# Amphishea: A Novel Persistent Buffer Management Strategy to Improve SMR Disk Performance
## 6 Experiment
### Performance Evaluation with the Different-Sized Chunk
The reasons why the number of RMWs on src2_2 decreases as the chunk size increases are the same as described above. **Normalized padding data volume. ** Figure 22 shows the trend in the amount of synthetic data padded for different chunk sizes, and all results are normalized to the chunk = 64 KB result. As can be seen from the figure, for workloads other than src2_2, the growth trend in the amount of synthetic data is consistent with the write amplification rate and the frequency of RMW operations, both of which are positively correlated with chunk size. For example, combining Figure 3 with Table 3 shows that hm_0 has a wide range of write request accesses and a small average write size, and thus introduces the most synthetic data. The src2_2 has a significantly higher write volume than the other workloads and a lower update frequency, which uses the chunk space more efficiently and therefore introduces the lowest amount of synthetic data. Figure 21: RMW and RMW-free counts. Figure 20: Normalized write amplification. **Normalized average victim zone size. ** Figure 23 shows the variation in the average victim zone size. We normalize all results to chunk = 64 KB. For most workloads, the average victim zone size decreases as the chunk size increases. This is attributed to the fact that as the number of chunks available in the LFB decreases, most of the chunks are evicted without accumulating enough buffer blocks, making it difficult to form larger victims. For src2_2, which features significant spatial locality, the average size of the victim zone is maximized when chunk = 128 KB. After that, increasing the chunk further causes the average size of the victim zone to gradually decrease. This indicates that chunk utilization is highest when the size is set to 256 KB, which makes maximum use of the sequential write capability of the disk. **Persistent buffer reallocation. ** To adapt to changes in workload access patterns, space in the HFB and LFB is periodically reallocated. Because a chunk in the LFB represents a set of contiguous LBAs in the zone, migrating it to the native zone does not involve multiple read/write operations. On the contrary, a block within a chunk in the HFB can come from several different zones, and its migration requires multiple reads and writes. In summary, if the HFB space is too large, subsequent space adjustments can add significant migration costs. Therefore, at system initialization, the HFB is set to 25% of the total persistent buffer size, and the rest of the space is allocated to the LFB. As shown in Figure 24, the bottom and top of the bar indicate the size of the reallocated space for HFB and LFB, respectively. It can be observed that the space of HFB is adjusted more than that of LFB in most of the traces. This is because HFB is given a smaller space at the beginning of the system, and HFB needs to be allocated more space to accommodate the hot blocks. Under web_0 and src2_2, there is less variation in space allocation, indicating that each buffer is well adapted to the access pattern of the workload at different stages. Figure 23: Normalized average victim zone size. Figure 22: Normalized amount of padded synthetic data.