Make ring's replica count adjustable

Registered by John Dickinson on 2013-01-15

Example:

$ swift-ring-builder account.builder set_replicas 4
$ swift-ring-builder rebalance

This is a prerequisite for supporting globally-distributed clusters,
as operators of such clusters will probably want at least as many
replicas as they have regions. Therefore, adding a region requires
adding a replica. Similarly, removing a region lets an operator remove
a replica and save some money on disks.

In order to not hose clusters with lots of data, swift-ring-builder
now allows for setting of fractional replicas. Thus, one can gradually
increase the replica count at a rate that does not adversely affect
cluster performance.

Example:

$ swift-ring-builder object.builder set_replicas 3.01
$ swift-ring-builder object.builder rebalance
<distribute rings and wait>

$ swift-ring-builder object.builder set_replicas 3.02
$ swift-ring-builder object.builder rebalance
<distribute rings and wait>...

Obviously, fractional replicas are nonsensical for a single
partition. A fractional replica count is for the whole ring, not for
any individual partition, and indicates the average number of replicas
of each partition. For example, a replica count of 3.2 means that 20%
of partitions have 4 replicas and 80% have 3 replicas.

Changes do not take effect until after the ring is rebalanced. Thus,
if you mean to go from 3 replicas to 3.01 but you accidentally type
2.01, no data is lost.

Blueprint information

Status:
Complete
Approver:
None
Priority:
Undefined
Drafter:
None
Direction:
Approved
Assignee:
None
Definition:
Approved
Series goal:
Accepted for grizzly
Implementation:
Implemented
Milestone target:
milestone icon 1.8.0
Started by
John Dickinson on 2013-02-28
Completed by
John Dickinson on 2013-03-05

Related branches

Sprints

(?)

Work Items

Dependency tree

* Blueprints in grey have been implemented.

This blueprint contains Public information 
Everyone can see this information.