Category Archives: VMware

What’s Block Size Got To Do With It?

(With apologies to Tina Turner.)

On WMware ESXi/Vsphere versions through 4.X, how big a file can be on a datastore is completely dependent on the block size specified when the datastore was formatted.  And this in turn drives the maximum size of any virtual hard disk.  For many virtual machines, this might not be an issue.  But if you want to create a virtual hard disk greater than 256 GB, then you need to pay attention!

VMFS-2 and VMFS-3

By default, VMware File System (VMFS) uses a 1 MB block size, which supports a maximum file size of 256 GB on VMFS-3 and 456 GB on VMFS-2.  (No, you read that right.  The maximum file size actually went down between VMFS-2 and VMFS-3.)  To support larger file sizes, you need to use a larger block size when the datastore is formatted.

Block Size VMFS-2 Disk Size VMFS-3 Disk Size
1 MB 456 GB 256 GB*
2 MB 912 GB 512 GB*
4 MB 1.78 TB 1 TB*
8 MB 2 TB 2 TB – 512 bytes
16 MB 2 TB ** Not Valid **
32 MB 2 TB ** Not Valid **
64 MB 2 TB ** Not Valid **

* On ESXi 4.0, 512 bytes is subtracted from the maximum file size for any block size on a VMFS-3 datastore.  On ESXi 4.1, this only occurs when the block size is 8 MB.

Once the block size is set, the only way to change it is to reformat the datastore.  Which means moving the existing data elsewhere as a formatting a datastore, like formatting any disk, destorys the existing data.


In ESXi/VSphere 5.0, VMware introduced VMFS-5 (there is no VMFS-4) which uses a unified 1 MB block size that cannot be configured.  But the maximum file size is now 2 TB – 512 bytes, so block size no longer matters.  As is the case with any new feature, it's only available on ESXi/VSphere 5.0 and higher.

If you upgrade an older ESXi/VSphere to 5.0 or higher and the existing datastores use VMFS-3, you can upgrade the datastores to VMFS-5.  This is a non-destructive process, meaning the upgrade can be done with live on the datastore.  You should always have backups in case anything goes wrong!  Upgrading from VMFS-3 to VMFS-5 will not give all the VMFS-5 features, but you will get most of them.

Further Reading

See VMware Knowledge Base Article 1003565 for details on block size and how it effects the maximum file size.

See VMFS-5 Upgrade Considerations for information about upgrading an existing VMFS-3 datastore to VMFS-5.

Posted in VMware.
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