Knowledge Base

Many applications are designed to issue large I/O requests for higher bandwidth. ESX Server 3.5 now supports increased limits for the maximum I/O request size passed to storage devices. ESX Server 3.5 passes I/O requests as large as 32MB directly to the storage device. I/O requests larger than this are split into several, smaller-sized I/O requests. Some storage devices, however, have been found to exhibit reduced performance when passed large I/O requests (above 128KB, 256KB, or 512KB, depending on the array and configuration). As a fix for this, you can lower the maximum I/O size ESX Server allows before splitting I/O requests.

If you have measured decreased storage performance in ESX Server 3.5 compared to a similar ESX 3.0.x system, try reducing the maximum I/O size, as described below, and see if performance improves. If your storage device does not have this problem (or if the problem does not go away when you reduce the maximum I/O size), you are better off leaving the maximum I/O size at its default 32MB setting because it will increase performance and (or) lower CPU utilization on your system.

One way to diagnose the problem is by looking at latency statistics reported by esxtop . Beginning in ESX Server 3.5, esxtop includes several detailed storage statistics that report time spent in various components. If storage devices are a problem, esxtop will display high device latencies. (For information about using esxtop , refer to the esxtop man page).

To reduce the size of I/O requests passed to the storage device using the VMware Infrastructure Client: