RAID 0 (Stripes and Concatenations)

RAID 0 devices allow you to use multiple slices or disks together as a single volume (containing a single filesystem) to provide additional space for data storage. Additionally, Enhanced Storage lets you expand a existing volume by adding additional slices.

Concatenations are RAID 0 devices that assume sequential access to data. For example, if you have a disk with three unused slices, you might combine the three into a single RAID 0 (concatentation) volume to make more efficient use of the space.

Stripes are RAID 0 devices that assume parallel access to data. This method can result in improved performance over either raw slices or concatenations, but only if the stripes are on separate drives and ideally also on separate controllers.

Neither stripes nor concatenations offer failure tolerance, so mirrors or RAID devices should be used if failure tolerance is a requirement.

Expanding Storage Capacity of Existing Filesystems

An additional use for RAID 0 devices is to expand the capacity of an existing volume or filesystem. Mounted or unmounted UFS file systems contained within a metadevice can be expanded without having to halt or back up your system. (Nevertheless, backing up your data is always a good idea.) After the metadevice is expanded, you grow the file system with the growfs(1M) command.

After a file system is expanded, it cannot be decreased. Decreasing the size of a file system is a UFS limitation.

Applications and databases using the raw metadevice must have their own method to "grow" the added space so that the application or database can recognize it. Enhanced Storage does not provide this capability.

You can expand the disk space in metadevices in the following ways:

Note that adding slices to a RAID 5 device provides additional capacity, but there is no fault tolerance in the extra space that was added, making this a poor long-term solution.