Open Stack

On Thu, 26 Feb 2015 12:40:32 -0800
Shrinand Javadekar <shrinand at maginatics.com> wrote:

> Let's say I start with two disks of 10GB each [...]
> At a later point, I increase the capacity of the cluster by adding one
> 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)?

Yes, it will.

The thing to understand there is that the dispersion (and thus your
data safety) and the utilization are in direct conflict in the scenario
that you outlined. This problem is not unique to Swift, BTW. Any
replicated storage faces it.

The way out is to realize that you created the problem in the first place.
You could've bought 2 200 GB disks instead.

We have knobs such as weight, and now the overload, that help hapless
administrators who are cornered by the circumstances and the decisions
of their predecessors. But fundamentally, IMHO you need to realize that
the solution can only come from capacity expansion that takes into
account the characteristics of a replicated system.

-- Pete