iSCSI sequential read

In Figure 1 a sequential-read FIO workload is running against a file system using iSCSI connected devices. The workload is scaled from 1 to 128 jobs running on the KVM guest. The V7000 storage devices are directly attached to the KVM host system via iSCSI.

High throughput levels are observed for all protocols with a workload of 1 and 2 jobs, as the KVM guest page cache can contain the total file size being used for these measurement runs. The CPU load level on the guest and host closely tracks the number of workload jobs at 1 and 2 jobs. The close tracking of CPU load appears related to satisfying the I/O request from the guest page cache, reducing the number of Start Interpretive Execution (SIE) exits for I/O.

Starting at 4 jobs, the total file size exceeds what the KVM guest page cache can hold, leading to a substantial drop in throughput as page reclaim and read requests directly from the virtual device increase. This triggers device I/O in the KVM host, increasing the number of SIE exits for IO.