We have already seen two types of volume, a simple read-write volume, and a read-only volume such as a clone. When you create a full dump of a Coda volume, you can restore this as a read-only volume. Read-only volumes are also useful because you can make identical copies on several servers, and access to these read-only replicas may be much more efficient than access to a read-write volume. A replicated volume is fully usable as long as at least one running server has a copy of it; this may improve the availability of the data. (Coda seems to be dropping support for read-only replicas. It seems that a slight gain in efficiency is offset by the increased complexity of the code.)
Besides volumes of these kinds, which all Coda's cousins support, Coda also has read-write replicas. These are convenient because they give the advantage of high availability, though writing to them may be less efficient than writing to simple read-write volumes. Coda classes some volumes as read-write replicas even though they exist on only one server. Read-write replicas, whether they live on one server or on several, are the normal kind of volume in Coda cells, and you should plan your cell structure with this in mind.
Replication is something like having a highly configurable, flexible RAID system in software. It is true that there is some management overhead, but we find that hardware RAID systems can be down for a long time if the controller breaks, which has happened to us several times now.