VMware supports a massive VMDK file size, but why create a monster?

Practical uses for a 64 TB VMDK file are limited today, but as storage needs grow, you might find a reason to embrace monster VMs.

VMware increased the maximum limit for VMDK files from 2 terabyte (TB) in vSphere 5.1 to 64 TB in vSphere 5.5. But why would you need a 64 TB VMDK file size?

A 64 TB VMDK file size is an enormous virtual disk. To understand how big, let's put that into a perspective that everyone can relate to. A DVD holds 4.7 GB, so you would need 13,944 DVDs to equal 64 TB.

To be accurate, VMware Inc. only allows a volume of 62.9 TB for VMDKs. The maximum LUN size is 64 TB, but we still need to reserve some room for logs, .vmx and swap files.

So why have 64 TB VMDK files? I think the size comes down to a need to keep up with changing operating system capabilities. I wouldn't recommend creating a single 62.9 TB disk today. But as disk management on Windows matures and as Microsoft's new Resilient File System (ReFS) becomes more mature, we may see additional uses. Think about the difficulties of having a single disk that size:

  • Long waits to index or search for a file
  • Exceeding maximum path lengths
  • Difficulties repairing a disk that large

A more realistic approach would be to have a disk presented with multiple partitions. Remember, we can now have more than 128 partitions. Windows can only present 26 disks, but you can always create a mount point. Microsoft likes to call this an NTFS Junction Point or Symbolic Link. In this case, a Junction Point is just like an addition on your house. You cut a hole in the wall, build a room, and you now have more space. There's no need to move to a new house or change your address. That way, if you have a problem it will only take out part of the disk and not the whole thing, which means quicker repairs and faster searches and indexes.

Eventually, we will see even larger files sizes with the onslaught of cloud storage. I also think we will begin to see different infrastructure that takes advantage of the storage at the application layer using object-oriented storage.

So what do you need to do to use a 62.9 TB VMDK file size? First, you can only create this type of disk using the vSphere Web client. I assume most users will be doing this on Windows, in which case you must use GUID Partition Tables since you are exceeding the 2 TB boundary of a master boot record. You will also need to increase your block allocation size to above 16 K.

This was first published in January 2014

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Scott Gottesman asks:

Do you have a need for 64 TB VMDK file sizes?

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