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
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