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LUN Usage approaching 100%

I have a NetApp FAS2040 that is presenting two FC LUNs to an vSphere 4.1 Cluster.  One of the LUNs is currently at 97% utilization according to the VSC.  The LUN is 1.5TB on top of a 1TB volume with a-sis enabled.  The other utilization percentages are:

Datastore Usage - 75%

LUN Usage - 97%

Volume Usage - 68%

Aggregate Usage - 65%

I immediately have two concerns:

1.  Why doesn't the Datastore Usage % match LUN Usage % ?  I am consuming less space on the Datastore than the LUN as reported by the VSC.

2.  Do I need to worry if the LUN Usage % reaches 100%?  Will my Datastore be dismounted?


Re: LUN Usage approaching 100%


No need to worry. The reason that the LUN usage does not match the datastore usage is due to how VMFS handles blocks and reports on space. Lets start with an empty VMFS and a empty LUN. Lets say you build a new VM on there, fresh with no OS, just a 40GB VMDK file. Since you haven't really written anything to the LUN it still shows as empty while the datastore now has 40GB used. Lets say you now load an OS and fill that VM up. The Datastore and the LUN and now agreeing that 40GB has been used. Now say you deleted that VM, since VMFS is like most other file systems, it doesn't really delete the VM but rather just deletes the pointer to it. The data store will show 40GB being free but the LUN still thinks 40GB is used. No one told it the VM was delted.

So in your case it looks like at some point you moved or deleted some VMs which is why you LUN usage is higher than your datastore usage. No worry about that though, as long as you have room in your volume, all is well.


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Re: LUN Usage approaching 100%

So in other words over the long term a thin provisioned LUN will always approach 100%.

Suppose I wind up clearing up enough space on the Datastore so my utilization there is only 300GB.  Is it then posssible to shrink the LUN, or since the Filer thinks it is fully allocated is this not possible?

Re: LUN Usage approaching 100%

No, you cannot shrink the LUN but not for that reason. VMware has formatted that LUN with VMFS and shrinking the LUN would corrupt that. NOT good!  You can however always adjust the Volume size. Since the LUN is thin provisioned it's size does not matter, just the Volume size which should stay relatively small thanks to dedupe. You can always shrink that, releasing the space back to the Aggr and then optionally create a new VOL/LUN.


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