# ReZNS: Energy and Performance-Optimal Mapping Mechanism for ZNS SSD
## 4 Evaluation
### YCSB Benchmark  
To evaluate ReZNS on real-world workloads, we additionally compared the effectiveness of ReZNS on the Yahoo Cloud Serving Benchmark (YCSB) [29] on RocksDB. For a comprehensive evaluation, in this experiment, we used YCSB A, B, and F workloads; A and F are write-heavy workloads, and B represents read-heavy workloads. We performed an additional evaluation where the size of each zone in the ZNS SSD is configured with 1 GiB to understand the effectiveness of ReZNS in detail.  
Figures 7 and 8 plot the throughput and the number of zone-reset commands of all workloads. As expected, ReZNS is the winner in all cases (see Figure 7); it improves the baseline's performance by up to 35%, 19%, and 16% for YCSB-A, YCSB-B, and YCSB-F, respectively. As mentioned above, it is reasonable that ReZNS shows good performance compared with the baseline in the write-heavy workloads (i.e., YCSB-A and YCSB-F). Surprisingly, ReZNS also provides better performance on the YCSB-B workload. To understand the reason, we investigated I/O requests from the workload and figured out that the performance gain comes from the 5% write requests of the total YCSB-B workload. This is extremely important in that the small amount of writes can achieve 19% performance improvement. Meanwhile, Figure 7 shows different performance results even though YCSB-A and YCSB-F have the same read-write ratio of 1:1. The main reason is that the YCSB-A workload is composed of read and write operations, but YCSB-F includes a set of read-modify-write operations, which may have a different CPU overhead.  
Figure 5: Overall performance of Filebench. **(a)** Varmail; **(b)** Fileserver.  
Figure 6: The number of zone-reset commands on Filebench. **(a)** Varmail; **(b)** Fileserver.  
We notice that ReZNS dramatically reduces the number of zone-reset commands by up to 52%, 61%, and 54% in YCSB-A, YCSB-B, and YCSB-F workloads, respectively. This is a valuable result even if the reduced number of commands does not directly affect the performance (see Figure 7). This is because the zone-reset command is the only one that affects the lifespan of ZNS SSDs whose write amplification factor (WAF) is 1. Note that intensive and frequent zone-reset commands are harmful to the lifespan of ZNS SSDs. In summary, ReZNS can extend the lifetime of the NAND flash memory as long as possible by reducing the number of zone-reset commands.  
Finally, Figures 9 and 10 show the throughput and the number of zone-reset commands; the size of each zone is configured with a size of 1 GiB. As shown in Figures 9 and 10, ReZNS shows the same pattern as the evaluations in Figures 7 and 8. In other words, the benefits of ReZNS can be enabled regardless of the size of each zone. The reason behind this is that ReZNS maximizes the space utilization by reusing the available free space inside the zones instead of the time spent on the zone-reset commands.  
Figure 8: The number of zone-reset commands on YCSB workloads. In this evaluation, each zone is configured with a size of 64 MiB.  
Figure 7: Overall performance of YCSB workloads. In this evaluation, each zone is configured with a size of 64 MiB.