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Choosing a block size when creating VMFS datastores

VMFS datastore on VMware ESX servers are default 1MB block size. The block size determines the minimum amount of disk space that any file will take up on VMFS datastores. The block size also determines the maximum size that any file can be, if you select a 1MB block size on your data store the maximum file size is limited to 256GB. There is also no way to change the block size after you set it without deleting the datastore and re-creating it, which will wipe out any data on the datastore.

Because of this you should choose your block size carefully when creating VMFS datastores. The VMFS datastores mainly contain larger virtual disk files so increasing the block size will not use all that much more disk space over the default 1MB size. You have the following choices when creating a datastore:

  • 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

There is no noticeable I/O performance difference by using a larger block size. When you create your datastore, make sure you choose your block size carefully. 1MB should be fine if you have a smaller datastore (less than 500GB) and never plan on using virtual disks greater then 256GB. If you have a medium (500GB – 1TB) datastore and there is a chance that you may need a VM with a larger disk then go with a 2MB or 4MB block size. For larger datastores (1TB – 2TB) go with a 4MB or 8MB block size. In most cases you will not be creating virtual disks equal to the maximum size of your datastore (2TB) so you will usually not need a 8MB block size.

Choose carefully, once you create your datastore there is no changing it later without loosing the data on the datastore.