ZPool with different sized vdevs - cons?

[Dantron]

What are all the drawbacks/concerns of having a ZPOOL with different sized vdevs?

I currently have two zpools

[zpool 1] == 8x8TB + 1TB ssd cache
[zpool 2] == 8x10TB +1TB ssd cache

I am going to try making a single zpool and was currious how big the cons are, in doing this. It would be much nicer to just have a single volume, instead of two.

 

[sretalla]

What are all the drawbacks/concerns of having a ZPOOL with different sized vdevs?

I presume you have RAIDZ2, so mostly it's just file shares or media storage for apps/jails.

No real performance hit (even after rebalancing) and maybe even a little better IOPS (but don't read too much into that as the uneven sizes will detract from it a bit).

Probably mostly up-side to it except for scrub and resilver times (which will double or more).

 

[Arwen]

I don't think re-silver times will double, as a disk replacement occurs only in the affected vDev. Of course, the ZFS code could be doing more than just a single vDev walk, so it would likely be longer than 2 separate pools. But, double, or longer? Not so sure.

But, I otherwise agree with @sretalla that it is mostly better to have 1 pool with 2 vDevs. (Unless you have very special configs, 1 pool uses Mirrored vDevs and the other uses RAID-Z2.)

 

[Dantron]

thanks everyone for input!

The pools will be blown away after I get it backed up to source. It will be significantly easier to manage with just one volume.

I currently am using Z1, so in the merged setup, Z2 will definitely work

 

[victort]

Out of curiosity, what are the advantages to having 1 pool with multiple vdevs vs multiple pools of a single vdev?

 

[Davvo]

Out of curiosity, what are the advantages to having 1 pool with multiple vdevs vs multiple pools of a single vdev?

ZFS Storage Pool Layout

This amazing document, created by iXsystems in February 2022 as a "White Paper", cleanly explains how to qualify pool performance touching briefly on how ZFS stores data and presents the advantages, performance and disadvantages of each pool...

www.truenas.com

Different pools may have different layouts.

 

[Arwen]

A single pool allows all the storage to be shared between the datasets or zVols. You can still set Quotas or Reservations on datasets or zVols when you want limits in place. A single pool with 2 vDevs also have double the IOPS, as a single vDev has the IOPS of a single disk.

But, as I said earlier, if their is a need for different vDev configurations, or types of storage media, (large pool with HDD, smaller pool, with Mirrored vDevs & SSDs), then obviously 2 pools would be better.

This is VERY user & use case specific. Some TrueNAS users have NASes run for years, while they gain experience. Then determine during an upgrade to change things the 2nd, (or more), time around to fit their knowledge and experience better.