# FirepanIF: High Performance Host-Side Flash Cache Warm-Up Method in Cloud Computing
## 4 Evaluation
### Effectiveness of Firepan and FirepanIF
Next, Figure 6c shows the result of Firepan. Compared to the previous experiments, the IOPS value does not drop during the migration. However, this is only an illusion for the user-level application. As the target VM was paused during VM and flash cache migration and starts with its copied flash cache, the measured IOPS value in the VM shows high performance, as if no migration occurred. In the real world, the user suffers due to the VM and flash cache migration, which takes approximately 3 and 30 s, respectively. Thus, we further investigate the real effectiveness of Firepan by uncovering the I/O usages of the host machines. We trace the bandwidth (MB/s) of the read requests issued by the target VM on the two host systems, and show the results before and after VM migration in Figure 7. We use the bandwidth metric instead of the IOPS in this analysis because I/O requests from the VM are fragmented in the host system, which results in inaccurate measurement. The bandwidth is the sum of two I/O streams for the shared storage and flash cache device. The results of NoCacheMigration and Cachemior show similar trends. The performance drops after the migration and then increases slowly while the flash cache is warmed up. The results are similar to those in Figure 6a,b. In the case of Firepan, the bandwidth was zero during flash cache migration, which takes approximately 30 s. Then, the bandwidth immediately increases to the same level as that before the VM migration. We exclude the bandwidth used to migrate the flash cache in the results of Firepan and FirepanIF. We provide more detailed results in Table 1. When we compare the average bandwidth for 400 s after VM migration, Firepan provides 13.49% more bandwidth for the target VM despite the suspension for 30 s compared to NoCacheMigration. Even if we compare 800 s after VM migration, which is sufficient time for the cache warm up in the other methods, Firepan provides 9.93% and 2.73% more bandwidth compared to NoCacheMigration and Cachemior, respectively. The benefit is derived from the immediate restoration of the entire cache state while the other methods gradually restore the cache state. Finally, the result for FirepanIF is shown in Figure 6d. After the VM migration, the target VM is executed immediately without its host-side flash cache while the flash cache migration is in progress. Thus, the performance drops for approximately 30 s. The average IOPS value during this period is approximately 6K, which is lower than the performance of NoCacheMigration immediately after the VM migration (i.e., 8K) in Figure 6a. As flash cache migration requires additional bandwidth for the network and host-side flash cache, the four FIO threads on the target VM are affected. After the flash cache migration is completed, the performance immediately recovers to the same level as that when  
Figure 7: Bandwidth before and after VM migration with NoCacheMigration, Cachemior, Firepan, and FirepanIF. the flash cache is fully warmed up. As shown in Figure 7 and Table 1, FirepanIF provides 16.68% more bandwidth for the target VM after 400 s following the VM migration and 12.00% after 800 s. This result shows that FirepanIF is the most effective method for host-side flash cache warm-up among the four methods.