Size
Nominal drive capacity. This parameter directly determines not only the amount of data that can fit on the device, but also its cost; many SSD models even come in several versions that differ in capacity. Therefore, when choosing, it is worth considering the real needs and features of the application — otherwise you can overpay a significant amount for volumes that are not needed in fact.
In terms of actual values, a capacity of
120 GB or less is considered small these days. This can also be equated with
a 240 GB SSD. Average values are already considered
500 GB, increased —
1 TB(in the range of which
SSDs fall 400 and
800 GB). And the most capacious modern SSDs can accommodate
2 TB,
4 TB and
even more.
Write speed
The highest speed in write mode characterizes the speed with which the module can receive information from a connected computer (or other external device). This speed is limited both by the connection interface (see "Connector"), and by the characteristics of the device of the SSD itself.
Read speed
The highest data exchange rate with a computer (or other external device) that the drive can provide in read mode; in other words —
the highest speed of information output from the drive to an external device. This speed is limited both by the connection interface (see "Connector"), and by the characteristics of the device of the SSD itself. Its values can vary from 100 – 500 MB / s in the slowest models to 3 Gb / s and higher in the most advanced ones.
MTBF
The drive's time between failures is the time that it is able to continuously work without failures and malfunctions; in other words — the operating time, after which there is a high probability of failures, and even failure of the module.
Usually, the characteristics indicate some average time derived from the results of conditional testing. Therefore, the actual value of this parameter may differ from the claimed one in one direction or another; however, in fact, this moment is not particularly significant. The fact is that for modern SSDs, the MTBF is estimated at millions of hours, and 1 million hours corresponds to more than 110 years — while we are talking about pure operating time. So, from a practical point of view, the durability of a drive is often limited by more specific parameters — TBW and DPWD (see below); and the manufacturer's warranty generally does not exceed several years. However, data on the MTBF in hours can also be useful when choosing: other things being equal, more time means more reliability and durability of the SSD as a whole.
Write IOPS
The IOPS provided by the drive in write mode.
The term IOPS refers to the highest number of I / O operations that an SSD module can perform per second, in this case, when writing data. By this indicator, the speed of the drive is often evaluated; however, this is not always true. Firstly, the IOPS values of different manufacturers can be measured in different ways — by the maximum value, by average, by random write, by sequential write, etc. Secondly, the benefits of high IOPS become noticeable only with some specific operations — in in particular, the simultaneous copying of numerous files. In addition, in fact, the speed of the drive may be limited by the system to which it is connected. In light of all this, it is generally acceptable to compare different SSD modules by IOPS, but the real difference in performance is likely not to be as noticeable as the difference in numbers.
As for specific values, for the write mode with IOPS
up to 50K is considered relatively modest,
50 – 100K — medium,
more than 100K — high.
Read IOPS
The IOPS provided by the drive in read mode.
The term IOPS refers to the maximum number of I / O operations that an SSD module can perform per second, in this case, when reading data from it. By this indicator, the speed of the drive is often evaluated; however, this is not always true. Firstly, the IOPS values of different manufacturers can be measured in different ways — by the maximum value, by the average, etc. Secondly, the advantages of high IOPS become noticeable only with some specific operations — in particular, when copying numerous files at the same time. In addition, in fact, the speed of the drive may be limited by the system to which it is connected. In light of all this, it is generally acceptable to compare different SSD modules by IOPS, but the real difference in performance is likely not to be as noticeable as the difference in numbers.
For modern SSDs in read mode, an IOPS value of
less than 50K is considered a very limited indicator, in most models this parameter lies in the range of
50 – 100K, but there are also
higher numbers.
TBW
The abbreviation TBW stands for drive time between failures, expressed in terabytes. In other words, this is the total amount of information that is guaranteed to be written (rewritten) to this module. This metric measures the overall reliability and lifespan of a drive—the higher the TBW, the longer the device will last, all other things being equal.
Note that knowing the TBW and the warranty period, you can calculate the number of rewrites per day (DWPD, see the relevant paragraph), if the manufacturer did not specify these data. To do this, use the formula: DWPD = TBW / (V * T * 365), where V is the drive capacity in terabytes, T is the warranty period (years). As for specific numbers, there are a lot of drives on the market with a relatively low TBW —
up to 100 TB ; even these values are often sufficient for everyday use for a considerable amount of time. However, models with TBW at the level
of 100 – 500 TB are more common. Values of
500 – 1000 TB can be classified as "above average", and in the most reliable solutions this figure
is even higher.
