Re: [Qemu-devel] [RFC] Disk integrity in QEMU

[Chris Wright <chrisw@redhat.com>]

* Anthony Liguori (anthony@codemonkey.ws) wrote:
> Mark Wagner wrote:
>> So I guess the real issue isn't what the default should be (although
>> the performance team at Red Hat would vote for cache=off),
>
> The consensus so far has been that we want to still use the host page  
> cache but use it in write-through mode.  This would mean that the guest  
> would only see data completion when the host's storage subsystem reports  
> the write as having completed.  This is not the same as cache=off but I  
> think gives the real effect that is desired.

I think it's safe to say the perf folks are concerned w/ data integrity
first, stable/reproducible results second, and raw performance third.

So seeing data cached in host was simply not what they expected.  I think
write through is sufficient.  However I think that uncached vs. wt will
show up on the radar under reproducible results (need to tune based on
cache size).  And in most overcommit scenarios memory is typically more
precious than cpu, it's unclear to me if the extra buffering is anything
other than memory overhead.  As long as it's configurable then it's
comparable and benchmarking and best practices can dictate best choice.

thanks,
-chris