This article follows the presentation to know everything about the SSD.
Where mechanical hard drives have a theoretical lifespan only limited by the inherent fragility of the mechanics of its components (write head, platter, motor, etc.), the operation of SSDs requires limited durability.
This lifespan is defined in number of write cycles. Indeed, although the SSD memory chips and their electrical operation guarantee high read / write speed, they can only receive a certain number of new data after which, their use being exceeded, they will be unusable.
Lifetime and TeraByte Written
To characterize the lifespan of an SSD, two parameters must be taken into account:
- TBW (TeraByte Written): expressed in terabytes and corresponds to the maximum number of bytes that can be written to an SSD during its operation (different from the storage capacity)
- MTBF (Mean Time Before Failure): expressed in number of hours of operation, its order of magnitude is generally expressed in millions of hours, which corresponds to more than a hundred years for current use .
The TBW is calculated from three elements:
- SSD storage capacity in GB
- The number of write / erase cycles, which is the number of times the write / erase operation can be repeated on a memory area before it degrades. This number therefore depends on the technology used within the SSD (SLC, MLC or TLC type).
- WAF (Write Amplification Factor): is the ratio between the number of bytes actually written to the SSD and the number written, seen from the host. This figure varies depending on the type of controller used and the host system (We also find, in some manufacturers’ documentation, the term Write Acceleration Index – WAI)
Which gives us the following formula:
As for MTBF, its calculation depends on several parameters which are often known only to the manufacturer (algorithm used in the controller, etc…).
To give an idea of the lifespan of an SSD, hardware manufacturers often give the corresponding TBWs for their drives. However, it has been shown by experience that these SSDs have a much higher TBW than advertised (probably to take into account variations in the operating environment).
For example the SSD of Samsung 850 Pro announces a TBW of 600 TB against 9.1 PB (Petabytes) measured! That is more than 15 times the number announced.
When it comes to industrial computing, SSDs often come with an MTBF like all the components in the field.
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