Why SSDs don't slow down from fragmentation

HostIt used to be a common thing to let the computer sit for hours and clean itself up. You would watch a screen full of tiny colored blocks move around just to get back a little bit of speed. But these days, we never talk about doing that anymore. Why did that chore just disappear with newer computers?

GuestIt comes down to how much the physical world used to hold us back. Those old drives were basically stacks of spinning plates with a moving arm, sort of like a record player. When you saved a file, the drive would try to put it in one neat line. But over time, as you deleted and added things, the drive would've to break files into bits to fit them into the empty gaps. If a single file was split into ten bits across that spinning plate, the metal arm had to physically jump ten times to read it. That takes a lot of time. It's like trying to bake a cake when the flour is in the attic and the eggs are in the basement. You spend more time walking than you do baking.

HostSo that cleaning process was just moving the flour and the eggs back onto the same shelf so the arm didn't have to move as much.

GuestExactly. You were putting the data in a straight line so the arm could stay still while the disk spun. But a solid state drive, or SSD, has no moving parts. There's no arm and no spinning plate. It's all just electrical signals moving through chips. No matter where a bit of data is, the drive can grab it almost instantly.

HostI get that it's fast, but it still feels like it would be better to have things in a row. Even with electricity, isn't a straight line easier to follow than a bunch of scattered dots?

GuestYou would think so, but at the size we're talking about, distance almost doesn't exist. To the chips in your drive, grabbing a bit of data from the very top takes the same amount of time as grabbing it from the bottom. It's like having a light switch for every single grain of sand in a sandbox. Flipping one switch is just as fast as flipping any other, no matter where they are. Being in a row gives you zero speed boost.

HostThat makes sense for the speed, but it still feels messy. Why would the drive even bother splitting things up if it doesn't have to? Why not just keep them together to be safe?

GuestThis is the part that feels backwards. For these new drives, being messy is a way to stay alive. The chips inside can only be written to a certain number of times before they wear out and die. Think of it like a piece of paper. You can write on it and erase it, but if you do that in the exact same spot a thousand times, you'll eventually poke a hole right through the page. If the drive always kept things neat and used the same spots at the front over and over, those spots would die in a year while the rest of the drive stayed brand new.

HostSo it spreads the work out so one part of the chip doesn't get tired?

GuestThat's it. The drive uses a secret map to spread the wear across every single chip. When you save a file, the drive looks for the freshest spots that haven't been used much and puts the data there. It might put one piece of your photo in the corner and another piece right in the middle. To the computer, it looks like one solid file, but inside the drive, it's scattered everywhere on purpose. There's a middleman chip that lies to your computer. When you ask for your file, the chip looks at its map, grabs the pieces from all over the place, and hands them to you before you even notice.

HostSo if I used one of those old cleaning tools to put everything back in a row, I would actually be hurting the drive.

GuestYou would be killing it. Every time you move a file, it counts as a write. Those old tools would force the drive to move every single piece of data just to make them look tidy. You would be rubbing the eraser on the paper over and over for no reason. You would be wearing out the chips and shortening the life of the drive just to fix a speed problem that isn't even there.

HostIt's strange that the very thing that used to save our computers is now the one thing that can break them.

GuestIt shows how we have moved from the world of objects to the world of signals. We spent forty years trying to solve the problem of space because moving through space is slow. Now that we use electricity to store things, where something is located doesn't matter anymore. We have traded the stress of moving parts for the stress of wearing out parts.

HostThe drive is a giant grid of tiny buckets, and as long as the map knows where the water is, it doesn't matter which bucket it's in.

GuestThose little colored boxes on the screen are gone because the drive finally learned how to be messy and fast at the same time.