vSAN Stretched Cluster: parity with RAID-5

[KevinDS95]

Hi everyone,

I have a question in vSAN Stretched cluster. If I have a RAID-5 policy with dual site mirroring and 4 hosts per site. Is there 8 parity ? 4 for primary fault domain (i.e AZ1) and 4 parity for second fault domain (i.e AZ2) ? And metadata in Witness ? I understood if there is no witness but I'm confused is there is witness because I think I don't understand really the role of witness in term of parity/metadata because I have found 2 VMware guides with different explanation.

First, this link explain we have 1 parity per host (chapter Data Placement)

https://core.vmware.com/resource/vmware-vsan-design-guide

Second link, explain we have 1 parity for all 4 hosts (chapter Host number calculation):

https://core.vmware.com/resource/vsan-stretched-cluster-guide

And I have another question please (I don't find the answer in Google). If I have the RAID-5 policy with 8 hosts per site for example, is there data spread into all hosts or only for the minimum prerequisites hosts (4) ?

Thank you
Kevin

[depping · Accepted solution]

The way RAID-5 Erasure Coding works is that you have 3 data blocks and 1 parity block, so 4 in total. If you stretch an object you will have an X+X configuration with a witness component in the third site. The witness component is used to determine who has the most up to date version of the data etc, basically a quorum mechanism. So the witness in this case doesn't hold any parity, the two sites contain Data+Parity.

Same applies to RAID-6, but with RAID-6 you would need 6 hosts in each location and you would have 2 parity blocks instead of 1.

[KevinDS95]

Thank you for your help.

I better understand how witness work