M K S E T

The MKSET utility is designed to control MK (SCSI) tape behavior when the
$QIO IO$_SKIPFILE function is used.

In the past, OpenVMS has always skipped files on tape by using skip record
commands to actually do the motion. This has essentially simulated skips by
filemark, and while it functions correctly, this sort of skipping is
sometimes quite slow compared with what a tape can achieve by using its
skip-filemarks commands.

In V7.1, OpenVMS allows the tape driver to be set up to use skip by
filemarks where the user permits. Modern tape drives keep track of the end
of logical data, so MKdriver can use these facilities to position correctly
if these features exist on a tape drive. (If they do not, the old skip by
records skipping is used.)

The skipping by filemarks, when used, will correctly sense end of tape
provided the tape ends in double EOF. Utilities such as Backup and file
structured copy, which use ANSI formatted tape, need the end positioning to
be set correctly and work fine with the new skip method. They already
continue skip-file operations across null files if any such exist.

Some third party utilities may, however, depend on the documented behavior
of stopping a skip by files on double EOF marks on tape.  To accommodate
this, OpenVMS will by default assume the use of the old skip by records
form of tape motion.

Because this form of positioning may be as much as 100 times slower than
skip by files, however, OpenVMS provides two additional features.

1. The magtape ACP and Backup will use an added function modifier in their
IO$_SKIPFILE commands to tape (IO$M_ALLOWFAST) which will permit them to
use the skip by filemarks command, which is usually faster. This modifier
will also be transmitted across TMSCP if all systems are at the 7.1 level.

2. A new utility, MKSET, is provided in SYS$ETC to control behavior.  It
works by issuing an IO$_SETCHAR function to the local SCSI magtape, with a
null P1 argument and one of three function modifiers:
IO$M_ALLOWFAST_NEVER, ALLOWFAST_PER_IO, or IO$M_ALLOWFAST_ALWAYS. These are
selected by the MKSET commands

$ MKSET/PER_IO  MKcuuu:
$ MKSET/ALWAYS  MKcuuu:
$ MKSET/NEVER   MKcuuu:

which set the controls. (MKcuuu: means MK unit with controller letter c
and unit number uuu:, such as MKB300: where controller letter is B and
unit number is 300.)

The settings have the following meanings:

PER_IO		This setting (the default) will allow an MK device to
		use the skip by filemarks where the drive supports it
		(it must be able to do read position and report blank
		check at end of data), and where the IO$M_ALLOWFAST
		function modifier is supplied with IO$_SKIPFILE. Otherwise
		the tape will skip files by skipping by records.

ALWAYS		This setting allows an MK device to skip files by
		filemarks (the new method) instead of by skipping records
		provided the tape supports it, and without needing any
		modification to the IO$_SKIPFILE function.

NEVER		Never use skip by filemarks but always skip by
		skipping records, regardless of whether the tape
		drive supports skip by filemarks and regardless of
		whether the IO$_SKIPFILE function has the modifier
		IO$M_ALLOWFAST or not. If using a utility that depends
		on the old semantics of skipping with skip-records,
		you may need to use this, since it will also cause Backup
		or COPY to use the older positioning.

This utility must be run on the system to which the locally direct attached
SCSI magtape is connected. The utility will modify a device's
characteristics even if the device is allocated to a different user.

Note that if a tape does NOT support skip by filemarks, the older skip
	by records method will be used. If errors occur during skip
	by filemarks, the MKdriver support is turned off, so that the
	old algorithm is used when a device appears to have difficulty
	with the SCSI commands needed to use skip by filemarks positioning
	correctly. If the "skip by filemarks supported" flag within
	MKdriver is cleared, it remains cleared until a reboot. The
	MKSET settings only permit (or fail to permit) use of the skip
	by filemarks algorithm where it is marked as supported.

	Whenever skip by filemarks is used for IO$_SKIPFILE, for whatever
reason, the MT$M_FASTSKIP_USED flag will be set to 1 in the DEVDEPEND
longword in the MK UCB and will be returned in the second longword in the
IOSB for that operation. Thus an application can use this information to
determine which skip algorithm was used, which may permit it to adjust its
positioning appropriately.

The MKSET image requires PHY_IO privilege to be used, and should be set
up as a foreign command:

$ MKSET :== $SYS$ETC:MKSET

prior to use.



As a bit of extra information, if MKSET had a separate CLD file it would
look something like this:

! Set up MK: skipfile behavior.
DEFINE VERB MKSET
     Image sys$etc:MKSET
     parameter p1,prompt="MKn:",value(required,type=$device),label=UNIT
        qualifier ALWAYS
! Always means we always skip by filemarks if the drive allows
        qualifier PER_IO
! Per_io means we skip files by skip-record by default, but may use
!  skip by files if the function modifier is present
        qualifier NEVER
! Never means never use skip by filemarks, regardless.
  Disallow any2 (ALWAYS, PER_IO, NEVER)