A customer was trying to back up a VM with a 20GB OS vmdk and a 1TB vmdk on two different VMFS datastores over iSCSI with a 1MB block size and 8MB data size respectively, and received the error below when trying to snapshot the VM:
File <unspecified filename> is larger than the maximum size supported by datastore <unspecified datastore>
To check the block size of a VMFS datastore go to the VI Client, with vSphere:
1.Select an ESX host that contains the datastore.
2.Click the Configuration tab.
4.Click on the datastore.
5.The block size is identified in the Details window under the Formatting subheading
The basic block size to virtual disk size is below ( in vSphere, which is DIFFERENT than ESX 3.X )
|Block Size||VMDK Size|
To fix the issue, the OS VMDK residing on the 1MB block size VMFS datastore must be migrated to a datastore with a 8MB block size (It is a best practice is to have all VMFS datastores with the same block size to avoid such issues.)
Note this issue does not apply to NFS datastores.