Re: wip-librbd-caching

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On Thursday, April 12, 2012 at 12:45 PM, Sage Weil wrote:
> On Thu, 12 Apr 2012, Martin Mailand wrote:
> > The other point is, that the cache is not KSM enabled, therefore identical
> > pages will not be merged, could that be changed, what would be the downside?
> >  
> > So maybe we could reduce the memory footprint of the cache, but keep it's
> > performance.
> I'm not familiar with the performance implications of KSM, but the  
> objectcacher doesn't modify existing buffers in place, so I suspect it's a  
> good candidate. And it looks like there's minimal effort in enabling  
> it...

But if you're supposed to advise the kernel that the memory is a good candidate, then probably we shouldn't be making that madvise call on every buffer (I imagine it's doing a sha1 on each page and then examining a tree) — especially since we (probably) flush all that data out relatively quickly. And RBD doesn't currently have any information about whether the data is OS or user data… (I guess in future, with layering, we could call madvise on pages which were read from an underlying gold image.)
Also, TV is wondering if the data is even page-aligned or not? I can't recall off-hand.

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