Zvol replication and slowly making progress :)

[jms123]

So I have been trying to work out why my zvol will not replicate to a remote TrueNAS due to out of space issues and I think I have worked out why it is doing it but not the reasoning behind it or if there is a way round it.

So firstly my zvol uses 80% of the pool in a test environment which I know is not recommended and it should be 50% because of write performance but I am wondering if there is another reason for the 50% rule ?

pool size 899 GB
zvol size 721 GB

80% of pool = 179.8GB

As long as the amount of actual data within the zvol amounts to less than 179.8GB it will replicate but if I go over as in -

Code:

root@truenas[~]# zfs list -o space TestVPS-Pool/TestVPS-Zvol
NAME                       AVAIL   USED  USEDSNAP  USEDDS  USEDREFRESERV  USEDCHILD
TestVPS-Pool/TestVPS-Zvol   721G   721G        0B    180G           544G         0B

then it reports an out of space error and will not replicate.

I don't understand why this is the case. The actual data is simply streamed between the primary and backup servers as far as I understand it and the snapshot is very small (in the KB or MB) so why will it not replicate. What does it think it needs the space for ?

Obviously if I only used 50% of the pool for the zvol then based on the above it looks like it would replicate but all of the recommendations for 50% are for performance and replication is never mentioned.

Does anyone know why it does this and if there is way to override it ?

 

[Arwen]

I don't have any specific answers, zVols are a bit of magic to me when it comes to space usage.

But, their is an interaction with the zVol block size, verses the Pool ashift size, verses any RAID-Zx stripe width.

So, please post the following
zfs get volblocksize SOURCE_POOL/ZVOL zpool status SOURCE_POOL zpool status DESTINATION_POOL zpool get ashift SOURCE_POOL zpool get ashift DESTINATION_POOL zfs list SOURCE_POOL zfs list DESTINATION POOL

Someone else may be able to go on from here

 

[Patrick M. Hausen]

How much space is there on the remote TrueNAS?