I’d like to add a 3rd VDEV to my pool, but my case is out of drive slots.
I have a SAS expander with external ports installed, but will longer cables to the new drives be a problem?
External cable length, (and really internal cable lengths), really depend on the protocol. SATA really starts to have trouble with 1 meter long cables, so the shorter the better.
A SAS expander to external SATA drives basically need shorter cables. Except if you have a more complicated setup. For example, you have an external SAS 4 port. Then feed it 4 x SATA drives, so cable should be short. However same external SAS 4 port to SAS drives, (or 2nd SAS expander), would be 2 meter cable length.
Thus, it both depends on the drives you intend to use, and the external disk chassis, (which could have a SAS expander in it).
I’m using SAS drives, thanks.
Suppose I were to have 3 identical VDEVs in one pool, but two of the VDEVS operating off a second expander daisy-chained to the first: Would that cause any issues?
Theoretically no, it should not cause any connection problems.
Performance might suffer depending on how many pass through ports go from the first SAS expander to the second. And how many SAS ports go from the SAS HBA to the first SAS expander.
Meaning if you have 8 SAS ports to the first SAS expander. Then use only 4 SAS ports to the second SAS expander, disks on the first SAS expander may appear to have more simultaneous performance that the disks off the second SAS expander.
Anyway, sometimes it is worth time to examine bandwidth numbers. The end result may suggest using a SAS expander with more ports. Or second SAS HBA.