swapping tapes between Suns and DECstations

[Frank J. Nagy:VAX Guru,Wizard&Loose Cannon]

>>It seems that there is no standard format for 8mm tapes, so that
>>an 8mm tape written on, say, a DECstation 5000 cannot be read on
>>a Sun and vice versa.  

Actually, there are two basic formats for 8mm tapes (all the 8mm
drives can read/write either, its up to higher level software to
select the correct mode).  The drives can write in either variable
block or fixed block mode; tapes written in one mode cannot be read
in the other.  All VAXes (VMS) write tapes in variable block mode for
instance since this provides the correct emulation of normal magnetic
tape operations (user records can be any size).  In fixed block mode,
records written to the tape must be multiples of 1024 bytes (the basic
stripe size of data on the tape - in variable block mode the drive
firmware manages fitting user data to/from the 1024 byte stripes).

We've discovered than some UNIX systems (Sun's for instance) only
provide [at this time] drivers that use fixed block mode.  Other system,
such as SGIs, provide drivers that handle either fixed or variable block
mode.

We've had no problems interchanging 8mm tapes here at Fermilab between
numerous systems: VAXes running VMS with several different 8mm systems
from different vendors [8mm on VS3100 SCSI, QBus-to-SCSI and even
HSC-based 8mm drives] between themselves and with SGIs (Personal Irises
and several 4D/240s, using variable block mode) and a DECStation 3100.

= Dr. Frank J. Nagy   "VAX Guru & Wizard"
= Fermilab Computing Division/Distributed Computing Dept/Special Projects Grp
= HEPnet/SPAN: FNDCD::NAGY (43123::NAGY) or FNAL::NAGY (43009::NAGY)
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