[Sheepdog] The distribution of objects is not even enough ?

[Huxinwei]

> -----Original Message-----
> From: Liu Yuan [mailto:namei.unix at gmail.com]
> Sent: Thursday, April 26, 2012 5:08 PM
> To: Huxinwei
> Cc: sheepdog at lists.wpkg.org
> Subject: Re: [Sheepdog] The distribution of objects is not even enough ?
> 
> On 04/26/2012 04:53 PM, Huxinwei wrote:
> 
> > Hi list,
> >
> > I made some tests against the consistent hashing algorithm of
> > sheepdog. Here are some samples: ============================
> number
> > of objects: 600000, replication is 3, total is 1800000 the cluster
> > contains 100 nodes expecting 18000 objects per node acceptable
> > variation is 600 (objects per node) With 2 (2.00 percent) nodes add,
> > 33915 objects (1.88 percent) need to be relocated. 34.00 per is
> > underload, with least as 14757 32.00 per is overload, with most as
> > 23376
> >
> > number of objects: 600000, replication is 3, total is 1800000 the
> > cluster contains 600 nodes expecting 3000 objects per node acceptable
> > variation is 100 (objects per node) With 2 (0.00 percent) nodes add,
> > 5881 objects (0.33 percent) need to be relocated. 33.00 per is
> > underload, with least as 2342 31.00 per is overload, with most as
> > 3790
> >
> > number of objects: 200000, replication is 3, total is 600000 the
> > cluster contains 200 nodes expecting 3000 objects per node acceptable
> > variation is 100 (objects per node) With 2 (1.00 percent) nodes add,
> > 6363 objects (1.06 percent) need to be relocated. 33.00 per is
> > underload, with least as 2432 32.00 per is overload, with most as
> > 3623 ============================
> >
> > The object ID is generated via standard random() call. As you can
> > see, currently algorithm is good enough to handle adding nodes.
> > However, the distribution of objects on nodes is not even enough. The
> > worst case is about 25% more/less objects on a single node. This also
> > means we are going to waste about 25% disk space of the total.
> >
> > Could anyone comment on the testing result ? Am I testing it wrong ?
> > Or there's really something to improve here.
> >
> 
> 
> Sheepdog internally have a virtual nodes mechanism to handle this very
> problem. That is, one physical node will be virtually distributed on the
> hash ring by a specified number (default 64).

The test above is done with 64 vnodes per node.
It actually calls nodes_to_vnodes and obj_to_sheep directly.

> With vnode mechanism, we also get a another feature: weighted storage
> for each node.
> 
> Thanks,
> Yuan