I just added a second disk to a single disk ZFS pool. I increased the slots to 2 and assigned the drive. There is no sign of the new configuration till the array is started. This is rather scary as one does not know what will happen.

I request that the new proposed configuration be shown so we can have confidence in the change we are making.

Thank you,

Morris

10 hours ago, Morris0 said:

There is no sign of the new configuration till the array is started. This is rather scary as one does not know what will happen.

I'm sorry, but I don't follow, can you expand? It will show the new device assigned, the the data there will be deleted, but it can only be added to the pool after the array has started.

12 hours ago, Morris0 said:

There is no sign of the new configuration till the array is started. This is rather scary as one does not know what will happen.

What do you mean by "sign of the new configuration"? If you mean the type of pool (mirror/raidz/stripe...) then it'll be what you'll have set it to in the pool settings prior to adding the drive.

On 12/17/2025 at 3:04 AM, JorgeB said:

I'm sorry, but I don't follow, can you expand? It will show the new device assigned, the the data there will be deleted, but it can only be added to the pool after the array has started.

I feel we should see the proposed change to the pool before starting the array. Starting the array inserts the drive into the pool. How do we know the system will do what we wish to the pool? For example, if one is adding a spare rather than expanding how would this work?

11 hours ago, Morris0 said:

For example, if one is adding a spare rather than expanding how would this work?

For that, you would need to add a spare subpool first and add the device there, if it's added to the main pool, it will be used to expand it, if supported for the current pool profile.

I come from TrueNAS and in that environment nothing is committed without first displacing what will happen. I find this approach rather risky, particularly for those that are not experienced. I looked over the documentation for ZFS:

I did not see anything about a spare subpool. I just gave pool expansion quite a bit of thought and it gets quite interesting if one has a Raid Z1 of 3 drives and wants to add 3 drives to this pool. There are two possible configuration: 2 VDEV of 3 drives each or a single VDEV of 6 drives. If one increases the drive slots from 3 to 6 and then starts the aray, it will ask for the drives. Will it show the user that it's going to create a pool of 2 VDEV of 3 drives each? That's what I'd guess would be done as growing the VDEV has to be one drive at a time which was in the Read ME for the 7.x release yet is not in the documentation which is rather thin and that's what concerns me about the lack of a preview of what will happen. Furthermore, if you don't get the desired results, there is no way back, or at least I don't see a non destructive way of doing so.

9 hours ago, Morris0 said:

if one has a Raid Z1 of 3 drives and wants to add 3 drives to this pool. There are two possible configuration: 2 VDEV of 3 drives each or a single VDEV of 6 drives. If one increases the drive slots from 3 to 6 and then starts the aray,

If you want to add another vdev, you must add the 3 devices at the same time; if you want to expand the current on, you must add one device at a time.

10 hours ago, Morris0 said:

Will it show the user that it's going to create a pool of 2 VDEV of 3 drives each?

Again, it's only option if you add 3 new devices.

But I do see your point; it could be useful to better show what is going to happen for users not so familiar with ZFSZ and its limitations.

10 hours ago, JorgeB said:

If you want to add another vdev, you must add the 3 devices at the same time; if you want to expand the current on, you must add one device at a time.

Again, it's only option if you add 3 new devices.

But I do see your point; it could be useful to better show what is going to happen for users not so familiar with ZFSZ and its limitations.

Thank you Jorge,

Happy Holidays,

Morris