[Qemu-discuss] Question about cache flushes' implementation in cache=writeback mode

[Michal Soltys]

Hi,


If we have cache=writeback (default since 2012-8) enabled, does it imply
that essentially any request in guest with REQ_FLUSH (REQ_PREFLUSH now)
set is equivalent of doing sync() on the host (as the exposed storage
basically has a gigantic cache in form of host's page cache) ? Or is
there some subtle/more fine grained control over this ?

I'm asking, as in case of for example host with multiple VMs using
separate physical disks, single flush from one VM causes expensive
host-wide sync of everything - and in such scenario cache=none sounds
like much more sensible approach.

Or maybe there's something more fundamental I'm misunderstanding here ..