[Openstack] [Swift] Using unevenly sized disks in a cluster

Shrinand Javadekar shrinand at maginatics.com
Thu Feb 26 20:40:32 UTC 2015


I have a question about using unevenly sized disks in a Swift cluster
configured to do 2x replication.

Let's say I start with two disks of 10GB each and configure the rings
so that both the disks have the same weight (say 10). In this case,
one replica of each object will be on each device.

At a later point, I increase the capacity of the cluster by adding one
500GB disk. I also assign it a weight 500.

This means that my 500GB disk should have 50x more data than the 10GB
ones. Swift will move some replicas of existing objects onto this
500GB disk.

How does Swift place data such that it the "as unique as possible"
policy is maintained. Beyond a point, won't it have to place both
replicas on the same device (the 500GB disk)?

If yes, to overcome the above problem, does Swift recommend always
adding a pair of evenly sized disks (for 2x replication)?

Thanks in advance.

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