Re: Cache Tiering Investigation and Potential Patch

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On Wed, 25 Nov 2015, Nick Fisk wrote:
> Hi Sage
> 
> > -----Original Message-----
> > From: Sage Weil [mailto:sage@xxxxxxxxxxxx]
> > Sent: 25 November 2015 17:38
> > To: Nick Fisk <nick@xxxxxxxxxx>
> > Cc: 'ceph-users' <ceph-users@xxxxxxxxxxxxxx>; ceph-devel@xxxxxxxxxxxxxxx;
> > 'Mark Nelson' <mnelson@xxxxxxxxxx>
> > Subject: Re: Cache Tiering Investigation and Potential Patch
> > 
> > On Wed, 25 Nov 2015, Nick Fisk wrote:
> > > Presentation from the performance meeting.
> > >
> > > I seem to be unable to post to Ceph-devel, so can someone please
> > > repost there if useful.
> > 
> > Copying ceph-devel.  The problem is just that your email is HTML-formatted.
> > If you send it in plaintext vger won't reject it.
> 
> Right ok, let's see if this gets through. 
> 
> > 
> > > I will try and get a PR sorted, I realise that this change modifies
> > > the way the cache was originally designed but I think it provides a
> > > quick win for the performance increase involved. If there are plans
> > > for a better solution in time for the next release, then I would be
> > > really interested in working to that goal instead.
> > 
> > It's how it was intended/documented to work, so I think this falls in the 'bug
> > fix' category.  I did a quick PR here:
> > 
> > 	https://github.com/ceph/ceph/pull/6702
> > 
> > Does that look right?
> 
> Yes I think that should definitely be an improvement. I can't quite get 
> my head around how it will perform in instances where you miss 1 hitset 
> but all others are a hit. Like this:
> 
> H H H M H H H H H H H H
> 
> And recency is set to 8 for example. It maybe that it doesn't have much 
> effect on the overall performance. It might be that there is a strong 
> separation of really hot blocks and hot blocks, but this could turn out 
> to be a good thing.

Yeah... In the above case recency 3 would be enough (or 9, depending on 
whether that's chronological or reverse chronological order).  Doing an N 
out of M or similar is a bit more flexible and probably something we 
should add on top.  (Or, we could change recency to be N/M instead of just 
N.)
 
> Would it be useful for me to run all 3 versions (Old, this and mine) 
> through the same performance test I did before?

If you have time, sure!  At the very least it'd be great to see the new 
version go through the same test.

> Also I saw pull request 6623, is it still relevant to get the list order 
> right?

Oh right, I forgot about that one.  I'll incorporate that fix and then you 
can test that version.

Thanks!
sage
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