Re: [PATCH] virtio-blk: set QUEUE_ORDERED_DRAIN by default

From: Avi Kivity
Date: Fri Aug 28 2009 - 02:33:49 EST

On 08/28/2009 04:15 AM, Rusty Russell wrote:
On Thu, 27 Aug 2009 08:34:19 pm Avi Kivity wrote:
There are two possible semantics to cache=writeback:

- simulate a drive with a huge write cache; use fsync() to implement
- tell the host that we aren't interested in data integrity, lie to the
guest to get best performance
Why lie to the guest? Just say we're not ordered, and don't support barriers.
Gets even *better* performance since it won't drain the queues.

In that case, honesty is preferable. It means testing with cache=writeback exercises different guest code paths, but that's acceptable.

Maybe you're thinking of full virtualization where we guest ignorance is
bliss. But lying always gets us in trouble later on when other cases come

The second semantic is not useful for production, but is very useful for
testing out things where you aren't worries about host crashes and
you're usually rebooting the guest very often (you can't rely on guest
caches, so you want the host to cache).
This is not the ideal world; people will do things for performance "in

We found that cache=none is faster than cache=writeback when you're really interested in performance (no qcow2).

I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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