# Amphishea: A Novel Persistent Buffer Management Strategy to Improve SMR Disk Performance
## 6 Experiment
### Performance Evaluation with the Different-Sized Chunk  
To evaluate the impact of different chunk sizes on system performance, we adjust the chunk size from 64 KB to 1024 KB to observe the variation of each metric. In addition to the basic metrics mentioned in Section 6.2, the size of the space reallocated by the space allocator for the HFB and LFB is shown. **Normalized Average Response Latency. ** All results are normalized to the response latency by chunk = 64 KB. As shown in Figure 18, the response latency increases gradually with chunk size for most traces. By analyzing the traces, we find that their average write requests are small, and increasing the chunk size results in lower space utilization in the LFB. The limited number of chunks and lower space utilization are the reasons for the increased number of LFB cleanups. The astute reader will notice that Amphishebaena's I/O latency under src2_2 gradually decreases as the chunk size increases. Note that src2_2 is a spatial locality-dominated workload, it contains many write requests with larger sizes, which contributes to improving the space utilization of a chunk. Therefore, the I/O latency  
Figure 16: The amount of padded synthetic data. Figure 17: Average victim zone size. for such workloads gradually decreases as the chunk size increases. Although a larger chunk size under src2_2 is beneficial for improving the write efficiency and reducing garbage collection time, the space utilization of the chunk decreases when its size exceeds a certain threshold. For example, the response latency under src2_2 starts to increase at chunk = 256 KB. The reason for this, as described above, is that a larger chunk increases the frequency of buffer cleanup, resulting in an increase in latency. **Write hit rates. ** As described in Section 5.2, the top and bottom of the bars indicate the write hits of the LFB and HFB, respectively. Figure 19 shows the variation of the write hit rate for different chunk sizes. We can see that the write hit rate is not sensitive to the chunk size. Although a larger chunk size potentially increases the cleaning frequency in the LFB, it does not affect the HFB. We can see from the results that the write hits are mainly contributed by the HFB, while the LFB is only used as a temporary storage for cold blocks. The space allocator does not reduce the size of the HFB, even though the number of free blocks is limited. Therefore, most of the hot blocks remain in the HFB. Overall, increasing the chunk size does not significantly reduce write-hit rates. **Normalized write amplification. ** As shown in Figure 20, all results are normalized to chunk = 64 KB. It is clear that write amplification increases gradually with chunk size, except for src2_2. On the one hand, increasing the chunk size causes the LFB to be cleaned more frequently. On the other hand, if the gap between the chunk size and the average size of write requests is larger, the chunk may also be evicted before it is fully filled. The above factors are the reasons why increasing the chunk size leads to out-of-controllable write amplification. Also, most of the write requests in src2_2 are out of order, but Amphibaena's eviction policy causes the victims to be written back to the native zone in an organized way, thus eliminating write amplification. Figure 19: Write hit rates. Figure 18: Normalized average response latency. **RMW and RMW-free counts**. We record the number of RMWs for different chunk sizes and normalize all results to chunk = 64 KB. As shown in Figure 21, the larger the chunk size, the higher the number of RMWs. As the chunk size increases, the number of free chunks gradually decreases. Although the space allocator adjusts the space size of the LFB accordingly, the number of free chunks is still limited. Since the LFB is the only exit from the buffer, it must be cleaned frequently to ensure that there is enough space for new chunks, resulting in an increase in the number of RMWs.