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Sizing a vSAN Stretched Cluster

Duncan Epping · May 30, 2017 ·

I have had this question a couple of times already, how many hosts do I need per site when the Primary FTT is set to 1 and the Secondary FTT is set to 1 and RAID-5 is used as the Failure Tolerance Method? The answer is straight forward, you have a local RAID-5 set locally in each site. RAID-5 is a 3+1 configuration, meaning 3 data blocks and 1 parity block. As such each site will need 4 hosts at a minimum. So if the requirement is PFTT=1 and SFTT=1 with the Failure Tolerance Method (FTM) set to RAID-5 then the vSAN Stretched Clustering configuration will be: 4+4+1. Note, that also when you use RAID-1 you will need at minimum 3 hosts per site. This because locally you will have 2 “data” components and 1 witness component.

From a capacity stance, if you have a 100GB VM and do PFTT=1, SFTT=1 and FTM set to RAID-1 then you have a local RAID-1 set in each site. Which means 100GB requires 200GB in each location. So 200% required local capacity, 400% for the total cluster. Using the below table you can easily see the overhead. Note that RAID-5 and RAID-6 are only available when using all-flash.

I created a quick table to help those going through this exercise. I did not include “FTT=3” as this in practice is not used too often in stretched configurations.

DescriptionPFTTSFTTFTMHosts per siteStretched ConfigSingle site capacityTotal cluster capacity
Standard Stretched across locations with local protection11RAID-133+3+1200% of VM400% of VM
Standard Stretched across locations with local RAID-511RAID-544+4+1133% of VM266% of VM
Standard Stretched across locations with local RAID-612RAID-666+6+1150% of VM300% of VM
Standard Stretched across locations no local protection10RAID-111+1+1100% of VM200% of VM
Not stretched, only local RAID-101RAID-13n/a200% of VMn/a
Not stretched, only local RAID-501RAID-54n/a133% of VMn/a
Not stretched, only local RAID-602RAID-66n/a150% of VMn/a

Hope this helps!

Related

Server, Software Defined, Storage, vSAN 6.6, sizing, stretched cluster, virtual san, vsan

Reader Interactions

Comments

  1. Wojciech Marusiak says

    2 June, 2017 at 17:29

    Thanks Duncan.

  2. Phil H says

    7 June, 2017 at 16:39

    Do you have any feedback on how a 2+2+1 stretched cluster would fit into this? You have a 1+1+1 but not 2+2+1.

    • Duncan Epping says

      13 June, 2017 at 09:33

      The article provides the minimum node count for a certain policy. With 2+2+1 you can do:
      PFTT = 1
      SFTT = 0

      As for SFTT=1 you need 3+3+1 as the table indicates

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About the author

Duncan Epping is a Chief Technologist in the Office of CTO of the Cloud Platform BU at VMware. He is a VCDX (# 007), the author of the "vSAN Deep Dive", the “vSphere Clustering Technical Deep Dive” series, and the host of the "Unexplored Territory" podcast.

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