This section reports file system metrics (XQP) collected from OpenVMS performance statistics.
The number of times the executive I/O subsystem successfully maps a virtual to logical segment, without needing to invoke XQP services.
The number of times the XQP updates the Window Control Block (WCB). Window turns occur when the executive I/O subsystem fails to map a virtual-to-logical segment using the current contents of the Window Control Block (WCB). The XQP updates the WCB with virtual-to-logical mapping information by reading a new portion of the file's header from disk or cache and reissues the I/O transfer. A large number of window turns usually indicate that a file or volume is fragmented. If the WCB is regarded as a cache of file mapping pointers, each window turn indicates a cache miss.
A very large file may cause excess window turns due to its size, even if the file is contiguous. This is because the maximum size of a window control block pointer is 65K blocks. If you encounter this case, you should provide a larger default window size when mounting the disk.
The number of times the executive must map and queue a segment in a multi-segment request to a driver. A split I/O occurs when the executive I/O subsystem cannot map a single logical I/O request as a single physically contiguous request and must split the logical request into multiple physical segments. Usually Split I/Os result from transfers occurring on fragmented disks.
The number of disk erase I/O operations per second (for example, when the DCL commands DELETE/ERASE or PURGE/ERASE are used).
This is the number of file open requests during the reporting interval.
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