## 5 Evaluation  
### Apply Differentiated Data Protection Policy  
Figure 8 shows the error rates of the four models for each cluster classified in Figure 4 based on Equations (1)-(6) in Section 4. In the figure, RBER represents the initial BER at 3000 P/E cycles, whereas ECC represents the BER of a conventional SSD that only uses ECC to protect data. Additionally, the model presented in this paper is Differentiated Protection. Differentiated Protection is a model that applies a differentiated data protection policy, such as the model presented in this paper. The BER is expressed by dividing the model into two parts: one without parity overhead and one with it. The initial RBER in Figure 8 was taken from the reference paper [22], and the error rate value indicated by each model in Figure 8 can be regarded as the average error rate value of each cluster. Furthermore, the black line near the error rate of \(10^{-15}\) represents the industry standard error rate for a typical SSD [23]. The differentiated data protection policy without parity overhead model and ECC model are expressed using Equations (1)-(6), and the procedure is as follows.  
1. For RBER, the value measured in 3D TLC NAND flash memory [22] is used for calculating our analytical model.
2. UPER is obtained through the ECC parameters of the reference paper [14] and Equations (2) and (4) of this paper. The UPER of Equation (4) obtained here is used to obtain the error rate of the ECC model without any parity data per stripe and the error rate of clusters 4 and 5 of the differentiated data protection policy.
3. Clusters 2, 3, and 6 with one parity data per stripe of the differentiated data protection policy can receive data protection by one parity, so they are derived through Equations (3) and (5).
4. Cluster 1 consists of two parities per stripe, and data protection can be obtained by two parities, so it can be calculated through Equation (6).  
We experimented with the financial workload on a 128 GB SSD in the work [14]. Through the experiment, we measure the amount of write including parity writes in  
Figure 8: Bit error rate by cluster and model at the end of an endurance test.  
the SSD caused by the proposed policy. The amount of parity writes directly affects the P/E cycles and error rates of flash memory, so it is important to analyze SSD reliability. Compared with the case where the number of parity data per stripe is 0, the WAF increases by 20% when the number of parity data per stripe is 1 when the number of parity data per stripe is 2, the Differentiated Protection model considering parity overhead in Figure 8 is shown by referring to the result of 50% increase in WAF.  
Looking at Figure 8, compared with the existing RAID model that uses only ECC to protect data, it can be seen that the model to which the differentiated data protection policy including parity overhead presented in this paper is applied shows the same or low error rate in all clusters. In clusters 4 and 5, it can be seen that the model that only uses ECC to protect data and the Differentiated Protection model have the same BER. As parity data are not added to clusters 4 and 5, there is no parity overhead in the Differentiated Protection model, and data is protected with the same protection strength as the model that protects data using only ECC.