vSphere Virtual Flash?

TeeJayHoward

Limpness Supreme
Joined
Feb 8, 2005
Messages
11,603
Could someone please explain to me how Virtual Flash Host Swap Cache and Virtual Flash Read Cache work? I get that they speed up VMs somehow through the use of a local SSD. What I want to know is what's behind the magic. HOW does it work?
 

bmh.01

Gawd
Joined
Mar 28, 2002
Messages
610
vFRC is a local host based read cache.

Host swap cache allows the host to swap (which should be avoided) to an SSD rather than to a presumed slower datastore.
 

TeeJayHoward

Limpness Supreme
Joined
Feb 8, 2005
Messages
11,603
vFRC is a local host based read cache.
Could you please explain this to me like I'm five? Does it just store a copy of the .vmdk on the local SSD and execute reads from there? What block size do I want? What happens if I give it 1GB of vFRC for a 16GB disk? What about 16GB of vFRC? 32GB?

Host swap cache allows the host to swap (which should be avoided) to an SSD rather than to a presumed slower datastore.
So if you overprovision RAM and it needs to swap due to the host running out of physical memory, it will do it to an SSD rather than the .vswp file on the other datastore?
 

bmh.01

Gawd
Joined
Mar 28, 2002
Messages
610
Could you please explain this to me like I'm five? Does it just store a copy of the .vmdk on the local SSD and execute reads from there? What block size do I want? What happens if I give it 1GB of vFRC for a 16GB disk? What about 16GB of vFRC? 32GB?

You assign an eligable (SSD & Local) drive to the FRC pool of a host which can then be allocated to the disks of VMs running on that host. You give each disk of each vm an amount of cache and the block size for it to use. So it isn't necessarily a one click feature to enable you need to monitor the guest to be given cache to see it's access patterns to ensure that you don't waste cache space allocating too large a block which won't be efficiently filled.

The reads carried out by the VM will then be cached so that it can be retrieved (if not modified) without going to shared or slower local storage.

So if you overprovision RAM and it needs to swap due to the host running out of physical memory, it will do it to an SSD rather than the .vswp file on the other datastore?

Once it has gone through all the other stages (compression, page sharing, balloning, guest swapping) then yes. Although if you have to swap then it's better to do it in the guest (where it knows what it is swapping) rather than the host which should be a last resort. So I can't imagine this is a highly used feature.
 

FireExit

Weaksauce
Joined
Sep 28, 2005
Messages
80
Once it has gone through all the other stages (compression, page sharing, balloning, guest swapping) then yes. Although if you have to swap then it's better to do it in the guest (where it knows what it is swapping) rather than the host which should be a last resort. So I can't imagine this is a highly used feature.

Don't forget that it only does those stages if VMTools is installed on the VM. If it doesn't, then the hypervisor goes straight for "I own your RAM"
 
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