In ESXi 5.5, the host config has a number of similar sounding settings related to swapfiles and caches of various types.

I understand the principles of write-back/write-through disk cache and swapping VM swap files to the host instead of their remote VMDKs, and I'm reading the docs, but it's still pretty confusing which of these is which, and how they inter-relate within ESXi.

For information, my system has a single standalone host with 96GB RAM and three datastores - a local boot store, an iSCSI main store, and a 250 GB NVMe SSD for swap/cache use.

A clear explanation of the differences and how they work together would be really useful right now :)

  • What are the specific settings you're referring to? Maybe I can help you if you're a bit more specific there.
    – Mario Lenz
    May 3 '17 at 21:39
  • 1
    If only there were publically-available documentation explaining all this on the vendor's website sigh...
    – Chopper3
    May 5 '17 at 9:26

Host cache: deprecated.

Use Flash Read Cache instead. You can reserve a Flash Read Cache for any individual virtual disk. The Flash Read Cache is created only when a virtual machine is powered on, and it is discarded when a virtual machine is suspended or powered off. Obviously, this cache can only operate in Write-Through mode.

VM Swap & Host Swap

Read this article for better understanding: http://pawellakomski.pl/blog/types-of-swapping-on-esx-hosts/

Web Archive https://web.archive.org/web/20161116081952/https://pawellakomski.pl/blog/types-of-swapping-on-esx-hosts/


I wouldn't touch any of the Swap settings or modify from default. Ideally you wouldn't be swapping because you planned for enough RAM to handle your VM workload, right?

vFlash is just a read cache that can lower latency for network-based storage or frequently-read data. It's set per-VMDK (not just per-VM).

If you want the best performance for your single-host setup, use internal disks.

  • 1
    I'm aiming for enough RAM for direct host needs + VM guest RAM and I'm happy to add more if needed. But I'm provisioning an NVMe 250GB as well out of pure caution (I don't expect to need a fraction of it), and because usage tends to expand to use all resources. Also because host cache might have other uses (e.g., disk cache which reflects host disk access efficiency rather than VM memory demands). That said, what I asked was clarification - what each is, more exactly, and how they interrelate - and not really asking advice about using them. Any chance of expanding your answer for that?
    – Stilez
    Apr 30 '17 at 11:20

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