9 track tapes and block sizes

[Chuck Guzis]

On 9/16/20 10:29 PM, shadoooo via cctalk wrote:
> Hello,
> I have a question about 9 track tapes and block sizes.
> What I know is that tape is subdivided in files by means of marks, and each
> file is subdivided in blocks of equal size.
> Programs like tar use a specific block size to create files on tape.
> However files can have different block sizes like bootloader file,
> installation dumps and root file system copy on 2.11BSD.
> Now suppose you find and unknown tape you want to preserve: using dd you
> could easily 1:1 copy tape files to hard disk files using a SCSI drive and
> Linux.
> But: how you know which block size is on the tape?

Variable length blocks can be handled using the mt(1) command on linux.
"defblksize 0" generally will do this, but this varies between distros.
Some use the "setblk 0" parameter.  Not all hardware and not all drivers
support variable block size.

As far as 9 track maximum block size, I've seen 132K blocks--and on old
CDC 6000 7-track systems, the 1LT driver would permit a block to be as
long as a reel of tape, provided the application reading or writing
could keep up.

1LT was a bag of worms.  ECS accesses could torpedo a transfer; it made
the DD60 flicker like crazy.  It was a great source of PSRs.

--Chuck