Wednesday, November 16, 2011

NetApp Insights: Sequential Reads

In the spirit of customizing NetApp technology for your workload and needs, one thing to talk about is sequential reads. The vast majority of customers don't have to worry about this because WAFL is designed to meet 99% of normal use cases, but in some unique situations it pays to optimize.

 For example, if your workload is 20 minutes of random writes followed by 20 minutes of sequential reads, and you don’t use any reallocate technology, you will likely see declining performance on your NetApp system over time. Who has this workload?  Basically nobody.  But for less extreme cases, you may find performance boosts through customizing your volume for sequential reads like this guy, who got a 6% throughput increase.  

Here are a couple steps you can take:

  • set vol option read_realloc.
  • option wafl.optimize-write-once=false. 
  • schedule a regular reallocate scan.

About read reallocation (Quote)

    For workloads that perform a mixture of random writes and large and multiple sequential reads, read reallocation improves the file's layout and sequential read performance. When you enable read reallocation, Data ONTAP analyzes the parts of the file that are read sequentially. If the associated blocks are not already largely contiguous, Data ONTAP updates the file's layout by rewriting those blocks to another location on disk. The rewrite improves the file's layout, thus improving the sequential read performance the next time that section of the file is read. However, read reallocation might result in a higher load on the storage system. 

   Also, unless you set vol options vol-name read_realloc to space_optimized, read reallocation might result in more storage use if Snapshot copies are used. If you want to enable read reallocation but storage space is a concern, you can enable read reallocation on FlexVol volumes by setting vol options vol-name read_realloc to space_optimized (instead of on). Setting the option to space_optimized conserves space but results in degraded read performance through the Snapshot copies. Therefore, if fast read performance through Snapshot copies is a high priority to you, do not use space_optimized. 

   Read reallocation might conflict with deduplication by adding new blocks that were previously consolidated during the deduplication process. A deduplication scan might also consolidate blocks that were previously rearranged by the read reallocation process, thus separating chains of blocks that were sequentially laid out on disk. Therefore, since read reallocation does not predictably improve the file layout and the sequential read performance when used on deduplicated volumes, performing read reallocation on deduplicated volumes is not recommended. Instead, for files to benefit from read reallocation, they should be stored on volumes that are not enabled for deduplication. The read reallocation function is not supported on FlexCache volumes. 

If file fragmentation is a concern, enable the read reallocation function on the original server volume.  (/Quote)

More reading here.  Interesting quote from this additional reading: "For write-intensive, high-performance workloads we recommend leaving available approximately 10% of the usable space for this optimization process."  Luckily that 10% includes any white space, including unused space carved out for thick provisioned LUN's.

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