Questions regarding VSAN component placement.

[dongjh]

Hello,

I have 3 questions, can anybody give me the answers ? Thanks.

1.If 'Number of Failures to Tolerate' = 1, and 'Number of Disk Stripes' = 1, can a VSAN object be placed on more than one hosts?

2.If 'Number of Failures to Tolerate' > 1, and 'Number of Disk Stripes' > 1, can the components of a replica be placed on more than one hosts?

3.If 'Number of Failures to Tolerate' > 1, and 'Number of Disk Stripes' = 1, can the components of a replica be placed on more than one hosts?

[TheBobkin]

Hello dongjh​,

1. Assuming we are talking about 'normal' Objects (e.g. NOT a 12TB vmdk on a 6-node cluster with only 10TB per node) Object will be placed like so: 1st data-replica on one host, 2nd data-replica on another host, witness component on another host.

2.+3. Assuming you are talking FTT=2 with RAID1 (which requires a minimum of 5 nodes/Fault-Domains for component placement) then this will be placed like so: 1st data-replica on one host, 2nd data-replica on another host, 3rd data-replica on another host,  1st witness component on another host, 2nd witness component on another host. If using Stripe-Width>1 it will aim to stripe each data-replica across that many number of disks on the same node/Fault-domain (e.g. Stripe-Width=3 would be 3 RAID-0 sub-components per data-replica spread over 3 disks on one node/Fault-Domain).

Bob

[dongjh]

Thank you for your reply.

Assuming there's a big object(suppose the object is mirrored and striped), the size of one replica is larger than the free space of any host.,  but the size of one component(stripe) is smaller than the free space of any host, will VSAN put different components(stripes) of a replica on different hosts ?

Another word, does VSAN hold components like this: if a host have enough space to hold one replica of a object, the whole replica will stay on one host(no matter if the object is striped or not), however, if one host does not have enough free disk space to hold a striped replica. it will place some components of the replica to other hosts ? I.e.  VSAN will do it's best to hold the whole replica on one host ?

Of course any two components of different replicas can not be placed on one host.

[dongjh]

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[dongjh]

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[vpradeep01 · Accepted solution]

Hello,

Please find the answer:

Q

Assuming there's a big object(suppose the object is mirrored and striped), the size of one replica is larger than the free space of any host.,  but the size of one component(stripe) is smaller than the free space of any host, will VSAN put different components(stripes) of a replica on different hosts

Yes, this should work.

Ex:

4 Node vSAN cluster

FTT 1; SW 1

VMDK 200 GB

Component C1 with 200 GB will be placed on host 01

Component C2 would be further broken into two LSOM components and will be placed on host 02 - 100 GB and Host 03 - 100 GB since we do not have one replica space of 200 GB on rest of the hosts.

Host 04 would have witness component.

Thanks