Choosing an SSD is not always easy. These are products that have been on the market for a few years and for less than a couple of years are marketed at much more affordable prices than in the past.

SSDs with a capacity of 240-256 GB are probably the most attractive for most users because they are usually the most affordable in terms of cost per megabyte, at least at present.

Sequential read/write speed values indicate how fast an SSD is in both operations, for example when copying several gigabytes of data from one storage medium to another. This is a parameter, however, to be taken with the springs because it is difficult, for example, to write a single file, large in size, in a completely sequential way.

The performance of each SSD is then presented by offering the data relating to the speed of read/write 4K random.

In this case, the performance of the SSD is estimated by taking into account multiple read/write operations on non-sequentially stored files. Reading and writing operations are performed by working on 4 KB of data to reflect the typical situation: in real situations, in fact, most of the accesses on SSDs are random and act on small blocks of data.

This information is typically expressed in IOPS, i.e. in operations per second.

Running a benchmark software on a traditional magnetomechanical hard disk and on an SSD, using the same interface (for example, SATA 3), you can easily realize the differences in performance.

What is an SSD?

Let’s start by saying that no traditional hard disk, magnetomechanical, can even remotely approach the performance offered by an SSD.

First of all, to refer to SSDs, never use the term “disks”. An SSD, in fact, unlike traditional hard disks, does not contain moving plates and elements but integrated circuits for data storage.

The memory modules that equip an SSD are nothing more than NAND flash chips that use the so-called tunnel effect to change the electronic state of the cells. It is therefore no longer necessary to rely on magnetic and mechanical solutions.

The disadvantage of using SSDs is that they have a finite number of write cycles when using flash memory. As explained in the article Is the durability of SSDs a parameter to worry about? However, while avoiding considering SSDs as long-term data storage media, all newer products guarantee a lifetime of many years, thanks to the ability to write petabytes of data without difficulty (often much more data than claimed by the manufacturers).

Even in terms of data retention (the ability to retain stored data, even without being fed for a long time), SSDs offer excellent guarantees.

For safety, you should never keep an unsupplied SSD for three consecutive months, but except for this precaution, any other fears are out of place.

In this regard, we invite you to read the article High temperatures damage the SSDs? Data retention.

Our advice, however, is to install the operating system and applications on the SSD, configured as the main drive and save the data, instead, on a larger traditional secondary hard disk (perhaps scheduling the backup of data on a NAS server connected to the local network).

Three parameters are generally sufficient for the “identikit” track of an SSD:

1) IOPS (Input/Output Operations for Second). As mentioned above, the number of read/write operations the unit can perform in every second.

2) Latency. The time it takes for the unit to start the required operation.

3) Data transfer rate. The speed in MB/s at which data is transferred to the storage device or copied. Hardware and software are waiting for information from the storage units much longer than with any other local data source.

DRAMs can transfer data at over 20 GB/s. For a good SSD, (sequential) data transfer performance of more than 400 MB/s is expected.

Benchmark software such as CrystalDiskMark or AS SSD Benchmark allow you to test traditional hard drives and SSDs with multiple read and write tests.

In general, you will find that in the sequential read/write (I/O) of a 16 MB file, the differences between traditional hard drives (hereinafter referred to as HDDs) and SSDs are not so marked (an SSD is 3.5 times more than a HDD) in terms of IOPS.

The picture changes radically, instead, if you work with 4 KB block read/write operations in a random way. Among other things, this is the typical situation we are confronted with on a daily basis.

In this case, the performance of an SSD is better than one or two orders of magnitude compared to HDDs. To give some numbers (do not take the data as cast gold because the performance of the SSDs are constantly evolving), just to offer a yardstick, for 4K random I/Os, the SSDs are 50-60 times faster than the HDDs in reading and 100-120 times faster in writing (always in terms of IOPS).

Even more so, there is no history with a 512 bytes random I/O test: here the SSDs “tear” heavily the HDDs.

Analyzing the benchmark results in terms of troughput in MB/s, the same differences between HDDs and SSDs should be noted.

Looking at the SSD data you will notice what was mentioned in the introduction that at a very high data transfer rate (in MB/s), v will correspond to the performance in practice more contained in the random read/write 4K tests. Compared to HDDs, however, an SSD will always have 50-60 times better read throughput and 100-120 times better write throughput (roughly the same value as the IOPS).

The access or latency time of an SSD, then, is enormously better than that of a HDD.

A HDD can offer an access time of 14-16 ms while an SSD of 0.03 ms is over 500 times better.

By equipping yourself with a tool like HD Tune Pro, you will be able to see how the performance of a traditional hard drive tends to degrade as you continue with the transfer of data.

As you can see in the image below, however, the data transfer rate remains substantially constant in the case of an SSD.

Then, let’s also analyze the data related to the access or latency time (in yellow): in an SSD it remains very contained and constant, in a HDD it is over 500 times higher and it is variable.

Finally, two words about NAND memories used in SSDs.

These can be of various types and vary depending on the product: “Single Level Cell” (SLC), “Multi Level Cell” (MLC) and “Triple Level Cell” (TLC).

The three types differ based on the number of bits that can be stored by each cell and reflect the voltage levels that can be assumed. The TLC is the cheapest and allows you to store three bits; the MLC (stores two or more bits) is a bit more expensive; the SLC (a single bit stored for each cell) is the most expensive but at the same time durable and fast.

In the evaluation of an SSD there are certainly other parameters to consider but those presented so far allow you to get an idea of the performance of each unit.

SSD Drive Formats

SSDs are generally marketed in 2.5-inch formats, so they can also be installed on notebooks as well as in mSATA and M.2 formats.

To replace a notebook’s hard disk with an SSD, we suggest you refer to the article How to replace a notebook’s hard disk with an SSD.

An mSATA SSD is often used on ultrabooks or other devices where space needs to be minimized. The connector that distinguishes a mSATA SSD is similar, at least at first glance, to the PCI Express Mini Card interface but is not electrically compatible with it (you need a SATA controller and not a PCI Express controller).

More recently, the first SSDs based on socket M.2 have started to be marketed.

Socket M.2 has been designed to enhance the performance of SSDs and is interesting not only because it offers the possibility for expansion cards to connect to the PCI Express bus but also to many other buses. Developed to replace the mSATA and mini PCI Express interfaces (two old standards used for connecting SSDs and Wi-Fi cards in many notebooks), M.2 allows the use of different connection schemes that can allow connection to other buses such as USB 2.0, USB 3.0 SATA 3.0, DisplayPort and so on.

With M.2 SSDs, storage units become small “modules” to connect to the motherboard, just as you would with any other card.

The SATA III or 3.0 specifications (6 Gbps in terms of data transfer, at least in theory) were defined, in fact, at a time when magnetomechanical hard disks, those of the traditional type, were still the masters. Now, in addition to the SATA interface, the M.2s are looking to PCI Express (PCIe) with the precise intention of further improving performance.

The most recent M.2 SSDs are credited with reading and writing speeds of 1,400-1,500 MB/s and 1,000 MB/s, respectively. For the first time, therefore, you exceed the gigabyte per second in writing to a storage unit.

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