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Why your SSD suddenly slows down: SLC cache explained

Why your SSD suddenly slows down: SLC cache explained

Have you noticed your SSD starts copying a huge file at amazing speed, then suddenly slows down? Many people think the drive is failing, but usually the reason is a clever trick inside the SSD called SLC Cache. Here it is, simply explained.

First: an SSD is built from cells

An SSD stores your data in flash memory (NAND) cells. According to Kingston, there are types based on how many bits each cell stores:

  • SLC: 1 bit per cell. The fastest and most durable; Kingston says about 100,000 program/erase cycles.
  • MLC: typically 2 bits per cell, about 10,000 cycles.
  • TLC: 3 bits per cell, roughly 3,000 to 5,000 cycles. Kingston says it is the standard in most PCs, laptops and consoles.
  • QLC: 4 bits per cell, about 1,000 cycles. It makes big capacities cheaper, but Kingston says writing to it is significantly slower.

The simple rule: more bits per cell means more capacity and lower cost, but slower writes.

So what is SLC Cache?

Here is the trick. According to Kingston, most modern consumer SSDs use a pSLC Cache. It is not a different physical cell type, but a small portion of the TLC or QLC cells themselves, run as if they were SLC and storing only 1 bit. That area takes in new data quickly, then moves it to the slower, high-density area. The result, says Kingston: the drive feels much faster in daily use.

Samsung calls its version Intelligent TurboWrite. According to Samsung, data is written to an SLC buffer first, and when the SSD is not reading or writing, the data moves to the multi-bit cells, freeing up the buffer again.

Why does speed drop on huge copies?

Because the cache is limited. In normal use (installing an update, saving files, loading a game) everything usually fits in the cache and you see high speed. But if you copy very large files in one go and exceed the cache size, writes can go straight to the slower area, and you notice the speed drop. This is normal design, not a fault.

Does this affect games?

While playing, an SSD mostly reads data rather than writes it, and the cache is about writing. So the slowdown we are talking about shows up more when you copy or download huge files, not while you play.

Practical tips

  1. Do not fill the SSD to the last byte: on many drives the cache size depends on free space, so leave enough room.
  2. If you edit video or move huge files often, choose a TLC drive over QLC, and check the cell type on the maker's website.
  3. Give it time to rest: according to Samsung, the buffer frees up when the drive is idle.
  4. Do not judge an SSD by one speed number: the speed on the box is usually measured in the best conditions.

SSDs available at GamerZone

Bottom line

An SSD slowing down during a long copy is usually not a fault: it is the end of the fast cache and the start of writing to slower cells. Know the cell type, leave free space, and pick the drive for your use. Not sure which SSD suits you? Message us on WhatsApp 0564121558 or visit our Olaya or Aqiq branch in Riyadh. Installments available with Tamara and Tabby.

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