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Maximum $qio I/O Size Limits?

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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
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?
/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.

answer written or last revised on ( 7-JAN-2004 )

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