Hi Loic, thanks for the education! I’m also trying to understand the new ‘indep’ mode. Is this new mode designed for Ceph-EC only? It seems that all of the data in 3-copy system are equivalent and this new algorithm should also work?
Sincerely, Yuan On Mon, Jan 13, 2014 at 7:37 AM, Loic Dachary <l...@dachary.org> wrote: > > > On 12/01/2014 15:55, Dietmar Maurer wrote: > > From the docs: > > > > > > > > step [choose|chooseleaf] [firstn|indep] <N> <bucket-type> > > > > > > > > What exactly is the difference between ‘firstn’ and ‘indep’? > > > Hi, > > For Ceph releases up to Emperor[1], firstn is used and I'm not aware of a > use case requiring indep. As part of the effort to implement erasure coded > pools, firstn[2] and indep[3] were separated in two functions. The firstn > method is best suited for replicated pools. The indep method tries to > minimize the position changes in case an OSD becomes unavailable. For > instance, if indep finds > > [1,2,3,4] > > and after a while 3 become unavailable, it is very likely to replace it > with > > [1,2,5,4] > > It matters to erasure coded pools because > > [4,5,2,1] > > (i.e. the same OSDs but in different positions), implies more I/O. Another > difference is that in the case of a mapping failure (i.e. unable to find > the required number of OSDs), firstn will return a short list ( for > instance [1,2,3] when 4 are required ) and indep will return a list with a > placeholder at the missing position ( for instance [1,2,CRUSH_ITEM_NONE,4] > ). > > Cheers > > [1] implementation in releases up to Emperor > https://github.com/ceph/ceph/blob/v0.72/src/crush/mapper.c#L295 > [2] firstn https://github.com/ceph/ceph/blob/v0.74/src/crush/mapper.c#L295 > [3] indep https://github.com/ceph/ceph/blob/v0.74/src/crush/mapper.c#L459 > > -- > Loïc Dachary, Artisan Logiciel Libre > > > _______________________________________________ > ceph-users mailing list > ceph-users@lists.ceph.com > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com > >
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