Regarding Space release after deleting contents from LUN and CIFS/NFS

[praveen121b]

Hi All,

       Got a doublt please help me understand the concept.

When I delete all the contents from LUN NetApp Storage still shows the LUN as full since the blocks are being held up by snapshots and we need Snapdrive to release the space.

But in the same way if i delete data from NFS or CIFS shares it gets reflected the released/deleted space in NetApp without Snapdrive, but as per my knowledge here also snapshots have the data backedup.

Please explain this concept.

Praveen. 

[mikkisse]

Hello!

If you using thick provision technology, your LUN will take all space, what you specified in LUN size and doesn't matter have you data or not.

If you using thin provision technology, your LUN will take from the storage only real used space. But, if you delete something from thin LUN, this doesn't give you free space at storage, because NetApp haven't information about real used space in your system which uses LUN. I heared about some features that can send to storage information about real used LUN size, but i don't use it. 

Be careful with thin LUNs, there is no good way, if you overcommit space at storage.

[praveen121b]

Hi Mikkisse,

           Thanks for your reply.

You mean to say if my LUN is thick provisioned then once the data is deleted from LUN it will reflect in NetApp storage as free space without snapdrive?

One more query, I do not know wheather it is related to above topic or not, lets say I have CIFS Share and I have snapshot enabled on it. Once I delete the data from CIFS share from my Windows Hosts the snapshot still holds the blocks on storage and show it as full or it will release the block?

[mikkisse]

If you delete data from lun, you does not free up your storage, you just free up your lun. Thick luns are space guaranted and they are reserve at storage size equal of space, which you specified at lun creation step.

[mikkisse]

If you delete file from CIFS share, you will free up your volume, but this file will kept in your snapshot. Snapshot takes space from your volume. 

[praveen121b]

Let's say I have 100GB LUN mapped to my Windows server with NetApp snapshot enabled and I have used total 100GB.

After deleting 50GB out of 100GB from host side could I put 50GB data again, or does snapshot still holds that 50GB blocks and do not allow me to use freed 50GB.

[mikkisse]

Ok, for example.

Thick lun with enabled snapshots:

You have Volume 200GB, free 200GB.

You create thick LUN with 100GB Size. Free size at volume: 100GB, because thick LUN reserve size.

You map LUN to windows server and put 50GB file on it. Now you have 50GB free at Volume, because 100GB LUN + 50GB Snapshot.

You delete 50GB file from system. Now you have 100GB free space at Windows server, but you still have 50GB free at your NetApp volume.

You delete Snapshot. Now you have 100GB free at Volume.

Thin lun with enabled snapshots:

You have Volume 200GB, free 200GB.

You create thin LUN with 100GB Size. Free size at volume: 200GB, because thin LUN does not reserve space.

You map LUN to windows server and put 50GB file on it. Now you have 100GB free at Volume, because 50GB used space at LUN + 50GB Snapshot.

You delete 50GB file from system. Now you have 100GB free space at Windows server, but you still have 100GB free at your NetApp volume, because deleting files from thin LUN does not free up your storage.

You delete Snapshot. Now you have 150GB free at Volume.

But, as i know, there is some SCSI technologies, that can help you to free some disk space from storage, using thin LUN.

[praveen121b]

Question on Thick Provisioned LUN:

1) After deleting 50GB from host side how much data can I put again, 50GB or 100GB, if I am not deleting the snapshot on storage?

2) Do I need to create volume double the size of LUN to utilize the full capacity of the LUN as same size of space would be used by snapshot? 

[mikkisse · Accepted solution]

Question on Thick Provisioned LUN:

1) After deleting 50GB from host side how much data can I put again, 50GB or 100GB, if I am not deleting the snapshot on storage?

You can put 100GB of data on your 100GB thick Lun. You can create additional 100GB Lun in your 200GB Volume, but there will not be space for snapshots.

2) Do I need to create volume double the size of LUN to utilize the full capacity of the LUN as same size of space would be used by snapshot? 

No, you don't need. Percent of free space at volume depends by your snapshot policy, data changerate etc. In my practice there are around 20% of volume, when snapshot technology used. By the way you can setup snapshot autodelete policies, when (for example) volume space near 5%. 

[praveen121b]

Thanks for all your replys and your patience.

One last question regarding CIFS shares, as you said even if I delete data from CIFS share it will be still available in snapshot.

Let's say I have 100GB share size on a thick provisioned volume and 5GB snapshot space reserved. Lets say I put 80GB data in that share and a snapshot I  have deleted 80GB data from the share then how would my snapshot keep the 80GB data in 5GB snap reserve space?

If it is through pointers to the disk block then still my 75GB is held up by snapshot and it's not free. Only if I delete that specific snapshot I would be able to put another fresh 80GB data, Am I right?