Are you thin, thick or just lazy?

vSphere 5 is almost upon us and I’ll try to start throwing out nuggets (now that I’ve learned the Beta NDA is over).

One of my favorite tidbits is the new way to create hard drives (VMDKs) for virtual machines. While there are many types of VMDK files suported only three can be created from the GUI in 4.x and 5.x : Thin, Thick (Lazy) and Thick (Eager).

In vSphere 4.x there were two checkboxes when creating a VMDK files:

The checkboxes were a little misleading, because you could only choose one. It also only referred to one of the types by name (Thin) and you don’t really get an understanding of what would happen if you didn’t choose either (the default is Thick, Lazy) and the third option seemed to be tied directly to a little-used feature (Fault Tolerance).

Enter vSphere 5:

Now it lists all 3 types, you see radio buttons and the default is obvious.

Quick definitions:
Thin Provisioning creates the VMDK with just the header information, but it does not allocate or zero blocks.
Thick (Lazy) creates the VMDK and allocates all the blocks for the VMDK but doesn’t write zeroes them.
Thick (Eager) creates the VMDK, allocates and zeros all the blocks.

Best write speed: Eager (note this is more “performance white paper” territory than real-world difference)
Best creation time: Thin
Least space taken on disk if it’s not needed: Thin
Historic default: Lazy

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