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SSD overprovisioning in vSphere

SSD vendors like Samsung or Intel often provide tools to reserve some space on your SSD that can be used by the internal algorithms for better wear-leveling, longevity, garbage collection, and performance. My own Samsung SSD comes with a tool to do that on Windows, for example.
But on vSphere, you'll have to do that manually: I see two possibilities:

  1. don't format the SSD "full" but instead use "partial", and specify only the amount of the disk that you want to use. The rest is left unpartitioned, and therefore unallocated. You can only do this if you haven't formatted the SSD as VMFS yet. If you have, this second possibility can still help you:
  2. create a thick-provisioned lazy-zeroed VMDK on the SSD-backed VMFS, either with vmkfstools or by creating a dummy VM. Those blocks are allocated, but remain unwritten. You're not going to attach the disk to a running VM, so they won't ever be written to. This effectively reduces the amount of blocks that can be written to, leaving the rest open. Caveat: if you've previously removed VMDKs from the volume, you have no guarantee that the underlying blocks are in an unwritten state.

P.S. Your SSD might have some overprovisioning space built in already. I've always assumed that's the reason some vendors sell 256GB while others sell 240GB models. The latter vendors sell the same amount of flash as the former, but leave 1/16th unallocated.


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