VSAN Part 27 – VM Memory Snapshot Considerations

I’ve done a few posts in the past which discuss the VM Home Namespace object. To recap, the VM Home Namespace is where we store all the virtual machine configuration files, such as the .vmx, .log, digest files, memory snapshots, etc. I also highlighted that the VM Home Namespace is limited to 255GB in size. This led one reader to raise the following observation:

It means that it is not possible to do a snapshot with memory for a VM with 256 GB of RAM.

This is indeed correct. If you attempt to snapshot a VM (with memory) that has a memory configuration that does not fit into the VM Home Namespace (in this case I created a VM with 256 GB of memory), you will hit the following “insufficient disk space on datastore” error:

vsan - insufficient - 2The reader then had a follow-up question:

Are there any plans to decrease VM Home Namespace?

I can’t say too much about futures at this point, but what I can say is that the issue is recognised, and we certainly plan to address it going forward. However, in the meantime, if you have VMs with large memory requirements, and you wish to be able to snapshot those VMs along with the memory, be cognizant of this restriction.

  1. a question from “VSAN PART 23 – WHY IS MY STORAGE OBJECT STRIPED?” as I cannot post it there. I understand the concern and issue of the article, but generally, issn;t striping a good thing ? Shouldn’t we want to have as much as more striping for our data ?

    • Generally striping is good, but remember that with VSAN all I/O goes to flash first, and is then destaged to spinning disk. So a stripe will only help on occasion, such as a read cache miss or destaging from SSD to spinning disk.

      The other concern is component count. There is currently a limit of 3,000 components per host. Striping will increase the component count per object, so you need to keep this in mind when designing your VSAN.

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