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vSAN performance enhancements delivered with vSphere 6.0 Update 3 and vSphere 6.5.0d(vSAN 6.6) (2149127)

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Symptoms

Virtual machine guest I/O performance may deteriorate:
  • During heavy write-intensive workloads
  • During large file delete operations while data services such as Deduplication and Compression is enabled on a vSAN enabled Cluster

Purpose

This article outlines performance enhancements introduced in vSAN.

Starting with vSphere 6.0 Update 3 and vSphere 6.5.0d (vSAN 6.6), multiple issues are addressed to optimize vSAN I/O path for improved performance. 


Cause

  • In vSAN, every I/O operation is logged before being processed. Without efficient log management in place, the size of the log entries can grow very quickly. When the log size approaches the max allocated size limit (16GB), this triggers unnecessary I/O operations to process the entries. This may impact the performance of the cluster. There are various determining factors in play to determine the actual impact, such as the current health of the cluster, pending I/Os and other metrics.

  • Deletes in vSAN cluster where data services are turned on need to be managed to maintain object consistency. The I/O processes involved in processing the delete operations are maintained in a log based system. As such the log can grow very large very quickly (even with enhanced log packing). This is especially true in scenarios where multiple files spanning several terabytes in size are being deleted.

Resolution

To address the performance issues, the following enhancements are introduced: 
  • More logs much less space
    vSAN now has efficient log management strategies that allows more logging to be packed per byte of storage. This prevents the log from reaching its assigned limit too fast and too frequently. It also provides enough time for vSAN to process the log entries before it reaches it’s assigned limit thereby avoiding unnecessary I/O operations

  • Pre-emptive de-staging for large file deletes in scenarios where data services are enabled
    vSAN has built in algorithms that de-stages data on periodic basis. The de-staging operations coupled with efficient log management significantly improves performance for large file deletes including performance for write intensive workloads

  • Checksum  Improvements
    vSAN has several enhancements that made the checksum code path more efficient. These changes are expected to be extremely beneficial and make a significant impact on all flash configurations, as there is no additional read cache look up (all reads occur in capacity tier). These enhancements are expected to provide significant performance benefits for both sequential and random workloads.

Additional Information

For more information about the changes, refer to the ESXi 6.0 Update 3 Release Notes.

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