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Unraid as TrueNas Scale VMs backup server?


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Currently with a ZFS plugin I have created ZFS pool mirror from 2 x SSD (I guess I would be able to import that as native ZFS pool after 6.12). I have another server (main) with TrueNas Scale in other location and running there couple of VMs. TrueNas creates their VM disks as ZVOL as far as I know. I am creating daily snapshots independently for each VM (snapshots of the whole zvols). I would like to  use 'zfs send | zfs recv' to replicate these snapshots as backup from TrueNas to Unraid. I think there should be no issue with that with a plugin installed (correct me if I am wrong).


Since, both Unraid and TrueNas Scale uses KVM, I was wondering if I could use Unraid VMs as temporary/backup VMs in case when there is some hardware failure on the main server - probably just for couple of hours? ZVOLs are block storages and I can see that they are being added as virtual block storage in /dev/zd(x). I can check which one is which by running 'ls -l /dev/zvol/<poolname>'. I have briefly tested that I could just add that block storage to the Ubuntu VM as:


    <disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='writeback'/>
      <source dev='/dev/zd0'/>
      <target dev='hdc' bus='sata'/>
      <boot order='1'/>
      <address type='drive' controller='0' bus='0' target='0' unit='2'/>


I would also add similar RAM/CPU resources as on TrueNas machine and I would already have a reverse-proxy in place to match CloudFlare/Domains (DNS) setup.


Couple of question here:

1. Should I use cache='writeback' or cache='none'?

2. Should I use bus='sata' or virtio?


After TrueNas VMs are operational again, I would manually move changes in databases (only timeseries data) from Unraid to TrueNas VM databases, so everything is up to date and I do not lose any data between the snapshot and the time when TrueNas machine failed. That way I do not have to deal with transferring snapshots from Unraid to TrueNas.


Is everything OK with this idea or should I be worried about something? The alternative would be to have an additional TrueNas server as fallback or some cloud hosting fallback architecture.

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