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StorNext File System Tuning
The Underlying Storage System
StorNext File System Tuning Guide 2
RAID Cache
Configuration 0
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.
RAID Write-Back Caching0
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
metadata operations throughput. This is easily observed in the hourly