Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive

[Paul Brook <paul@codesourcery.com>]

> On 05/11/2010 10:53 AM, Paul Brook wrote:
> >>> I disagree.  We should not be removing or rejecting features just
> >>> because they allow you to shoot yourself in the foot.  We probably
> >>> shouldn't be enabling them by default, but that's a whole different
> >>> question.
> >> 
> >> I disagree and think the mentality severely hurts usability.  QEMU's
> >> role should be to enable features, not to simplify every obscure
> >> feature.  In general, if someone wants to accomplish something, we
> >> should try to provide a mechanism to accomplish it.
> >> cache=none|writeback|writethrough is an example of this.  No one other
> >> than QEMU can control how we open a file descriptor so we need to
> >> provide a knob for it.
> > 
> > Doesn't the same argument apply to the existing cache=writethrough?
> > i.e. if you want to avoid data loss you should make sure your guest
> > issues flushes properly, and it's not something qemu should be trying to
> > hack round be adding an implicit flushe after every write.
> 
> cache is the host page cache acting as an extended disk cache.  In
> writethrough mode, the behavior is identical to writethrough on a normal
> disk cache in that all operations are completed only when sent down to
> the next storage layer.

IMO this is a bug. Making host pagecache writethrough but still having a 
volatile writeback disk cache seems like a complete waste of time. I can see 
the advantage of disabling host pagecache (avoid double caching in host RAM), 
but having different levels of cache be writethrough/writeback seems extremely 
suspect.

It's also occurred to me that you're also basing your arguments on the 
assumption that host pagecache is volatile.  On a machine with a good UPS this 
is not true. In the even of external power failure the UPS will flush the host 
page cache and cleanly shut the machine down. As with battery-backed RAID 
cards, it's entirely reasonable to consider the cache to be non-volatile 
storage and ignore the flush requests.

If you don't trust your host OS in this situation then you're into a whole 
different level of pain, and raises obvious questions about the firmware 
running on your storage subsystem.

Paul