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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance, Quality and Reliability | Redundant Arrays of Inexpensive Disks (RAID) | RAID Configuration and Implementation | Advanced RAID Features ]

Caching

Caching is a technique that is used to buffer discrepancies between the performance of different hardware components. It is used throughout the PC: cache is found within system processors, on motherboards, within hard disks themselves, and many other places. In every case, the goal of the cache is the same: to provide a temporary storage area that allows a faster device to run without having to wait for a slower one. For more on the theory of caching and how it works in PCs, see this explanation.

Most advanced RAID controllers include on-board cache, which in many ways acts exactly the same way that the cache within a hard disk does: it improves performance to some extent by storing information that was recently used, or that the controller predicts will be used in the future, so it can be supplied to the system at high speed if requested instead of necessitating reads from the slow hard disk platters. Since a RAID controller turns an array of hard disks into one "virtual hard disk", putting cache on the controller is a natural enhancement. Typically this is implemented as a slot on the controller that takes a standard PC memory module; some controllers can take an amount of cache exceeding the total system memory on most regular PCs! While caching does improve performance, as with cache size in hard disks, don't overestimate the performance impact of increasing the size of the cache.

One area where caching can impact performance significantly is write caching, sometimes also called write-back caching. When enabled, on a write, the controller tells the system that the write is complete as soon as the write enters the controller's cache; the controller then "writes back" the data to the drives at a later time. As described in detail in this page on write caching, this improves performance but imposes the risk of data loss or inconsistency if, for example, the power to the system is cut off before the data in the cache can be "written back" to the disk platters.

Tip: To avoid potential problems with power failures and write-back caching, some controllers actually incorporate a built-in backup battery! This battery will allow any unwritten data in the cache to be retained for a period of time until the power is restored to the system. A very neat feature--though if the server is connected to a UPS, as it should be anyway, its value is debatable.

The reason that write-back caching is so important with RAID is that while writes are slightly slower than reads for a regular hard disk, for many RAID levels they are much slower. The cache insulates the system from the slowdowns inherent in doing writes to arrays such as those using RAID 5, which can improve performance substantially. The bigger the gap between read and write performance for a given RAID level, the more that array will benefit from using write caching. It is recommended for high-performance applications using striping with parity (though it will improve performance somewhat for all RAID levels.)

Next: Drive Swapping


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