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High value of file system overhead observed in the vSAN capacity breakdown with dedup enabled (2150999)

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Symptoms

  • For ESXi hosts in a vSAN Cluster with deduplication and compression enabled, there is a large file system overhead reported by the vSAN capacity monitor.
  • The Used Capacity Breakdown (Monitor > vSAN > Capacity) for a vSAN cluster incorrectly displays the percentage of storage capacity used for file system overhead. This number does not reflect the actual capacity being used for file system activities.

Cause

Starting with vSAN 6.6, the vSAN platform includes file system overhead in the logical usage of the disk. The file system overhead is calculated as fixed 1.2% of the logical capacity, which is shown even for a cluster with no usage at all. This is a notional number being introduced in the platform to account for the space to be reserved for system to correctly operate in both dedup and non-dedup cases.

In case of non-dedup clusters, this appears very small because the logical and physical capacities are the same. Therefore, the usage will be 1.2% of the total usage and is negligible. However, in dedup clusters, as there is a 10x bloat of logical capacity, 1.2% of the logical capacity is comparatively large. Therefore, for an unused or minimally used cluster, it would appear large. Again, this being a notional number, no real capacity is actually reserved or in use in the cluster, it is only reported.

 

Resolution

This issue is resolved in ESXi 6.5 Update 1 (vSAN 6.6.1), available at VMware Downloads.

The File System overhead is now reported accurately in the Used Capacity Breakdown.

Tags

vSAN Capacity Breakdown, High value, File System Overhead, dedup

Update History

7/27/2017-Updated 6.5 U1 download link in Resolution

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