# FirepanIF: High Performance Host-Side Flash Cache Warm-Up Method in Cloud Computing
## 4 Evaluation
### Effectiveness of Firepan and FirepanIF  
The first experiment shows the effectiveness of Firepan and FirepanIF with a realistic I/O workload compared to Cachemior. Firepan and FirepanIF directly copy the cache contents from the source to the destination node during migration, whereas Cachemior only transfers the hot cache list to the cache warmer in the destination node, as explained in Section 3.1. At the beginning of the experiment, four VMs run on the source host, and three VMs, on the destination host. Each VM runs the FIO for 3600 s. Then, we migrate a VM from the source to the destination host. We call this VM the target VM. The six other VMs run the FIO workload with a flash cache for the entire experiment. Figure 5: Configuration for the FIO workload. The experimental procedure is as follows: At first, the target VM runs a workload without a flash cache for the first 400 s. Then, we attach a flash cache to the VM. After 1600 s, the target VM is migrated to the destination host, and then, we immediately allocate a flash cache to the VM. Next, the VM continues running a workload during the next 1600 s.  
We ran this experiment with four methods: (1) No cache migration (NoCacheMigration), and flash cache migration using (2) Cachemior, (3) Firepan, and (4) FirepanIF. Figure 6 shows the results of the I/O operations per second (IOPS) with the FIO benchmark. First, we explain the result of NoCacheMigration shown in Figure 5(a). For the first 400 s, FIO performs approximately 8K IOPS for read operations without a flash cache, which is akin to the baseline performance. After the flash cache is provided in the source node, the cache is warmed up for the next 800 s approximately. After 1400 s of running, the IOPS result seems to stabilize, and the flash cache hit ratio at this time is approximately 60%. The average IOPS value between 1400 s and the migration moment is more than 15K, which exceeds a 90% increment from the baseline performance. At 2000 s, the target VM is migrated, and this operation takes approximately 3 s. After the target VM is migrated, the flash cache on another host should be warmed up again. Thus, the IOPS value dramatically drops after the migration, and thereafter, the performance increases again when the flash cache is attached to the migrated VM. It also takes approximately 800 s for the benchmark to provide the highest performance. The result of the write operation is similar to that of the read case. Second, Figure 5(b) shows the result with Cachemior. Until VM migration at 2000 s, the result is similar to that seen in Figure 5(a) because there is no difference in the procedure. After the VM migration, the cache warmer starts to warm up the flash cache at the destination host. The flash cache migration continuously loads the hot data from the shared storage by using the cache metadata transferred from the flash cache management system running on the source host. In this experiment, we use one asynchronous I/O thread for data preloading. The thread creates up to 128 requests concurrently for the shared storage. Thus, after 2000 s, the FIO and warm-up thread compete for the shared storage until the thread ends at 2360 s. During the preloading period, the target VM suffers from low performance because of the preloading thread's activity. This is the main drawback of the  
Figure 6: Performance of the target VM with NoCacheMigration, Cachemior, Firepan, and FirepanIF. preloading approach. When we analyze the raw data, the VM and preloading thread compete with each other for the bandwidth allocated for the shared storage. After the warm-up ends, the IOPS value immediately increases to above 15K, which means that the flash cache migration was successfully completed. As shown in Figure 6b, however, the performance loss by data preloading is too large and outweighs the benefit. The average IOPS values after VM migration are 13,946 and 13,672 for NoCacheMigration and Cachemior, respectively. Thus, flash cache migration with Cachemior does not provide any benefit.