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Replication of ZFS that is used for VM storage

Western Digital Drives - The Preferred Drives of FreeNAS and TrueNAS CORE

mephisto

Dabbler
Joined
Aug 3, 2012
Messages
20
Hey guys,

I'm wondering how ZFS replication works as I'm not sure it works well for a storage node that runs VMs. I believe ZFS snapshot would be equivalent to an unexpected shutdown of the VM data being replicated as the OS and the hypervisor are not aware of it and in consequence does not flush any cache writes or trigger VSS in Windows for example (very important for consistent backups).

Promox also uses ZFS, so I'm wondering how different their VM replication is from Freenas ZFS replication and how does it apply to the consistency of the replicated data inside the VM?
 

RickH

Explorer
Joined
Oct 31, 2014
Messages
61
Hey guys,

I'm wondering how ZFS replication works as I'm not sure it works well for a storage node that runs VMs. I believe ZFS snapshot would be equivalent to an unexpected shutdown of the VM data being replicated as the OS and the hypervisor are not aware of it and in consequence does not flush any cache writes or trigger VSS in Windows for example (very important for consistent backups).

You're absolutely correct! Recovering a VM from a ZFS snapshot (either on the same machine or a replicated snapshot) taken with a hypervisor that is 'unaware' is pretty much the same as powering back on after an unexpected shutdown... If you're using VMware as your hypervisor, FreeNAS includes tools to notify ESXi of the snapshot (referred to as 'VMware Sync' when you're configuring the snapshot task) that will correctly flush the write caches before the snapshot is taken (VMware refers to this as a 'quiesed' backup). I'm not aware of any similar tools for other hypervisors, but I only use FreeNAS with ESXi so I may be incorrect...

I'm not familiar with Proxmox, but I would assume there is some task that runs prior to the ZFS snapshot that performs a similar task.
 

mephisto

Dabbler
Joined
Aug 3, 2012
Messages
20
I'll check on proxmox forum for that information, I guess their hypervisor must do something similar, otherwise all their backups would be at risk of being corrupted. thanks for your help :)
 
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