SSD has several key indicators to measure its reliability: UBER, RBER and MTBF.
UBER: Uncorrectable Bit Error Rate, uncorrectable error bit rate;
RBER: Raw Bit Error Rate, Raw Bit Error Rate;
MTBF: Mean Time Between Failure, mean time between failures.
1
Data reliability
Let's take a look at the screenshot of the Reliability item in the S3710 SSD manual above:
UBER: Uncorrectable Bit Error Rate, a irreparable error bit rate, a measure of data corruption rate, equal to the number of data reads per bit that are still generated after any particular error correction mechanism is applied. (Probability).
Why SSD to define UBER? Any storage device product includes an HDD, and the user is most concerned with the correctness of reading after the data is saved. Think about the consequences of data loss and damage on customers. Especially enterprise-level user data. How do you convince users that a storage device system is reliable? The UBER indicator describes the probability of a data error, giving the user the possibility of describing the wrong data with intuitive probability data. The lower the better, of course.
Why does it generate erroneous data, the storage medium of the SSD is a flash memory, and the flash memory has a natural data bit turnover rate. There are mainly the following reasons:
l Erasing (P/E Cycle)
l Read Disturb
l Program Disturb
l Data retention error (Data Retention)
Although the SSD host and firmware design will use error correction code (ECC) (may also include other methods, such as RAID), but the flash memory is still not correct under certain conditions, so UBER is required to let the user know Error-corrected reference data for data error correction.
After the original bit flip of the flash memory is added to the BCH code (an ECC correction algorithm) ECC check protection, the conversion to UBER can be calculated. The core of the impact UBER is the flash bit error rate, also known as RBER (Raw Bit Error Rate). Figure 1-23 translates from RBER, ECC Code Length, and Strength to UBER, from which we conclude:
With the same ECC coding length, the UBER decreases with the increase of the protection strength, and it is greatly reduced.
Figure 1-1 Relationship between UBER and error correction strength
With the same ECC coding length and protection strength, the lower the RBER, the lower the UBER, exponentially lower.
Figure 1-2 Relationship between UBER and RBER
RBER reflects the quality of flash memory. All flash memory comes with an RBER indicator at the factory. The RBER for enterprise-class flash memory and consumer-grade flash memory is obviously different, and the price is certainly different. The RBER indicator is not fixed, as shown in Figure 1-25. The error rate of the flash memory increases with the increase of the PE cycle. In order to challenge the limits, one must be prepared to deal with one bad situation for every 100 bits.
Figure 1-3 Relation between RBER and PE
RBER is also related to the internal structure of flash memory. The RBER of two adjacent flash blocks may be completely different. Figure 1-26 shows the RBER distribution of different flash pages in a single flash block. It can be seen that the Upper Page's RBER is two orders of magnitude higher than the Lower Page.
Figure 1-4 Lower&Upper page RBER
Finally, returning to the end of the UBER indicator, the UBER specifications of the data are different for enterprise-level and consumer-level SSDs. What is the UBER indicator for commercial enterprise-class and consumer-grade SSDs? As shown in Table 1-9:
Table 1-9 SSD UBER values ​​for enterprise and consumer SSDs
| UBER | |
| Enterprise (Enterprise) | 10-17 even 10-18 |
| Consumer (Client) | 10-15 |
2
MTBF
The industry MTBF indicator reflects the product's trouble-free continuous operation time and is also an indicator of product reliability.
Let's take a look at SSD MTBF compliance standards and calculation methods.
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