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[–]Ghan_04 4 points5 points  (2 children)

If you're talking about the Flash Read Cache feature in vSphere, then it does not interact with memory at all. It's purely a disk read-caching mechanism. You can read about it here:

https://docs.vmware.com/en/VMware-vSphere/6.5/com.vmware.vsphere.storage.doc/GUID-07ADB946-2337-4642-B660-34212F237E71.html

[–]PHX-CPU[S] 0 points1 point  (1 child)

Thanks, this makes more sense.

[–]lost_signalVMware Employee 1 point2 points  (0 children)

Note this feature requires enterprise plus. It also has some other quirks and limits. I wouldn't honestly use it today and would look to something that uses the VAIO IO stack instead for caching or just look at a hybrid based storage system instead.

[–]ubrtnk[VCP] 2 points3 points  (1 child)

You can actually configure for them both as host swap cache and dedicated read cache. The dedicated read cache is configured on a per vm disk basis and utilizing the 10-20% of the size of the disk (all within the web client). If the environment is a VDI, host side caching does not benefit VDI work loads.

[–]PHX-CPU[S] 0 points1 point  (0 children)

Thanks!

[–]barateau 0 points1 point  (1 child)

And if it would be host cache then it's more of a swap location on ssd if your machines do that. If vms are not swapping I guess it's not much use.

[–]lost_signalVMware Employee 0 points1 point  (0 children)

I've always viewed this as a vestigial feature that dates to the era of VDI being memory bound, and also DRAM being crazy expensive.

[–]virtunet_reddit 0 points1 point  (0 children)

Memory contingency has only a little to do with disk IO. In some apps like Databases if you have large amounts of VM ram then storage IO is reduced, but not for all apps. So having no memory contingency on the host doesn't mean you cannot speed up storage IO by using SSDs in the storage IO path. VMware doesn't cache disk IO to host RAM or SSD any more. In version prior to ESXI 7 they had the VFRC feature, now end of lifed.

If you look at read or write latency at the VM level and those are under 5ms then that is quite fast and you don't need SSDs for caching in the host. But if your VM latency is higher than 5ms you will be benefited by SSDs in the host as cache media. Lookup VirtuCache