Quantum 3.1.4.1 manual RAID Cache Configuration, RAID Write-Back, Caching

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RAID Cache

Configuration

The single most important RAID tuning component is the cache configuration. This is particularly true for small I/O operations. Contemporary RAID systems such as the EMC CX series and the various Engenio systems provide excellent small I/O performance with properly tuned caching. So, for the best general purpose performance characteristics, it is crucial to utilize the RAID system caching as fully as possible.

For example, write-back caching is absolutely essential for metadata stripe groups to achieve high metadata operations throughput.

However, there are a few drawbacks to consider as well. For example, read-ahead caching improves sequential read performance but might reduce random performance. Write-back caching is critical for small write performance but may limit peak large I/O throughput.

Caution: Some RAID systems cannot safely support write-back caching without risk of data loss, which is not suitable for critical data such as file system metadata.

	Consequently, this is an area that requires an understanding of
	application I/O requirements. As a general rule, RAID system caching is
	critically important for most applications, so it is the first place to focus
	tuning attention.
RAID Write-Back	Write-back caching dramatically reduces latency in small write
Caching	operations. This is accomplished by returning a successful reply as soon
	as data is written into cache, and then deferring the operation of
	actually writing the data to the physical disks. This results in a great
	performance improvement for small I/O operations.
	Many contemporary RAID systems protect against write-back cache data
	loss due to power or component failure. This is accomplished through
	various techniques including redundancy, battery backup, battery-
	backed memory, and controller mirroring. To prevent data corruption, it
	is important to ensure that these systems are working properly. It is
	particularly catastrophic if file system metadata is corrupted, because
	complete file system loss could result. Check with your RAID vendor to
	make sure that write-back caching is safe to use.
	Minimal I/O latency is critically important for metadata stripe groups to
	achieve high metadata operations throughput. This is because metadata

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