Innovation, in an effort to provide better, faster, and less expensive solutions, frequently blurs the lines that separated previous technologies.
Consider the InServ storage array from 3Par.
Unlike most other storage arrays, InServ does not require that a set of disks be configured at a specific RAID level.
Rather, each disk is broken into 256-MB “chunklets.”
RAID is then applied at the chunklet level. A disk can thus participate in multiple and various RAID levels as its chunklets are used for multiple volumes.
InServ also provides snapshots similar to those created by the WAFL file system.
The format of InServ snapshots can be read–write as well as read only, allowing multiple hosts to mount copies of a given file system without needing their own copies of the entire file system.
Any changes a host makes in its own copy are copy-on-write and so are not reflected in the other copies.
A further innovation is utility storage. Some file systems do not expand or shrink.
On these systems, the original size is the only size, and any change requires copying data.
An administrator can configure InServ to provide a host with a large amount of logical storage that initially occupies only a small amount of physical storage.
As the host starts using the storage, unused disks are allocated to the host, up to the original logical level.
The host thus can believe that it has a large fixed storage space, create its file systems there, and so on.
Disks can be added or removed from the file system by InServ without the file system’s noticing the change.
This feature can reduce the number of drives needed by hosts, or at least delay the purchase of disks until they are really needed.