The “Real” maximum size of a VMDK file

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What is the biggest size of a VMDK file?

“2TB – 512 bytes”

This has always been the official answer, and you would find this same answer in blogs, VMware communities, and other sources.

The fact is: this answer is completely correct, but in a “real world environment” is wrong.
No, I’m not going nuts, let me explain.

As I said, the official answer is totally correct, you can create a VMDK that big on any kind of vSphere datastore.
But when it comes to really using that VMDK file, some problems may arise. As you can read in this VMware KB article:

For ESXi 5.0, this error occurs when any individual flat .vmdk file exceeds 2,181,972,430,848 bytes. This is the size generated when adding a virtual disk with size of 1.984492366201720 TB is specified.

ESXi 5.0 is going to check if there is enough free space on the datastore to eventually create a snapshot of that VMDK file, so the actual size of the VMDK file needs to be lower that the official answer. The free space in fact needs to be up to 16 GB for the initial snapshot overhead. Previous version of ESXi did not checked this before creating the snapshot, potentially filling up all the disk and creating problems.

You always have the possibility to redirect the delta files to another disk, but this has some impact on features like sVmotion and Storage DRS.

So, if you need to take a snapshot of that file, probably because you are using VADP libraries to do your backups, you’re better create a smaller disk, and 1.98 TB (or 2032 GB) is the “real” maximum size to be used.

Also, if you wouls like to use a backup software based on VMware VDDK driver and HotAdd mode, please be aware that some tests made by different people has shown this limit is creating troubles, and a safe size is 1.98TB (2027.52 GB). Be careful !

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