openstack team mailing list archive

[Caitlin Bestler <Caitlin.Bestler@xxxxxxxxxxx>]

John Dickinson wrote:


> Here's how to find the usable space for swift

> marketing size of the drive (eg 2TB) * .92 (to account for formatted size * .8 (for 80% fullness) / replicas

> If you have 15 TB of formatted space with 3 replicas, that gives you 5TB of usable space. If you have 15TB of unformatted space with thre
 > replicas, that gives you a little less than 4TB of usable space.

> The reason I calculate it at 80% fullness is so that you have some headroom to expand as your cluster grows. You don't want to have your hard
> drives fill up completely before you decide it's time to buy some new ones.

Just to emphasize that point, Swift's logic does not include any heroic efforts to hunt down the last scrap of available free space.

The cluster is full the first time an object cannot be created because an Object Server it is assigned to is full.

So operating at 80% or less capacity is very important. The simplification that this enables is a good tradeoff for almost all deployments.