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Two raidz2 vdevs, Different Sizes, Added at Different Times

Joined
Oct 9, 2019
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#1
I am currently in the planning phases on how I should create the zpool on a new server I'm building. This new server will have 6x10TB in a raidz2. I have an older server that has 8x3TB in a traditional raid5 (~15TB used). My plan is to setup the new server and copy all the data (over 10Gb) from the 8x3TB raid5 to the 6x10TB raidz2. Once the copy is complete, I'd like to move the original 8x3TB into the new server as another raidz2 vdev. Ideally, I would love to expand the initial zpool created.

The first 15TB would obviously be all stored on the 6x10TB raidz2 vdev. For the sake of easy math, 40TB of raw space minus 15TB from the initial copy would leave 25TB of free space. Adding the second raidz2 vdev (8x3TB) would expand that pool by 18TB, resulting in 43TB raw available space. At this point, obviously my data is unbalanced but it was going to be anyway due to the different sized raidz2s.

My question(s) are whether I should be concerned about this? After the initial data copy, the two raidz2s will be closer in free space (25TB vs 18TB instead of 40TB vs 18TB). Is it correct to assume that new writes will balance better? Would I be better off keeping the vdevs separated into their own pools? Any gotchas I seem to be missing here?

Any guidance or advice would be much appreciated!
 

garm

FreeNAS Expert
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#2
It will impact performance as the drives differ en quality, the vdevs have different performance and the data is written to a single vdev. You can mitigate the last point by copying the data to a new dataset once both vdevs are in place. That will spread the data a bit more even although the old vdev will probably end up with a bigger piece
 
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#3
It would only affect performance when accessing the original data copy, correct? Most of that will be archived data and not regularly accessed so I'm not entirely concerned about performance for the first 15TB from the original copy. New writes after the two vdevs are combined would be spread more evenly and benefit from that, yea?
 

Chris Tobey

FreeNAS Aware
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Feb 11, 2014
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#4
I have a similar question to this. I have a current pool with 6x8TB RAID-Z2 vdev that is at 85%. I have 6x3TB that I want to use to extend that pool with a second vdev. When I went to do this I get this warning:
1570807505913.png

My understanding of ZFS would lead me to believe that the same size and type is only a recommendation, not a requirement.

Is there any problem with adding a 6x3TB vdev to a pool with a 6x8TB vdev?
 

Jailer

Not strong, but bad
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Sep 12, 2014
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#5
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