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HP OpenVMS Systemsask the wizard |
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The Question is: Dear Wizard: How much disk I/O can I do at one time? We have an existing application that uses SYS$QIOW to write data to disk using IO$_WRITEVBLK. I found in the I/O User's Guide (section 2.4: Disk Function Codes): "Non-DSA disk devices can read or write up to 65,535 bytes in a single request. DSA devices connected to an HSC50 can transfer up to 4 billion bytes in a single request." Our software only runs on OpenVMS V7.1 and above on Alpha hardware. Since the software is going to our customers, some of whom have quite modest systems, should I limit the transfers to 64K or can I assume that all disk controllers/drives connected to an alpha can handle the 4 gig transfers? Thanks /RC Bryan The Answer is : $qio[w] and $io_perform[w] are the common and core I/O services. You probably do not have DSA devices, you most likely have SCSI and/or ATAPI (IDE) devices configured on your Alpha system. OpenVMS device drivers typically advertise their maximal I/O transfer capabilities via the device UCB$L_MAXBCNT field, and yet-larger transfers can be possible with various drivers as OpenVMS can segment I/O transfers larger than the driver's specific capabilities. You may or may not see uniform I/O transfer rates, as processing of differing-size I/O operations can result in differing bandwidths. For devices involving buffered I/O transfers, also please see the MAXBUF system parameter, and please see the BYTLM process quota.
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