Prior to this commit, swift-ring-builder would place partitions on
devices by first going for maximal dispersion and breaking ties with
device weight. This commit flips the order so that device weight
trumps dispersion.
Note: if your ring can be balanced, you won't see a behavior
change. It's only when device weights and maximal-dispersion come into
conflict that this commit changes anything.
Example: a cluster with two regions. Region 1 has a combined weight of
1000, while region 2 has a combined weight of only 400. The ring has 3
replicas and 2^16 partitions.
Prior to this commit, the balance would look like so:
Region 1: 2 * 2^16 partitions
Region 2: 2^16 partitions
After this commit, the balance will be:
Region 1: 10/14 * 2^16 partitions (more than before)
Region 2: 4/14 * 2^16 partitions (fewer than before)
One consequence of this is that some partitions will not have a
replica in region 2, since it's not big enough to hold all of them.
This way, a cluster operator can add a new region to a single-region
cluster in a gradual fashion so as not to destroy their WAN link with
replication traffic. As device weights are increased in the second
region, more replicas will shift over to it. Once its weight is half
that of the first region's, every partition will have a replica there.
DocImpact
Change-Id: I945abcc4a2917bb12be554b640f7507dd23cd0da
6d77c379bd