# Building Reliable Massive Capacity SSDs through a Flash Aware RAID-Like Protection +
## 3 Motivation  
In this section, we introduce the motive of this study by showing the bit error rate according to the increase in the number of P/E cycles when RAID-like technology is built in an SSD.  
Figure 1 show the error rate of an SSD device with RAID protections. The x-axis shows the number of P/E cycles and the y-axis shows the page error rate. The horizontal red bar indicates the industry standard error rate, which refers to the error rate of the HDD [55].  
The error rate is calculated under the following assumptions. Three different RAID configurations are applied to the SSD simulator that is presented in Table 1. We ran the Financial workload in Table 2 and measured WAF of the SSD by simulating each RAID (i.e., RAID0, RAID5, and RAID6) configuration. The three configurations provide different levels of reliability by maintaining different numbers of parity data per stripe. That is, RAID0, RAID5, and RAID6 maintain zero, one, and two parities per stripe, respectively. We adopt the state-of-the-art RAID-like technique, which was proposed for flash memory friendly policy, in order to minimize the parity management overhead of RAID configurations in the SSD [14]. Recall that ECC is used by default and RAID-like configurations are introduced in order to supplement reliability. The ECC is capable of correcting eight-bit errors per 1 KB code word with BCH code [45] and a stripe consists of 16 pages for the RAID configurations. The raw bit error rate (\(RBER\)) of flash memory is based on \(3 xnm\) MLC products [38].The WAF of flash memory increases due to the parity write cost of RAIDs, which increases the number of P/E and bit error rate. Through the simulation, we observed that RAID5 and RAID6 increase WAF by 20 and 50%, respectively, as compared to RAID0 configuration, due to parity overhead.  
Figure 1a assumes that the WAFs of RAID0, RAID5, and RAID6 are all the same in order to find out the reduction in error rate that can be obtained with pure RAID protection. On the other hand, Figure 1b reflects the increase in WAF due to parity overhead. As a result of observation in the simulation using Financial workload, the WAF ratio of RAID0, RAID5, and RAID6 is 1:1.2:1.5, respectively. Recall that, as WAF increases, the number of P/E increases. We derive the uncorrectable page error rate (_UPER_) of each RAID configuration through Equations (1)-(5), as in Section 6.  
The procedure for deriving the _UPER_ in Figure 1 is as follows.  
(1)  
We first obtain the P/E cycles from the SSD simulator for Financial workload.  
Figure 1: Comparison of the error rate according to the parity write cost in an SSD.  
2. Subsequently, the \(RBER\) can be converted from the obtained P/E number through Equation (1) or [38].
3. With the given ECC parameter in Table 3, we calculate the \(UPER\) through Equation (2). This probability is the error rate when the ECC is applied, and it is the same for RAID0. This is because RAID0 has no error recovery function other than ECC.
4. RAID5 reduces the error rate by providing enhanced data protection through ECC and RAID parity. Therefor, the \(UPER\) for RAID5 is derived by Equations (3) and (4).
5. Because RAID6 maintains two parities per stripe, the error rate can be further reduced and it is calculated by Equation (5).  
We observe that the UPER of RAID5 and RAID6 exceed the UPER of RAID0 at 16K and 19K P/E cycles, respectively, in Figure 1b. The relationship and derivation of the corresponding models [14; 53] are described in detail in Section 6. Through this, we observe that protecting the data with RAID-like technologies greatly reduces the error rate, but the increase in WAF due to parity management overhead adversely affects the reliability of the SSD. Therefore, reducing the parity cost of RAID-like technologies is a major issue in terms of reliability. In addition, the cost of parity not only affects reliability, but also the performance and lifetime of SSDs [14; 28; 51].