Darel Rex Finley in 888

SSD TRIM Command In A Nutshell

2015.07.03   prev     next

WHEN an SSD writes a block of data, it must perform a two-step operation:

  1. Zero the block (i.e. set all of its bits to 0).

  2. Write the data (that the OS has requested be put in the block).

Due to the way flash memory works, step 2 can change bits only from 0 to 1, not vice-versa. That is why step 1 is necessary.

Skipping Step 1

The write operation can be much faster if step 1 can be skipped, so when you format (completely erase and start over) the SSD, it zeroes all the blocks, then keeps a persistent table of which blocks are zeroed (initially, all of them). When the OS asks the SSD to write data to a particular block, and the SSD’s internal table says that the block is already zeroed, then the SSD skips step 1.

The SSD also has a pointer table that assigns real blocks to virtual blocks. This means that when the OS asks the SSD to “Write this data to block 6202,” but in the SSD’s pointer table, pointer #6202 contains the value 6560, then the SSD is really writing the data to block 6560, and the OS is none the wiser. Since the SSD can change its pointer table at will, this gives the SSD absolute authority over where the data is actually written.

By moving the pointers around, the SSD can assure that every time the OS asks to write a block of data, it is always being written to an already-zeroed block, and thereby keep write operations nice-and-speedy.

All Good Things

But eventually the pre-zeroed blocks run out. This happens long before the user fills up the SSD with data, because every time the OS writes a block of data (even if it’s a block it has written many times before), the SSD is changing the pointers so that it can write to a different, already zeroed block. So the zeroed blocks run out when the total number of block-write operations that have been performed equals the total number of blocks in the SSD.

At that point, all write operations are suddenly much slower, as they must perform both steps 1 and 2 as described above. And they stay slower until the SSD is someday reformatted — if ever!

Re-Zero?

Wouldn’t it be great if the SSD, whenever it is idle, could spend that time zeroing unused blocks? Then fast-writing could go on forever! Yes, that would be great. The problem is that the SSD doesn’t know anything about “unused blocks.” Only the OS knows about that, because only the OS knows what file structure it’s using, where the files are, and such as the like. The SSD knows nothing of that. Since there are many OSes that might be using this SSD, plus different file/folder structures that any one OS can use, there is no way for the SSD to know which blocks are OK to zero, and which aren’t.

TRIM To the Rescue

Enter the TRIM (Table Reset Indication Marker) command. The TRIM command is a standardized way for the OS to inform the SSD about blocks that can be zeroed. So when the OS decides that some data is expendable (probably because it belonged to a file that was deleted), it issues TRIM commands to the SSD, informing it that certain blocks are OK to be zeroed. In effect, the TRIM command is a promise by the OS that the next time it accesses that block (perhaps far in the future), it will be writing, not reading the data.

When the SSD receives a TRIM command, it does not immediately zero the specified block, but rather marks that block (in its table) as OK-to-zero. Then, whenever it has idle time, it spends that time zeroing any blocks that are OK-to-zero.

That’s It

Now you know what the SSD TRIM command is! Doesn’t that give you a warm fuzzy?

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