[Sheepdog] [PATCH] sheep: disable object cache by default

[Christoph Hellwig]

On Thu, May 10, 2012 at 12:09:36PM +0800, Liu Yuan wrote:
> Hmm, DIO object cache doesn't need extra client(gateway node) memory,
> this should probably be the default option because then it becomes
> completely a disk cache that can survive the host crash.

Even then it it consumes disk resources on the gateway.  In a setup
where networking is very fast and the storage nodes provide low latency
I/O that's not nessecarily the right tradeoff - especially not if the
gateway nodes have close to no local storage dedicated to the gateway.

> Also I want to mention that, for starting up hundreds of (cloned) Guest
> VM concurrently, object cache seems to be a must on a massive nodes
> cluster. We also get the benefit that all the cloned VM on the same node
> would share large scale of cached objects.

As said, there totally are use cases for it - just making it
unconditional on the sheep side seems like a fairly bad idea.  I'd
actually love to be able to en/disable it on a per VDI basis.
Especially for read-only base VDIs that never get written to during
normal operation it seems extremely useful.