Quantum 6-01376-05 User Manual

  StorNext File System Tuning

The Underlying Storage System

StorNext File System Tuning Guide

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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. 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.

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Write-back caching dramatically reduces latency in small write 

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 

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