Get best of the week

Drive Anatomy: SSD

image

Part 1. Drive Anatomy: Hard Drives

As hard as stone


In the same way that transistors revolutionized the computer field by increasing the speed of switching and performing mathematical operations, the use of semiconductor devices as storage devices led to the same result.

The first steps in this direction were taken by Toshiba, which proposed the concept of flash memory in 1980 . Four years later, she created NOR memory, and in 1987, NAND memory. The first commercial flash drive ( solid state drive , or SSD) was released by SunDisk (later renamed SanDisk) in 1991.

Most people began their acquaintance with SSDs from so-called USB sticks . Even today, their structure as a whole resembles that of most SSDs.


On the left is a single SanDisk NAND memory chip. Like SRAM, it is used in CPU and GPU caches. It is filled with millions of "cells" created from modified floating gate transistors . They use high voltage to record and erase the charge in individual sections of the transistor. When reading a cell, a reduced voltage is applied to the section.

If the cell is not charged, then when a low voltage is applied, current flows. This makes the system understand that the cell has state 0; in the opposite case, it has state 1 (i.e., no current flows when voltage is applied). Thanks to this, reading from NAND memory is very fast, but writing and deleting data is not so fast.

The best memory cells called peer cells(single level cells, SLC), have only one amount of charge created on the transistor site; however, there are memory cells that can have several charge levels. In general, all referred to as multilevel cells (multi-level cells, MLC) , but the industry acronym NAND-MLC memory 4 denote production charge. Other types have similar names: three-level (triple level, TLC) and four-level (quad level, QLC) have, respectively, 8 and 16 different charge levels.

This affects how much data can be stored in each cell:

  • SLC - 1 level = 1 bit
  • MLC - 4 levels = 2 bits
  • TLC - 8 levels = 3 bits
  • QLC - 16 levels = 4 bits

Etc. QLC seems to be the best cells, right? Unfortunately, it is not. The currents are very small and sensitive to electrical noise, therefore, to determine different charge levels of the cell, you need to read the value several times to confirm it. In short, SLCs are the fastest cells, but occupy the most physical space, and QLCs are the slowest, but for your money you get more bits.

Unlike SRAM and DRAM, when the power is turned off, the charge in the flash memory is saved and its leakage is veryslow. In the case of system memory, cells are discharged in nanoseconds, and therefore must be constantly updated. Unfortunately, the use of voltage and the supply of charge damage the cells, and therefore SSDs wear out over time. To combat this, tricky procedures are used to minimize the rate of wear; they usually make the use of cells the most uniform.

This function is controlled by the control chip shown on the right. It also performs the same tasks as the LSI chip used in the HDD. However, drives with spinning disks have separate chips for the DRAM cache and Serial Flash firmware, and both controllers are integrated in the USB flash drive. And since they are designed to be cheap, you won’t get much functionality from them.

But due to the lack of moving parts, we can confidently expect that the performance of flash memory will be higher than that of the HDD. Let's look at the metrics using CrystalDiskMark :


At first, the results are disappointing. The sequential read / write and random write speeds are much worse than the tested HDD; however, random reading is much better, and this is the advantage that flash provides. Writing and deleting data is quite slow, but reading is usually done instantly.

However, this test has another imperceptible feature. The USB memory test provides connectivity only according to the USB 2.0 standard, which has a maximum transfer speed of only 60 MB / s, and the HDD used the SATA 3.3 port, which provides throughput 10 times more. In addition, the used flash memory technology is quite simple: the cells are of the TLC type and are arranged in long parallel stripes; this arrangement is called flat(planar) or two-dimensional (2D).

The flash memory used in the best modern SSDs is of the SLC or MLC type, that is, it works a little faster and wears out a little slower, and the stripes are bent in half and lined upright, forming a vertical or three-dimensional cell structure. They also use the SATA 3.0 interface, although they are increasingly using a faster PCI Express system via the NVMe interface.

Let's take a look at one such example: the Samsung 850 Pro , which uses these tricks with a vertical layout.


Unlike the heavy 3.5-inch Seagate drive , this SSD is only 2.5 inches in size and is much thinner and lighter.

Let's open it (thanks to Samsung for using such cheap Torx bolts that almost fell apart when dismantled ...) and see why:


There is almost nothing in it!

No disks, no levers, no magnets - just one printed circuit board consisting of several chips.


So what do we see here? Small black chips are voltage regulators, and the rest perform the following functions:

  • Samsung S4LN045X01-8030: ARM Cortex R4-based tri-core processor for processing instructions, data, error correction, encryption and wear management
  • Samsung K4P4G324EQ-FGC2: 512 MB DDR2 SDRAM used for cache
  • Samsung K9PRGY8S7M: each chip is 64 GB of 32-layer NAND type MLC vertical flash memory (in total 4 chips, two are located on the other side of the board)

We have 2-bit flash memory cells, several memory chips and a lot of cache, which should provide increased performance. Why? Recall that writing data to flash memory is a rather slow process, but having multiple flash chips allows you to record in parallel. A USB stick does not have much DRAM for storing data ready for recording, so a separate chip will also help. Back to CrystalDiskMark ...


The improvement turned out to be huge . The speed of both reading and writing has become much higher, and the delays are much less. What else is needed for happiness? Smaller and lighter, no moving parts; SSDs also consume less power than mechanical disk drives.

Of course, all these advantages have a price, and here the word "price" is used literally: do you remember that for $ 350 you can buy a 14 TB HDD ? If you take the SSD, then for this amount you will be able to purchase only 1 or 2 TB . If you want a drive of the same level, so far the best thing you can do is spend $ 4,300 on one enterprise-level SSD with a capacity of 15.36 TB!

Some manufacturers madehybrid HDDs - standard hard drives with a bit of flash memory on their circuit boards; it is used to store data on disks, which are often accessed. Below is a board from a Samsung 1TB hybrid drive (sometimes called SSHD ).


In the upper right corner of the board are the NAND chip and its controller. Everything else is about the same as in the Seagate model, which we examined in a previous post.

We can use CrystalDiskMark for the last time to see if there is any tangible benefit from using flash memory as a cache, but the comparison will be unfair, since the disks of this drive rotate at a speed of 7200 rpm (and the HDD WD, which we used for autopsy - from just 5400 rpm):


The performance is slightly better, but the reason for this is probably the increased rotation speed - the faster the disk moves under the read / write heads, the faster you can transfer data. It is also worth noting that the files generated by the benchmark test will not be recognized as actively read by the algorithm, which means that the controller most likely will not be able to correctly use flash memory.

Despite this, better testing showed an improvement in HDD performance with an integrated SSD. However, cheap flash memory is likely to fail much faster than a high-quality HDD, so hybrid drives are probably not worth our attention - the drive industry is much more interested in SSDs.

Before we move on, it's worth mentioning that flash memory is not the only technology used in solid-state drives. Intel and Micron jointly invented a system called 3D XPoint . Instead of writing and erasing the charges of charges in the cells to create states 0 and 1, to generate bits in this system, the cells change their electrical resistance.

Intel touted this new memory under the Optane brand, and when we tested it, the performance was outstanding . Like the price of the system, but in a bad sense. An Optane drive for just 1 TB today costs more than $ 1,200 - four times more than an SSD of the same size based on flash memory.

The third and final drive, which we will explore in the next article, will be optical drives.

More articles:


All Articles