What NOT to put on an SSD?

If you worry about write cycles, you won't get anywhere.

You will have data on your SSD that changes frequently; your home, your configs, your browser caches, maybe even databases (if you use any). They all should be on SSD: why else would you have one, if not to gain speed for the things you do frequently?

The number of writes may be limited, but a modern SSD is very good at wear leveling, so you shouldn't worry about it too much. The disk is there to be written to; if you don't use it for that, you might just as well use it as a paperweight and never even put it into your computer.

There is no storage device suited for swap space. Swap is slow, even on SSD. If you need to swap all the time, you're better off getting more RAM one way or another.

It may be different for swap space that's not used for swapping, but for suspend-to-disk scenarios. Naturally the faster the storage media used for that, the faster it will suspend and wake up again.

Personally, I put everything on SSD except the big, static data. A movie, for example, doesn't have to waste expensive space on SSD, as a HDD is more than fast enough to play it. It won't play any faster using SSD storage for it.

Like all storage media, SSD will fail at some point, whether you use it or not. You should consider them to be just as reliable as HDDs, which is not reliable at all, so you should make backups.

Ok, so the goal is to get as much bang for the buck as possible - Speed vs. the price of replacement hardware (assuming a single large harddisk and medium-size SSD, which seems to be the norm). To simplify you can to weigh how much you notice the speed increase from moving a file to the SSD against the number of sectors written to move that file to the SSD.

• Files which need to be read a lot and written to rarely (such as the OS and programs) would probably be the most obvious to move to the SSD.
• Files which are written once and read to many times at a fixed data rate where the HDD is fast enough (for example music, video) should probably stay there. They are usually not modified, but consider that they are written to a lot of sectors.
• Small files which are modified a lot (such as some temporary files) are more complicated. For example, given a sector size of 512 bytes, you can overwrite a single-sector file 20,000,000 times before "consuming" the same amount of writes as writing a single 1 GiB file once. If the SSD takes care of wear leveling these should be equivalent.

Of course, even the best calculations also use up the most precious resource of all, time. So in the long run you're probably best off keeping it simple and buying new hardware slightly more often than the absolutely ideal case.

Agreeing with others, you should put pretty much everything except may be very large (video) files to avoid wasting expensive SSD space.

However, you should also make sure TRIM is enabled:

• Your SSD supports TRIM
• Your partition is aligned on a multiple of EBS
• Your file system supports TRIM on your file system (ext4 usually does)
• You run fstrim regulary (probably in a cron weekly)
• You keep at least 25% free disk space[1]

Remember to back-up your data.

UPDATE:

• Arch recommends not putting the swap on SSD to avoid wear off