RE: chooseleaf may cause some unnecessary pg migrations

[Sage Weil <sage@newdream.net>]

On Mon, 19 Oct 2015, Xusangdi wrote:
> 
> > -----Original Message-----
> > From: ceph-devel-owner@vger.kernel.org [mailto:ceph-devel-owner@vger.kernel.org] On Behalf Of
> > Chen, Xiaoxi
> > Sent: Monday, October 19, 2015 9:11 AM
> > To: xusangdi 11976 (RD)
> > Cc: ceph-devel@vger.kernel.org
> > Subject: RE: chooseleaf may cause some unnecessary pg migrations
> >
> > Sorry but not following...
> >
> > > then shut down one or more osds (please don't touch the crushmap, just stop the osd service or kill
> > its process).
> >
> > In this case, OSD is only down but not out, but will be marked out after 300s.
> >
> > So in what case your patch is helping?
> >
> >       If you said your patch helps on "down and out" , then my experiment is exactly the case,
> >
> 
> I am afraid it is probably not. Could you tell me how did you simulate 
> the osd "down and out" situation using crushtool? If it was done by 
> arguments such as '--remove-item' or 'reweight-item', it modified the 
> crushmap and is not what I'm aiming for. 

There is a --weight argument (noted in usage near --test, which is the 
only piece that uses it).  The crush map is not modified--only the weight 
vector that is passed in when a mapping is calculated (which is the 
equivalent of the in/out state in Ceph's OSDMap).  This should let you 
simulate this case.

When I'm debugging/understanding these issues I usually change the dprintk 
#define at the top of crush/mapper.c and use crushtool or osdmaptool to 
calculate a single mapping, comparing the log before and after a 
particular change.

sage