Writeback option - Ram inside VM or Host?

[FcbInfo]

Hi there.

I tried to find some about where the "writeback" cache is write, then I have found something says that is write in Ram.

When you config some KVM VM with writeback option, this will use Ram inside this VM or will use Ram of the Host (main server)?

Thanks!

 

[FcbInfo]

dd if=/dev/zero of=test bs=64k count=16k conv=fdatasync

On Debian guest (VM),

If I use writeback unsafe: 1.5gbps
If I use writeback: 93mbps (the same of no cache).

I'm confused!

 

[Mitya]

I've read that witeback is cache inside host, no-cache is cache inside guest. Unsafe writeback is just writeback without forwarding flushes from guest to host.

 

[FcbInfo]

Well, but on Windows VMs, writeback and writeback unsafe, doesn't do too much difference. They have almost the same speed, but on Linux guest, from 93mbps to 1500mbps, does hard difference.