN to Thick LUN

DJ-Potsdam — Fri, 05 Oct 2018 08:17:15 GMT

Hi,

Its something to do with Fractional reserve and snapshot reserve to gurantee space for the LUN when you convert to thick. NetApp handles thin and thick provisioning using reservers which gurantees space.

To understand more on fractional reserve you may have to read a bit since its not easy to describe for me over communities , sorry about that.I hope you should be able to understand it fast and justify the extra space 🙂

Some details from Doc Center for you http://docs.netapp.com/ontap-9/index.jsp?topic=%2Fcom.netapp.doc.dot-cm-sanag%2FGUID-226B1E60-4ACF-4A63-9AE5-09E8605D53A2.html

This FlexVol volume and file or LUN configuration combination provides the ability to use storage efficiency technologies and does not require you to actively monitor your free space, because sufficient space is allocated up front.

The following settings are required to configure a space-reserved file or LUN in a volume using thick provisioning:

Volume setting	Value
Guarantee	Volume
Fractional reserve	100
Snapshot reserve	Any
Snapshot autodelete	Optional
Autogrow	Optional; if enabled, aggregate free space must be actively monitored.

File or LUN setting	Value
Space reservation	Enabled

Thanks

Re: Converting Thin LUN to Thick LUN

DJ-Potsdam — Fri, 05 Oct 2018 08:20:11 GMT

https://community.netapp.com/t5/Cloud-Articles-and-Resources/From-Thick-to-Thin-and-Back-Again-A-Whirlwind-Tour-of-Thin-Provisioning-on/ta-p/86015

you can try to read through this, its a bit old but will give you can idea on whats happening.