> 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?
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
> >
> >
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