# Compression-Assisted Adaptive ECC and RAID Scattering for NAND Flash Storage Devices
## 4 Evaluation
### Analysis of Adaptive ECC and RAID Scattering
The experimental results are plotted in Figure 9. In all the figures, in accordance with the P/E cycle, RAID-Stripe4, RAID-Stripe8, and RAID-Stripe16 represent UBER for the original RAID configuration with stripe sizes four, eight, and 16, respectively, while Scatter4, Scatter8, and Scatter16 represent UBER for the RAID-scattering scheme with stripe sizes four, eight, and 16, respectively. We have also plotted Legacy UBER to compare results with the RAID-scattering scheme, as well as the RAID system. The Legacy shows the original uncorrected page error rate of the original fixed ECC scheme. From Figure 9a,b,e, we identify that RAID scattering gives UBER enhancement over the legacy RAID system for F-1, F-2, and F-5. This is due to the high compression ratio of these traces. In the RAID system, the error-containing page can be recovered using the parity page and other data pages within the corresponding stripe. If more than one of those pages also has an uncorrected page error, the error-containing page fails to be recovered. As data are compressed and scattered within a stripe in the RAID-scattering system, there is less probability for overlap between error-containing pages than in the original RAID system. This results in less UBER for RAID-scattering systems for traces F-1, F-2, and F-5. For F-1 and F-2, we identify that RAID scattering with all stripe sizes give less UBER than legacy RAIDs with even smallest stripe size, that is, four. This means that even if the stripe size is large  
Figure 8: Adaptive ECC scheme for IO traces from F-1 to F-5 in comparison with legacy. and the parity covers a wider range, the error-correction capability is better than of the original RAID with a smaller stripe size. However, for IO traces with lower compression rates such as F-3 and F-4, as shown in Figure 9c,d, there is no big difference between UBER of the RAID-scattering scheme and UBER of the existing RAID scheme. As compression is less effective for F-3 and F-4, the number of pages to be recovered in the corresponding stripe has an error rate that does not differ much from that of the original RAID scheme. Even so, the figure shows that there is some performance improvement for each P/E cycle. Finally, the RAID-scattering scheme with the adaptive ECC is compared with the legacy RAID system. In this experiment, each IO trace is modeled as compressed, encoded with adaptive ECC, and stored as scattering within the stripe in the RAID system. Like for the previous evaluation, we have simulated those by varying stripe sizes with four, eight, and 16. For all RAID and RAID scattering with adaptive ECC configurations, UBER is evaluated as the P/E cycle increases from 0 to 5000. The results for traces from F-1 to F-5 are plotted from Figure 10a-e. Similar to the results of the RAID  
Figure 9: Experimental results for RAID scattering in comparison with original RAID with various stripe sizes. In accordance with P/E cycle, UBERs of RAID Scattering and original RAID are estimated along with stripe sizes four, eight, and 16. (**a**) RAID Scattering for F-1, (**b**) RAID Scattering for F-2, (**c**) RAID Scattering for F-3, (**d**) RAID Scattering for F-4, (**e**) RAID Scattering for F-5. scattering only scheme, all figures show plots for UBER for RAID-Stripe4, RAID-Stripe8, RAID-Stripe16, Adp.ECC-Scatter4, Adp.ECC-Scatter8, and Adp.ECC-Scatter16 configurations in accordance with the P/E cycle. From the figures, we identify that RAID scattering with adaptive ECC shows a lower UBER than that of legacy RAID, as well as RAID-scattering scheme. For all cases, it definitely results in less UBER than legacy RAID systems, regardless of the compression rate of the IO data. Another noticeable point is that the UBER for RAID scattering with adaptive ECC is less than that of the legacy RAID even though the stripe size is larger, which implies that the RAID-scattering scheme can enlarge the stripe size for the RAID system configuration with a higher reliability.