vSAN Memory Overhead

If you have been reading more on sizing your vShpere Clusters with vSAN storage, I am sure you would have factored all the sizing aspects from Compute Perspective (CPU and Mem overcommitment ratios, HA Admission Control etc) and Storage Perspective (Disk Groups, No of Disks, FTT etc)

One of the common questions I get from the customers is Mem overhead on ESXi hosts which are a part of vSAN enabled cluster.

If you have come across Hardware Requirements for vSAN page, you would have observed Memory Requirements depends on number of diskgroups and number of devices which are managed by an ESXi host

Memory Requirements for vSAN
To calculate vSAN memory consumption in SAN 6.2 (ESXi 6.0 Update 3 and later) and vSAN 6.6 (ESXi 6.5.0d and later), use this equation:
BaseConsumption +
(NumDiskGroups * (DiskGroupBaseConsumption + (SSDMemOverheadPerGB * SSDSize))) +
(NumCapacityDisks * CapacityDiskBaseConsumption)
  • BaseConsumption: is the fixed amount of memory consumed by vSAN per ESXi host.
  • NumDiskGroups: is the number of disk groups in the host, this value should range from 1 to 5.
  • DiskGroupBaseConsumption: is the fixed amount of memory allocated to each individual disk group in the host. This is mainly used to allocate resources used to support inflight operations on a per disk group level.
  • SSDMemOverheadPerGB: is the amount of memory allocated per GB of SSD.
  • SSDSize: is the size of the SSD disk in GB.
  • NumCapacityDisks: is the number of capacity disks in the host (across all the diskgroups).
  • CapacityDiskBaseConsumption: is the amount of memory allocated per capacity disk.

Note: The size of the SSD for all flash is capped at 600GB, and thus using SSDs larger than 600 GB will not consume additional memory.

  • BaseConsumption = 5426 MB
  • DiskGroupBaseConsumption = 636 MB
  • SSDMemOverheadPerGB (hydrid) = 8 MB
  • SSDMemOverheadPerGB (allflash) = 14 MB
  • CapacityDiskBaseConsumption= 70 MB
For Example: Three disk groups per host, all flash configuration:
BaseConsumption +
(NumDiskGroups * (DiskGroupBaseConsumption + (SSDMemOverheadPerGB * SSDSize))) +
(NumCapacityDisks * CapacityDiskBaseConsumption)
5426 MB + (3 * (636 MB + (14MB * 600))) + (9 * 70 MB)
33164 MB

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Christie Borges

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Why you ase 600GB as SSDsize, is it refers to capacity of cacheSSD or CapacitySSD