vSphere Block Sizes

One of the issues I have come across with vSphere users is related to their VMFS block sizes and the implications of the various choices. The block size chosen determines how large a file can be written to the file system. In this case since we are talking about vSphere and VMFS file systems it would be how larger a virtual disk file (vmdk) can be written.

VMware has a good KB article which explains the block size limitations but these are basic maximum sizes allowed.

• 1 MB Block Size           256 GB max file
• 2 MB Block Size          512 GB Max file
• 4 MB Block Size               1 TB Max file
• 8 MB Block Size               2 TB Max file

VMware Block Size KB Article

Besides the file size limitations on lower than 8 MB block size one area where users can have issues is in snapshot operations or other backup type operations like doing a Hot-Add of a virtual disk.

The primary consideration is that vSphere will look in the Working Directory for the VM, which usually is where the configuration file (vmx) exists for the block size of that VMFS when doing a snapshot or other dealing with virtual disks. If you are in a situation where you have split your vmfks across multiple vmfs file systems be aware of the block sizes especially if they are different.

Consider this example of a VM with 2 vmdks ( one an O/S disk with a 40 GB vmdk and the other with a Data disk that is 1 TB ). The virtual disks exist on two different VMFS Luns.

Disk 1 40 GB     Lun1  ( 1 MB block size  )   – this is where the vmx file is

Disk 2    1 TB     Lun2   ( 8 MB block size)

When you go to add a snapshot vSphere will show an error similar to this :

File is larger than the maximum size supported by data store

 Initially it was thought that there were performance issues with larger block sizes so some admins kept the block sizes smaller, also 1 MB was the default. However I don’t believe that to be the case and the recommendation would be to always create your VMFS partitions with a 8 MB block size. In this way you can support the maximum file size and also will not have any issues with snapshots and other type operations. This also provides the most flexibility.

Pete