From:	STAR::COUGHLAN     "Tom"  7-OCT-1996 16:37:35.62
To:	EVERHART
CC:	
Subj:	Design #3 for Skipfiles Modifications

From:	EVMS::TGOODWIN      4-OCT-1996 13:19:04.76
To:	COUGHLAN
CC:	
Subj:	Updates to design from meeting



			Design #3 for Skipfiles Modifications

	The approach which we have decided to take is a merger of options 2.4.1
and 2.4.2 of the investigation report with modifications from Lenny's design.
This means that we will be creating a function code modifier for IO$_SKIPFILE
(IO$M_ALLOWFAST) which will be used to enable skip by filemarks for a single QIO
operation and we will be creating a user mode QIO interface (part of SETMODE)
which will be used to enable and disable skip by filemarks as the default
operation for IO$_SKIPFILE and to enable and disable the use of the
IO$M_ALLOWFAST modifier.

Following unit init of a device, skip by filemarks will always be disabled by
default for IO$_SKIPFILE and the new IO$_SKIPFILE modifier will be enabled. 
During a packack sequence, MKDRIVER will set or clear the SKIPFILE_SUPPORTED bit
of UCB$L_MK_FLAGS using the same algorithm which is in place currently
(reference CHECK_SKIPFILE_SUPPORTED routine).  This design will define a two new
bits in UCB$L_MK_FLAGS called SKIPFILE_ENABLED and SKIPFILE_DEFAULT which will
be initialized to the enabled and disabled states, respectively, during unit
init.

The IO$_SETMODE QIO inteface will use three modifiers to control the current
skip by filemarks settings.  The modifier IO$M_ALLOWFAST_OFF will be used to
clear both ENABLE_ALLOWFAST and FORCE_ALLOWFAST in UCB$L_MK_FLAGS.  The
modifier IO$M_ENABLE_ALLOWFAST will clear the FORCE_ALLOWFAST flag and set the 
ENABLE_ALLOWFAST in UCB$L_MK_FLAGS.  The modifier IO$M_FORCE_ALLOWFAST will be
used to set both ENABLE_ALLOWFAST and FORCE_ALLOWFAST in UCB$L_MK_FLAGS.  
The ENABLE_ALLOWFAST and FORCE_ALLOWFAST flags will only be looked at by
IO$_SKIPFILE if the SKIPFILE_SUPPORTED bit is set.  This will prevent the
possibility of enabling this on drives which we know we cannot support skipfiles
on.

Anytime the skip by filemarks is used by IO$_SKIPFILE, whether due to
IO$M_ALLOWFAST modifier being present or by forcing the allowfast functionality,
the MT$M_FASTSKIP_USED flag in the devdepend longword of the UCB will be set and
returned in the IOSB.  This will be used as positive acknowledgement to an
application which algorithm was used to move the tape.

For the changes to the IO_SKIP_FILE routine in MKDRIVER, I feel it is easiest
to show the change in the enabling algorithm using pseudocode:

OLD ALGORITHM
-------------
IO_SKIP_FILE:

If SKIPFILE_SUPPORTED then
    <skip by filemarks>
Else
    <skip by record>
Endif


NEW ALGORITHM
-------------
IO_SKIP_FILE:

<clear MT$M_FASTSKIP_USED in UCB$L_Devdepend>
If (SKIPFILE_SUPPORTED) then
    If (FORCE_ALLOWFAST) then
       <set MT$M_FASTSKIP_USED in UCB$L_Devdepend>
       <skip by filemarks>
    Else
	If (ENABLE_ALLOWFAST) then
	    If (IO$M_ALLOWFAST set for IO$_SKIPFILE function code) then
		<set MT$M_FASTSKIP_USED in UCB$L_Devdepend>
		<skip by filemarks>
	    Else
	        <skip by record>
	    Endif
	Else
	    <skip by record>
	Endif
    Endif
Else
    <skip by record>
Endif




Total Work to MKDRIVER
----------------------
- Add two bit definition to UCB
- Add one FDT routine and associated bit definitions
- Change code in IO_SKIP_FILE routine to support new algorithm

Work outside MKDRIVER
---------------------
- Utility to enable,disable and force skip by filemarks for IO$_SKIPFILE
- Modifications to OpenVMS utilites such as BACKUP and MTAACP to use
  IO$M_ALLOWFAST whenever possible.
- Documentation of skip by filemarks behaviour with respect to tape positioning
  for new algorithm and how to use the IO$M_ALLOWFAST modifier and
  MT$M_FASTSKIP_USED return indication.

By default the new interface, IO$M_ALLOWFAST modifier will be enabled and the
skip by filemarks default in IO$_SKIPFILE will be disabled.  When a user wants
to run an old application with the skip by filemarks feature, he will run the
utility with the FORCE_ALLOWFAST setting.  IO$_SKIPFILE calls from all
applications to the drive will now be performed as skip by filemarks until the
utility is used to disable the feature.

When a user has a new application which uses the IO$M_ALLOWFAST modifier and
desires to revert to the old functionality due to non-standard media, he must
do the following. First he will run the utility with the ALLOWFAST_OFF setting to
disable all skip by filemark operations.  The application will revert to using
skip by record with the IO$_SKIPFILE interface.

Running the utility with the ENABLE_ALLOWFAST setting will return the
functionality back to the boot up settings of using skip by filemarks only when
the IO$M_ALLOWFAST modifier is present.


Additional Issues arising from Design Review
--------------------------------------------

1) The utility/setmode interface must have some level of privilege checking prior
   to changing the skip by filemark settings.

2) The utility must check that the device is a local MK device (not a served MK
   device) before issuing the IO$_SETMODE with the skip by filemarks modifiers.

3) A further investigation into the effect of this design on served tapes is
   needed.

4) Some method (SHOW MAGTAPE for example) should be present for indicating the
   state of the SKIPFILE_SUPPORTED bit in UCB$L_MK_FLAGS.