VSAN inqury

[ChevUribe]

Based on the VSAN default storage policy, FTT = 1 so it means, every VM with applied with this policy will have another copy in the datastore for HA.

1.What if the original VM is deleted, will the secondary copy of the VM will also be deleted?

If not, how do i verify who is the secondary copy of the VM?

2. What if the secondary VM is deleted? Will VSAN automatically recreate another secondary VM?

3. If any instance, the original is renamed or moved, the secondary copy will still be the same?

[crosdorff]

Hi,

FTT set to 1 means that 2 copies of each object are stored on disk.

You can't remove the copies separate.

If you remove the VM, all objects, and so all copies, are removed.

[ChevUribe]

what if the copy is the one only deleted? will VSAN has a capability of error checking that a VM has no copy and then will trigger a recreate copy?

[Deeban]

In case of Default policy i.e. HFTT 1, SW 1 there are two data components and a witness component for each object.

If one of the data components is ABSENT due to a host down (Planned or Unplanned), VSAN waits for a default of 60 minutes before creating a new mirror for that component.

If the ABSENT component comes back within this 60 mins, VSAN will not create any new component, but it will do a resync if the recovered component is not up-to-date.

In case of a disk failure, VSAN will mark the component as "DEGRADED" and immediately create a new mirror copy provided there are sufficient resources to store this new copy.

Checkout a nice write-up on this by Duncan Epping at How VSAN handles a disk or host failure

[ChevUribe]

Thanks for that!

Lets say VM1 has already a copy by VSAN FTT=1 policy then the host where the copy is residing fails, if it took 60 minutes, VSAN created a new copy. What will happen if after 60 minutes the host where the original came back online; where does the original VM sync? to the original copy or the newly created copy?

Then from that scenario 3 instances is present. the original VM1 and the 2 VM1 copy? will VSAN delete one of them?

[Deeban]

Hi,

If I understand your question correct, " What happens when the old component comes back online when the resync for the new component is going on?"

VSAN will resync the recovered component as well and bring it back to active state. But this old component will be eventually deleted once the newly created component has been fully resynced.

Please refer : http://cormachogan.com/2015/10/28/vsan-resync-behaviour-when-failed-component-recovers/

Point 3, answers your question.

[elerium]

In my experience in VSAN 6.0-6.2, the following will happen if a copy disappears for 60 minutes and then returns later:

example: VM with FTT=1, 1TB VMDK

a host is left in maintenance mode for 60 minutes and components for this VM go absent

resync occurs at the 60 min mark, lose 1TB of disk space as the new copy is resynced across the cluster

maintenance mode host, exits maintenance mode and rejoins VSAN cluster

the resync across the cluster will take time to finish and then shortly after, the copy of the data from the maintenance mode host (that has exited maintenance mode by this time) will be deleted.

In this scenario, for a short period you will end up with 3 copies of the data, this can be problematic if the VMDKs are very large or you are close to running out of space. They mentioned that they may change this behavior in the future but it hasn't happened yet to reduce rebuild times from events like this, but I haven't seen it happen yet.

[ChevUribe]

thanks for that shared experience!