On 24 October 2011 09:17, Kevin Wolf <kw...@redhat.com> wrote: > Am 24.10.2011 09:53, schrieb Paolo Bonzini: >> I think it's not about "why is it there", but rather about "what is it >> useful for". My interpretation of it is "I do not need the image >> anymore unless the command exits cleanly": VM installations, qemu-img >> conversions, BDRV_O_SNAPSHOT (doesn't do it yet, but it could). Even >> SIGINT and SIGTERM would be excluded from this definition, but they cost >> nothing so it's nice to include them. > > I think another common interpretation is: "I don't run this VM in > production but for development. I want the VM to go faster and I can > recreate the image in the unlikely event that power fails during my > work. But it certainly would be nasty."
So at the moment the documentation (qemu-doc.html) says: "Writeback caching will report data writes as completed as soon as the data is present in the host page cache. This is safe as long as you trust your host. If your host crashes or loses power, then the guest may experience data corruption." and also: "In case you don't care about data integrity over host failures, use cache=unsafe. This option tells qemu that it never needs to write any data to the disk but can instead keeps things in cache. If anything goes wrong, like your host losing power, the disk storage getting disconnected accidently, etc. you're image will most probably be rendered unusable." which to me reads in both cases as "will not corrupt data if the guest crashes but may do so if the host crashes" and leaves me with no idea what the difference is. It might be nice if we could clarify that a bit... -- PMM
