In the ring builder, we place partitions with maximum possible
dispersion across tiers, where a "tier" is region, then zone, then
IP/port,then device. Now, instead of IP/port, just use IP. The port
wasn't really getting us anything; two different object servers on two
different ports on one machine aren't separate failure
domains. However, if someone has only a few machines and is using one
object server on its own port per disk, then the ring builder would
end up with every disk in its own IP/port tier, resulting in bad (with
respect to durability) partition placement.
For example: assume 1 region, 1 zone, 4 machines, 48 total disks (12
per machine), and one object server (and hence one port) per
disk. With the old behavior, partition replicas will all go in the one
region, then the one zone, then pick one of 48 IP/port pairs, then
pick the one disk therein. This gives the same result as randomly
picking 3 disks (without replacement) to store data on; it completely
ignores machine boundaries.
With the new behavior, the replica placer will pick the one region,
then the one zone, then one of 4 IPs, then one of 12 disks
therein. This gives the optimal placement with respect to durability.
The same applies to Ring.get_more_nodes().
Co-Authored-By: Kota Tsuyuzaki <tsuyuzaki.kota@lab.ntt.co.jp>
Change-Id: Ibbd740c51296b7e360845b5309d276d7383a3742
af72881d1d