@csmall Hello again. I’d second what @Haioken stated for a Rockstor Web-UI method.
You could also, if ports etc allow, add the new disk first and then remove the old disk, ie reverse the above order.
This Web-UI method is accessed via the Pool’s Detail page and then the “Resize Pool” which will then open a dialog based wizard that should step you through the available options. See the documentation entry in the Pools section entitled Pool resizing.
Note in the above you first logically remove the disk from the pool prior to physically removing / detaching it from the system, if that is the plan.
I would also suggest that a less risky move, if using a btrfs replace command via command line, is to leave the ‘disk to be replaced’ connected. This should be quicker also I think. But of course this may not be an option, ie if the disk is dead or there just isn’t sufficient ports; but in the later case it may still be advised to temporarily add a port rather than detach the disk from the system unnecessarily. If the ‘disk to be replaced’ is still working but is suspect, health wise, there is also the ‘-r’ switch where the ‘disk to be replaced’ is considered ‘read only’.
I think that the general opinion is that a ‘btrfs replace’ is the more preferred, read efficient, method to btrfs dev add, btrfs dev delete, or the other way around. But currently there is no ‘btrfs replace’ counterpart within the Rockstor Web-UI. But we do have this as an open issue:
Overall I think it is always best to not degrade a pool, ie physically removing a disk, prior to executing either a replace or a delete action as this, by definition, reduces redundancy and puts otherwise data at unnecessary risk, it also requires that a pool be mounted degraded (by btrfs design) which is a further complication. So always try to perform a logical removal before a physical removal is possible. I.e with 8 disks and sufficient free space (as @Haioken mentioned) you should be able to first remove via the Web-UI a disk, and once that procedure is complete (can take ages) you should have an un managed disk that can safely be physically removed. You can look to the balance tab for the pool, after initiating any device change, to see a progress report.
And that issue might be worth a look, info wise.
@Haioken, please correct me (double check) if I’ve gotten something wrong in the above, it’s rather off the top of my head.
Linking to a relevant btrfs wiki for context:
https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices
Hope that helps.