The following example shows RAID 5 volume that initially consisted of four disks (slices). A fifth disk has been dynamically concatenated to the metadevice to expand it.
The parity areas are allocated when the initial RAID 5 volume 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 device, 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 RAID 5 volumes are not suited for long-term use. Use a concatenated RAID 5 volume until it is possible to reconfigure a larger RAID 5 volume and copy the data to the larger volume.