||Donald K. Burleson
Oracle RAC Tips
Software RAID and Hardware RAID
As we have already mentioned, the RAID operations can be performed
either in the host or within the disk sub-system. When RAID is done within
a host, usually with the help of volume manager software or by means of a
device drive, it is referred to as ‘software RAID’. When it is implemented
within the storage system it is said to be ‘hardware RAID’.
Some of the issues surrounding the software RAID implementation are:
- Issue of Portability – Since the RAID software implementation has
some O/S specific components, these components have to be different for
each operating system.
- RAID operations share the kernel mode components and may add to the
system CPU load. Software RAID uses more system resources, as more disk
ports and channels are required, and it is subject to additional loads
during write and copy operations.
- Software RAID is relatively complex. Creating several dozen
redundant performance volumes across several dozen hard drives results
in several hundred “configuration records” that describe the layout.
When we look at the hardware RAID, we see many advantages, such as:
- The RAID firmware is executed on a dedicated processor within the
disk subsystem, and therefore does not share the system’s CPU.
- It is portable across all the operating systems. In the event of a
malfunction in the RAID firmware, the host system continues to operate
and gives a suitable report on the RAID issue. At the same time, if the
crash occurs at the system level the storage system functioning is
- Many of the RAID solutions are equipped with battery backup modules
that allow them to maintain cache coherency and complete outstanding
operations without loss of data integrity.
- RAID controllers or storage processors are specialized for enhancing
performance. Auxiliary processors are dedicated to calculating the
parity of the data blocks that are being written to disk, while the main
embedded processor concurrently fetches and executes RAID code.
However, there is one situation where software RAID becomes very
useful. When we want to mirror the drives from two different storage
units, only software mirroring at the host level can do it. For example,
after drives from two storage units are placed under the control of the
Veritas volume manager at host level, we can create a volume by mirroring
(using the RAID-1 level) two disks originating from two different disk
systems. By this means, even if access to one of the storage units is
lost, the volume can still continue to function.
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