On Wed, 25 Jan 2017, Wido den Hollander wrote:
> > Op 25 januari 2017 om 0:43 schreef Sage Weil <sweil@xxxxxxxxxx>:
> >
> >
> > On Wed, 25 Jan 2017, Loic Dachary wrote:
> > > Hi Sage,
> > >
> > > While documenting crush_reweight_bucket[1] I came accross something that
> > > I don't understand when reweighting uniform buckets[2]. The associated
> > > commit[3] is six years old but maybe your remember why the item_weight
> > > had to be adjusted with the average of the weight of the buckets... but
> > > only if there are more buckets than leaves ?
> >
> > I think it's just a half-hearted attempt to Do The Right Thing when the
> > situation is nonsensical. Uniform buckets are meant to be used with leave
> > (device) items of fixed weight (item_weight). If you (ab)use them with
> > bucket children, the algorithm can't really do the right thing because it
> > doesn't understand the child bucket weights. If there are a lot of bucket
> > children it resets item_weight to their average.
> >
> > This is probably pointless... we could just remove it, and perhaps warn
> > (or error out?) in CrushCompiler if a uniform bucket child is a
> > non-device.
> >
>
> So I know of a setup which uses something like this:
>
> datacenter dc1 {
> alg straw2
> hash 0
> item rack1
> item rack2
> }
>
> root ams {
> alg uniform
> hash 0
> item dc1
> item dc2
> item dc3
> item dc4
> }
>
> They want all 3 replicas over 3 different DCs and be able to handle a
> complete DC failure and recover from it. To prevent data shuffling when
> a weight is changed in a rack, but data may move inside the DC.
>
> From your comment I understand that was never the intention of uniform
> buckets?
It was certainly not the intention. I think it ought to work,
though, provided the code that tries to keep the weights summing
up the tree "behave" (are effectively a no-op) on the uniform buckets.
sage
>
> Wido
>
> > s
> >
> >
> > >
> > > Cheers
> > >
> > > [1] http://libcrush.org/main/libcrush/blob/wip-2-doxygen/crush/builder.h#L111
> > > [2] http://libcrush.org/main/libcrush/blob/wip-2-doxygen/crush/builder.c#L1282
> > > [3] http://libcrush.org/main/libcrush/commit/60f627f88c6314c5a89bb7119ead907ca8b8ef37
> > >
> > > --
> > > Loïc Dachary, Artisan Logiciel Libre
> > >
> > >
> --
> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in
> the body of a message to majordomo@xxxxxxxxxxxxxxx
> More majordomo info at http://vger.kernel.org/majordomo-info.html
>
>
