RAID Level, Segment Size, and Stripe Size
StorNext File System Tuning
The Underlying Storage System
unusual cases, it severely degrades typical scenarios. Therefore, it is unsuitable for most environments.
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
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. 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.
It can be useful to use a tool such as lmdd to help determine the storage system performance characteristics and choose optimal settings. For example, varying the stripe size and running lmdd with a range of I/O sizes might be useful to determine an optimal stripe size multiple to configure the SNFS StripeBreadth.
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