On 16.08.2012 16:57, Wietse Venema wrote: [] > * With KVM and QEMU, one would set "cache=none" to bypass the write > cache in the physical host and make fsync() work as expected in > a virtual machine.
FWIW, with QEMU (kvm is just an "accelerator" for qemu, the device model/emulation is exactly the same), cache flushes are supported with all cache modes except of "unsafe" (which is names that way for exactly this reason: flushes are ignored). So it is okay to run virtual machines with any cache mode (except of the obvious). Contrary to common misunderstanding, cache=none does NOT disable write cache. Only the default (up to current 1.1 version) cache=writethrough disables write caching completely. Instead, cache=none disables filesystem cache or pagecache on the host (hypervisor) only -- there's a cache in devices lower the stack (disk drives etc). So disk flushes are still needed and are properly implemented. cache=none corresponds to O_DIRECT (direct i/o, bypassing buffer cache), and cache=writethrough corresponds to O_SYNC. (there's anothe cache mode in recent qemu, directsync, which combines O_DIRECT and O_SYNC). /mjt
