Trim is something that was never required for HDDs, so it was a new command that was defined once SSDs became prevalent. The command is required because of one of those awkward encumbrances that NAND users must accommodate: Erase before write.
NAND flash bits cannot be altered the same way as an HDD. In an HDD a bit that’s currently set to a “1” can be re-written to a “0” and vice versa. Writing a bit either way takes the same amount of time. In NAND flash a 1 can be written to a zero, but the opposite is not the case. Instead, the entire block (4-16k bytes) must be erased at once, after which all bits are set to a 1. Once that has been done then zeros can be written into that block to store data. An erase is an excruciatingly slow operation, taking up to a half second to perform. Writes are faster, but they’re still slow.
Let’s say that a program needs to Continue reading “What is an SSD Trim Command?”