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StorNext File System Tuning
The Underlying Storage System
4 StorNext File System Tuning Guide
While read-ahead caching improves sequential read performance, it
does not help highly transactional 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, it severely degrades typical scenarios.
Therefore, it is unsuitable for most environments.
RAID Level, Segment
Size, and Stripe Size
Configuration settings such as RAID level, segment size, and stripe size
are very important and
cannot be changed after put into production
, so
it is critical to determine appropriate settings during initial
configuration.
The best RAID level to use for high I/O throughput is usually RAID5. The
stripe size is determined by the product of the number of disks in the
RAID group and the segment size. For example, a 4+1 RAID5 group
with 64K segment size results in a 256K stripe size. The stripe size is a
very critical factor for write performance because I/Os smaller than the
stripe size may incur a read/modify/write penalty. It is best to configure
RAID5 settings with no more than 512K stripe size to avoid the read/
modify/write penalty. The read/modify/write penalty is most noticeable
in the absence of “write-back” caching being performed by the RAID
controller.
The RAID stripe size configuration should typically match the SNFS
StripeBreadth
configuration setting when multiple LUNs are utilized in a
stripe group. However, in some cases it might be optimal to configure
the
SNFS StripeBreadth as a multiple of the RAID stripe size, such as
when the RAID stripe size is small but the user's I/O sizes are very large.
However, this will be suboptimal for small I/O performance, so may not
be suitable for general purpose usage.
RAID1 mirroring is the best RAID level for metadata and journal storage
because it is most optimal for very small I/O sizes. Quantum
recommends using fibre channel or SAS disks (as opposed to SATA) for
metadata and journal due to the higher IOPS performance and
reliability.
It is also very important to allocate entire physical disks for
the Metadata and Journal LUNs in ordep to avoid bandwidth contention
with other I/O traffic. Metadata and Journal storage requires very high
IOPS rates (low latency) for optimal performance, so contention can
severely impact IOPS (and latency) and thus overall performance. If
Journal I/O exceeds 1ms average latency, you will observe significant
performance degradation.