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

[MORITA Kazutaka]

At Fri, 11 May 2012 00:31:49 +0800,
Liu Yuan wrote:
> 
> On 05/10/2012 10:37 PM, Christoph Hellwig wrote:
> 
> >> What did you mean by en/disable it on a per VDI basis? I think object
> >> > cache does have this attribute, in terms of QEMU command line option. Or
> >> > I miss some context?
> > Have an attribute on the sheepdog_inode that the object cache is used
> > for access to this VDI, without having to coordinate all possible users
> > of it, e.g. a way to specify it when creating the vdi and having any
> > user simpliy do the right thing.
> 
> I don't think it make any sense. This just complicate the things, for
> now we have a global switch and per VDI switch already, any attempt to
> introduce yet another switch looks really crazy to me. Maybe it would

If we allow multiple cache features in future, I think it makes sense
to have such attribute because we cannot specify the detailed cache
control in the qemu command line options.  It enables us to provide
different cache levels according to the customer demand.

> help your use case, but I think you should be aware:
> 
> 1) Sheepdog originally targets for mixing computing and storage in one
> cluster to maximize the resource usage.
> 2) Sheepdog isn't inherently optimized for users who has extremely
> high-end and fast network.

Yes, Sheepdog aimed at that.  However, currently there are already
some users (one of them belongs to our NTT group) who want to use
Sheepdog without mixing computing and storage resources or try
Sheepdog with non-commodity hardware like Fusion-io or Infiniband.
As a sheepdog maintainer, I'd like to accept these requirements if it
doesn't bring much complexity to Sheepdog design and implementation.

Anyway, it's okay to me to enable the object cache by default if it
fits for 1) and 2) (I think we should document this restriction more
clearly for users not to enable it wrongly), but want to see other
cache features which fit for other requirements, too.

Thanks,

Kazutaka

> 
> I think we are coding sheepdog for general setup, where we have low-end
> servers with ordinary network connect and try to cluster them together
> to get a better utilized and elastic computing and storage.
> 
> I think with current global and per VDI switch for object cache, nothing
> technically stops you from achieving your personal target at your own will.
> 
> Thanks,
> Yuan
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