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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 unaffected.
  • 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.

For more information, see the book Oracle 11g Grid and Real Application Clusters 30% off if you buy it directly from Rampant TechPress . 

Written by top Oracle experts, this RAC book has a complete online code depot with ready to use RAC scripts.





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