uyozTic
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Enthusiast

SSD host cache - how much needed

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recently installed ESXi (free) for home use testing/study. ESXi boots from USB stick.

i have 2 SSD's for storage, one 1TB for VMs and one 64GB for host cache.

how much cache does the host require? i ask because i'd like to use part of the host cache disk to put a dir on it to store ISOs of OS's i want to install on the 1TB drive and test.

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JarryG
Expert
Expert

ESXi-host does not require any host-cache. VMs might. How much? Depends on how much you over-commit host-RAM. If you do not over-commit RAM, you do not need host-cache. And if you reserve all vRAM to VMs, you do not even need virtual machine swap file at all (with 100% vRAM reservation it will be not created).

In the worst case (when you do over-commit RAM, no sharing/compression/balooning works and all VMs use all vRAM you assigned to them) you might need SSD host-cache of the size:

"summary_of_vRAM_assigned_to_all_VMs" - "host_RAM_available_for_VMs"

But if your SSD host-cache is smaller, it will be used anyway. And when it is full, "normal" swapping will be used (with VM swap files).

_____________________________________________ If you found my answer useful please do *not* mark it as "correct" or "helpful". It is hard to pretend being noob with all those points! 😉

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JarryG
Expert
Expert

ESXi-host does not require any host-cache. VMs might. How much? Depends on how much you over-commit host-RAM. If you do not over-commit RAM, you do not need host-cache. And if you reserve all vRAM to VMs, you do not even need virtual machine swap file at all (with 100% vRAM reservation it will be not created).

In the worst case (when you do over-commit RAM, no sharing/compression/balooning works and all VMs use all vRAM you assigned to them) you might need SSD host-cache of the size:

"summary_of_vRAM_assigned_to_all_VMs" - "host_RAM_available_for_VMs"

But if your SSD host-cache is smaller, it will be used anyway. And when it is full, "normal" swapping will be used (with VM swap files).

_____________________________________________ If you found my answer useful please do *not* mark it as "correct" or "helpful". It is hard to pretend being noob with all those points! 😉

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yezdi
Virtuoso
Virtuoso

As per Duncan's article on Host cache, its good to keep math simple by allocating cache as same as the memory size.

Other option is monitor the 'swap to host cache value' and allocate the space according to the high value.

http://www.yellow-bricks.com/2011/08/18/swap-to-host-cache-aka-swap-to-ssd/

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Should give you a good hand:

https://www.vmware.com/files/pdf/techpaper/vfrc-perf-vsphere55.pdf

Kind Regards, Anatoly Vilchinsky
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uyozTic
Enthusiast
Enthusiast

thank you for your informative reply.

you and the other poster, in my mind, provided correct answer, can award to both or split? with two such helpful answers i hate to choose between.

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uyozTic
Enthusiast
Enthusiast

the other very helpful answer/reply. thank you for that.

how to award you each correct helpful answer?

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uyozTic
Enthusiast
Enthusiast

a very quick glance, i see i the SSD disk i put in for cache isn't even preferred for this function as its the the PCIe flash card type. that's funny!

i'll start to keep an eye on the statistics, more than my normal, but i suspect, as this is a home test/experiment type ESXi setup, i wont see much need for this cache --- now that i understand it better via all of your replies and your pdf link. most are linux VMs and one win2008r2 server &h swap files stored with respective VM. no databases per se except a tiny mysql just for personal goofing about.

guess it wouldn't hurt to allocate same amount of host RAM to SSD cache and be done with it. its only 64GB ssd drive.

i really learned a lot here folks, thanks much for replying and helping.

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No problemo:)

Let us know if you need anything else:)

Kind Regards, Anatoly Vilchinsky
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Ganguli
Contributor
Contributor

Couple of weeks back HP Flash FW Upgrade was failing from within ESXi till I turned on Host Caching. Second host same case. Sure it's required for VMs only?

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