Linux – How to migrate a Linux installation from HDD to SSD

I'd strongly suggest getting more memory installed so that you are not swapping. Any swapping just KILLS the performance of a Linux or UNIX(tm) system. So install enough memory to stop the swap!

While Marco's answer explained all the details correctly, I just want to focus on your last question/summary:

Is it a good idea to set up SSD + HDD in same pool, or is there a better way to optimize my pair of drives for both speed and capacity?

ZFS is a file system designed for large arrays with many smaller disks. Although it is quite flexible, I think it is suboptimal for your current situation and goal, for the following reasons:

• ZFS does no reshuffling of already written data. What you are looking for is called a hybrid drive, for example Apple's Fusion Drive allows to fuse multiple disks together and automatically selects the storage location for every block based on access history (moving data is done when there is no load on the system or on rewrite). With ZFS, you have none of that, neither automatically nor manually, your data stays were it was written initially (or is already marked for deletion).
• With just a single disk, you give up on redundancy and self-healing. You still detect errors, but you do not use the full capabilities of the system.
• Both disks in the same pool means even higher chance of data loss (this is RAID0 after all) or corruption, additionally your performance will be sub par because of the different drive sizes and drive speeds.
• HDD+SLOG+L2ARC is a bit better, but you need a very good SSD (better two different like Marco said, but a NVMe SSD is a good and expensive compromise) and most of the space on it is wasted: 2 to 4 GB for the ZIL are enough, and a large L2ARC only helps if your RAM is full, but needs higher amounts of RAM itself. This leads to sort of catch-22 - if you want to use L2ARC, you need more RAM, but then you can just use the RAM itself, because it is enough. Remember, only blocks are stored, so you do need not as much as you would assume by looking at plain files.

Now, what are the alternatives?

• You could split by having two pools. One for system, one for data. This way, you have no automatic rebalance and no redundancy, but a clean system which can be extended easily and which has no RAID0 problems.
• Buy a second large HDD, make a mirror, use the SSD like you outlined: removes the problem of differently sized disks and disk speeds, gives you redundancy, keeps the SSD flexible.
• Buy n SSDs and do RAIDZ1/2/3. Smaller SSDs are pretty cheap nowadays and do not suffer slow rebuild times, making RAIDZ1 interesting again.
• Use another file system or volume manager with hybrid capabilities, ZFS on top if needed. This is not seen as optimal, but neither is working with two single disk vdevs in a pool... at least you get exactly what you want, and some nice things of ZFS (snapshots etc.) on top, but I wouldn't count on stellar performance.