Overview of RAID-5 Volumes
RAID level 5 is similar to striping, but with parity data distributed across
all components (disk or logical volume). If a component fails, the data on
the failed component can be rebuilt from the distributed data and parity information
on the other components. In Solaris Volume Manager, a RAID-5 volume is a
volume that supports RAID level 5.
A RAID-5 volume uses storage capacity equivalent to one component in the volume
to store redundant information (parity). This parity information contains information about user data
stored on the remainder of the RAID-5 volume's components. That is, if you
have three components, the equivalent of one component is used for the parity
information. If you have five components, then the equivalent of one component is
used for parity information. The parity information is distributed across all components in
the volume. Similar to a mirror, a RAID-5 volume increases data availability, but
with a minimum of cost in terms of hardware and only a moderate
penalty for write operations. However, you cannot use a RAID-5 volume for the
root (/), /usr, and swap file systems, or for other existing file systems.
Solaris Volume Manager automatically resynchronizes a RAID-5 volume when you replace an existing
component. Solaris Volume Manager also resynchronizes RAID-5 volumes during rebooting if a system
failure or panic took place.
Example—RAID-5 Volume
Figure 14-1 illustrates a RAID-5 volume that consists of four disks (components).
The first three data segments are written to Component A (interlace 1), Component
B (interlace 2), and Component C (interlace 3). The next data segment that
is written is a parity segment. This parity segment is written to Component
D (P 1–3). This segment consists of an exclusive OR of the first
three segments of data. The next three data segments are written to Component
A (interlace 4), Component B (interlace 5), and Component D (interlace 6). Then,
another parity segment is written to Component C (P 4–6).
This pattern of writing data and parity segments results in both data and
parity being spread across all disks in the RAID-5 volume. Each drive can
be read independently. The parity protects against a single disk failure. If each
disk in this example were 2 Gbytes, the total capacity of the
RAID-5 volume would be 6 Gbytes. One drive's worth of space is allocated
to parity.
Figure 14-1 RAID-5 Volume Example
Example—Concatenated (Expanded) RAID-5 Volume
The following figure shows an example of an RAID-5 volume that initially consisted
of four disks (components). A fifth disk has been dynamically concatenated to the
volume to expand the RAID-5 volume.
Figure 14-2 Expanded RAID-5 Volume Example
The parity areas are allocated when the initial RAID-5 volume is created. One
component's worth of space is allocated to parity, although the actual parity blocks
are distributed across all of the original components to distribute I/O. When additional
components are concatenated to the RAID-5 volume, the additional space is devoted entirely
to data. No new parity blocks are allocated. The data on the concatenated
component is, however, included in the parity calculations, so the data 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. Then, copy
the data to the larger volume.
Note - When you add a new component to a RAID-5 volume, Solaris Volume
Manager “zeros” all the blocks in that component. This process ensures that the
parity protects the new data. As data is written to the additional space,
Solaris Volume Manager includes the data in the parity calculations.
