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vSphere FT disk format

Posted on Friday, April 24, 2009 | No Comments

Came across an interesting item on the vSphere training course earlier this week. In vSphere when you create a disk you are asked if you want to thin provision it or use it with Fault Tolerance (FT). If you select you want FT it will not let it be thin provisioned. But it makes you ask the question, what does it do different to the virtual disk if you do or do not check the FT option.

We dug until we got an answer. It eager zeros the disk, which you only used to be able to do through the command line. Eager zero means it pre-zeros out all of the blocks in the disk. This is rather than zeroing the block the first time its read, aka lazy zero which is the default.

One then wonders why FT wants this. The answer is that FT use multi-writer mode to access the disk. Formats other than eager zero have metadata which changes dynamically and this could create metadata consistency issues between the primary and secondary. Eager zeroed disks are static once the disk is initialized and hence there can be no consistency issues.

What if you don't tick the box when you create the disk? No problem. When you turn on FT for the VM it gives you a warning and converts the disk first.


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