File is larger than the maximum size supported by datastore '

I got this error when trying to take a snapshot of a VM with a 1TB Thin Provisioned HDD:



Create virtual machine snapshot VM NAME


File <unspecified filename> is larger than the maximum size supported by datastore '<unspecified datastore>


The used space was only around 9GB.

The problem here is VMFS block size. When I formatted the VMFS LUN I did it using 4MB block size (seeing as how my largest VMDK file would be 1TB).

However (and I should have known this), the file size was just over 1TB (1,073,742,000.00KB). Had I created it with 900GB I probably would have been ok.

The solution? I moved the VM to a VMFS datastore that had 8MB block size (this allows for 2TB files).

Depending on who you talk to people will say that its a waste of space. I dont think it is. Yes, you do have several files attached to a VM that take only 20-30k (but because of the VMFS block size are actually taking 8MB for each file) but in the long run if you need a large VMDK file you need to do this. You lose more space when creating the LUN (my 1.5TB LUN is actually 1.36TB once I format) than you would by selecting a bigger block size.

For those of you not familiar with block size, you have a few options:


• 1MB block size – 256GB maximum file size
• 2MB block size – 512GB maximum file size
• 4MB block size – 1024GB maximum file size
• 8MB block size – 2048GB maximum file size

The block size determines the maximum size for any one file on the VMFS datastore. So, if you think you will need a VM with a hard drive size of 256GB you will need to have a 1MB block size, 512GB you will need 2MB, so on and so forth. Do not get this confused with the files that are INSIDE the VM (ie, windows OS files, etc). The block size is for the VMDK file and the files associated with the VM (config files, snapshot files, etc.)