What I/O transfer size limits exist in OpenVMS?
The maximum transfer size is an attribute of the particular I/O device, controller and driver combination; there is no inherent limit imposed by OpenVMS (other than the fact that, today, byte counts and LBNs are generally limited to 32 bits). The maximum size of a device I/O request is limited by the value in UCB$L_MAXBCNT, which is set by the device driver based on various factors. (Also check the setting of the MAXBUF system parameter for buffered I/O transfers, and check the process quotas.) Currently, SCSI drivers limit I/O transfers to FE00(16) bytes, 65024 bytes (decimal). The reasons for this transfer size limitation are largely historical. Similarly, DSSI devices are limited to the same value, this for hardware-specific reasons. Transfers to HSC and HSJ device controllers via the CI are limited to 1,048,576 bytes. Client MSCP-served devices are limited to 65535 bytes-to help ensure that the I/O fragmentation processing happens on the client and not on the server system. Parts o
