Re: [Qemu-devel] [PATCH 0/2] block: Write out internal caches even with cache=unsafe

[Peter Maydell]

On 24 October 2011 09:17, Kevin Wolf <address@hidden> 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