Example -- Concatenated (Expanded) RAID5 Metadevice

Figure 2-7 shows an example of an RAID5 metadevice that initially consisted of four disks (slices). A fifth disk has been dynamically concatenated to the metadevice to expand it.

Figure 2-7 Expanded RAID 5 Metadevice Example

The parity areas are allocated when the initial RAID5 metadevice is created. One column's (slice's) worth of space is allocated to parity, although the actual parity blocks are distributed across all of the original columns to avoid hot spots. When you concatenate additional slices to the RAID, the additional space is devoted entirely to data; no new parity blocks are allocated. The data on the concatenated slices is, however, included in the parity calculations, so it is protected against single device failures.

Concatenated RAID5 metadevices are not suited for long-term use. Use a concatenated RAID5 metadevice unitl it is possible to reconfigure a larger RAID5 metadevice and copy the data to the larger metadevice.

When you add a new slice to a RAID5 metadevice, DiskSuite "zeros" all the blocks in that slice. This ensures that the parity will protect the new data. As data is written to the additional space, DiskSuite includes it in the parity calculations.