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Each storage pool is comprised of one or more virtual devices. A virtual device is an internal representation of the storage pool that describes the layout of physical storage and its fault characteristics. As such, a virtual device represents the disk devices or files that are used to create the storage pool.

Slow, overloaded virtual devices can delay accesses to the ZFS, thereby causing the user experience with the file system to suffer.

This test enables the administrators to isolate slow devices and understand how I/O load is distributed across devices, so that administrators are forewarned of slowdowns and/or abnormal load conditions.

Measurement	Description	Measurement Unit	Interpretation
VDevPoolAvail	Indicates the amount of data currently stored in this.	MB	This amount differs from the amount of disk space available to actual file systems by a small margin due to internal implementation details.
VDevPoolUsed	Indicates the amount of disk space available in this device.	MB	This amount differs from the amount of disk space available to datasets by a small margin.
VDevOperRead	Indicates the rate at which read I/O operations were sent to this device, including metadata requests.	Reads/Sec	High values of these measures are indicative of high levels of I/O activity on a device. Compare the values of these measures across virtual devices to identify the I/O-intensive devices.
VDevOperWrite	Indicates the rate at which write I/O operations were sent to this device.	Writes/Sec
VDevBWithRead	Indicates the bandwidth of all read operations (including metadata) to this device.	Reads/Sec	High values for these measures indicate high bandwidth usage by a virtual device. By comparing the values of these measures across devices, you can isolate those devices that consume bandwidth excessively, and also understand when they consume too much bandwidth - when reading? or writing?
VDevBWithWrite	Indicates the bandwidth of all write operations to this device.	Writes/Sec