If you don’t do much writing or mixed IO (e.g., things like a database), it doesn’t really matter how full you want to keep your SSD. Writing slows (and write amplification goes up) as the disk fills because garbage collection has to work harder. As you may well know, NAND media consists of a set of blocks, and each block contains a set of pages. Writes are at the page level, but erases are at the block level. As data is overwritten or trimmed, some data in a given block will no longer be valid. When a block is garbage collected, the penalty lies in the amount of still-valid data that needs to be copied to another block. The fuller you keep the SSD, the more laden each block is with data you still care about, which has to be moved each time garbage collection is invoked. But if you’re doing read-mostly work, it probably doesn’t matter.
If you don’t do much writing or mixed IO (e.g., things like a database), it doesn’t really matter how full you want to keep your SSD. Writing slows (and write amplification goes up) as the disk fills because garbage collection has to work harder. As you may well know, NAND media consists of a set of blocks, and each block contains a set of pages. Writes are at the page level, but erases are at the block level. As data is overwritten or trimmed, some data in a given block will no longer be valid. When a block is garbage collected, the penalty lies in the amount of still-valid data that needs to be copied to another block. The fuller you keep the SSD, the more laden each block is with data you still care about, which has to be moved each time garbage collection is invoked. But if you’re doing read-mostly work, it probably doesn’t matter.