Got a new SSD, don't want to wear it out but I still want to use some of it to store data files together with the old HDD

I bought a new 250GB Samsung 850 EVO SSD for my laptop that I want to use as primary storage device, together with the old but still functioning 250GB 7500 RPM HDD that I put in the former DVD bay with an adapter caddy.

Right now the HDD has only one big ext4 partition containing the OS, the applications and data files. I want to use the HDD for storing data, but I don't want to miss out on the opportunity to get the speed improvement of the SSD by doing so.

I want to combine say a 50GB or even smaller partition on the SSD and merge it with the partition on the HDD so that the least modified of the most accessed files are automatically moved to the SSD.

I've looked at caches like EnancheIO and Bcache, but they don't seem what I want, because (correct me if I'm wrong):

• The space occupied by the cache partition is subtracted from the amount of space available.
• The cache speeds up access to the most accessed files regardless of whether they're also the least often modified, which goes against the objective of not wanting to wear out the SSD.

Is the above correct, or could a cache (which one of those two?) help me reach my goal? If the above is correct, do you know of any other viable solution?

Would a union filesystem, like OverlayFS, be helpful here? If you monitored the HDD for the most accessed files (keeping track of their atime on a daily basis) and identified the least modified ones among them (keeping track of their mtime), in theory you could move those files to the SSD, freeing space on the HDD, while the union filesystem could make all that transparent to the user.

Would this work?

You have a few options depending on what you're trying to accomplish:

• Use bcache: You will be able to eat your cake, but not keep it.

  Yes, the amount of space you reserve for caching will be just like the opposite of a swap file: the amount you specify will be "taken away" from the total amount of disk space and given to the memory subsystem to be used as buffers for the other hard drive.
  To control which files do get cached, use something like vmtouch to fine-tune the bcache caching.

• Use LVM: You will get to keep your cake, but not eat it.

  You can use the Logical Volume Manager to create a volume that contains both SSD and HDD creating a large /home volume that contains the space from both, but:

  1. You will have no control over which file goes on the SSD and which one on the HDD
  2. If you lose one of the two drives, you lose all data and will need to restore from back-up!!!

• Use a manual system: You will be able to keep your cake, and eat it.

  Partition the drive into separate file systems: put / on the SSD and /home on the HDD. On top of this, you should put the all the files you want to go fast into /media/FastData and symlink the originals to the ones in /media/FastData if and only if these files reside in your /home (otherwise they already reside on the SSD anyway)

^{Note 1: I have a small SSD and a large HDD, so I use yet another system: / on the SSD and /home on the HDD and don't bother to optimise further...}
_{Note 2: A union file system will not help you any more then the manual system...
Note 3: Here are some more tips not to wear out your SSD from bullet point 4 and onwards}