Re: [libvirt] PATCH: Disable QEMU drive caching

Daniel P. Berrange wrote: > - It is unsafe on host OS crash - all unflushed guest I/O will be > lost, and there's no ordering guarentees, so metadata updates could > be flushe to disk, while the journal updates were not. Say goodbye > to your filesystem. This has nothing to do with cache=off. The IDE device defaults to write-back caching. As such, IDE makes no guarantee that when a data write completes, it's actually completed on disk. This only comes into play when write-back is disabled. I'm perfectly happy to accept a patch that adds explicit sync's when write-back is disabled. For SCSI, an unordered queue is advertised. Again, everything depends on whether or not write-back caching is enabled or not. Again, perfectly happy to take patches here. More importantly, the most common journaled filesystem, ext3, does not enable write barriers by default (even for journal updates). This is how it ship in Red Hat distros. So there is no greater risk of corrupting a journal in QEMU than there is on bare metal.