RAID Read-Ahead

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The Underlying Storage System

	operations involve a very high rate of small writes to the metadata disk,
	so disk latency is the critical performance factor. Write-back caching can
	be an effective approach to minimizing I/O latency and optimizing
	metadata operations throughput. This is easily observed in the hourly
	File System Manager (FSM) statistics reports in the cvlog file. For
	example, here is a message line from the cvlog file:
	PIO HiPriWr SUMMARY SnmsMetaDisk0 sysavg/350 sysmin/333 sysmax/
	367
	This statistics message reports average, minimum, and maximum write
	latency (in microseconds) for the reporting period. If the observed
	average latency exceeds 500 microseconds, peak metadata operation
	throughput will be degraded. For example, create operations may be
	around 2000 per second when metadata disk latency is below 500
	microseconds. However, if metadata disk latency is around 5
	milliseconds, create operations per second may be degraded to 200 or
	worse.
	Another typical write caching approach is a “write-through.” This
	approach involves synchronous writes to the physical disk before
	returning a successful reply for the I/O operation. The write-through
	approach exhibits much worse latency than write-back caching;
	therefore, small I/O performance (such as metadata operations) is
	severely impacted. It is important to determine which write caching
	approach is employed, because the performance observed will differ
	greatly for small write I/O operations.
	In some cases, large write I/O operations can also benefit from caching.
	However, some SNFS customers observe maximum large I/O throughput
	by disabling caching. While this may be beneficial for special large I/O
	scenarios, it severely degrades small I/O performance; therefore, it is
	suboptimal for general-purpose file system performance.

RAID Read-Ahead	RAID read-ahead caching is a very effective way to improve sequential
Caching	read performance for both small (buffered) and large (DMA) I/O
	operations. When this setting is utilized, the RAID controller pre-fetches

	disk blocks for sequential read operations. Therefore, subsequent
	application read operations benefit from cache speed throughput,
	which is faster than the physical disk throughput.
	This is particularly important for concurrent file streams and mixed I/O
	streams, because read-ahead significantly reduces disk head movement
	that otherwise severely impacts performance.

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Caching