Quantum 6-01376-05 manual RAID Read-Ahead Caching

RAID Read-Ahead Caching

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The Underlying Storage System

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 caching is a very effective way to improve sequential 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.

While read-ahead caching improves sequential read performance, it does not help random performance. Furthermore, some SNFS customers actually observe maximum large sequential read throughput by disabling caching. While disabling read-ahead is beneficial in these unusual cases,

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