Block Size.

The block size is a configurable parameter that specifies the unit of disk access and network transfer requests in the server. Its value affects the utilization of the devices, the overhead involved in the operation of the system, and the server throughput overall. In Figure 14, we illustrate the network throughput and miss ratio across different system loads in a block size range between 4KB and 1MB. We observe that both the measured metrics remain constant with block size larger than 16KB and 64Kb at low and high load, respectively. Low loads keep the miss ratio larger than that of high loads, because file requests arrive more infrequently and are less likely to need the same block concurrently. Smaller block sizes increase the disk access overhead, and lead to substantial block search overhead thus raising the processor utilization. In general, we found the block size equal to 64KB to perform well, and we used it in all the other experiments.

We also compared the unmodified ftpd implementation against the Circus prototype having the active region advancement functionality turned off. We only found a minor throughput discrepancy of less than 10% between the two. We concluded that the value of the block size, when simply incrementing the cursor of each client independently of the others, has less dramatic effect to the server network throughput due to much lower processor utilization involved, and inherently lower disk access locality.