[Gllug] [bit OT] what block size is best for a tape drive?

[t.clarke]

Chris asked whether 'NOT specifying the block size does the right thing these
days'.

All I can say is my experience with Unix (SCO Openserver  - dont groan !)
is that specifying a reasonably large block size on a DAT or QIC drive
definitely speeds things up.  I am unsure as to whether a block size
specified to a utility such as tar cpio dd etc actually sets the physical
block size on the device or simply defines defines the size of each atomic
'write' operation passed to the kernel.  On SCO Unix tape devices are used
as 'raw' devices, so , as I understand it, no blocking/buffering is done by
the kernel; each 'write' done by a user program results in a physical write
command to the drive.  What the drive actually does with the command is
anyones guess  - I believe some drives 'do their own thing' as far as
physical tape-block size is concerned (whereas the old open-reel tapes would
always write a block of exactly the length you specified - including a zero
length one).

A quick google reveals that Linux apparently has a command for setting the
physical block size of the tape being written; so presumably the programs
idea of block size and the actual physical block size on the tape can be two
different things !

All somewhat confusing.   I just leave the tape device set to defaults and
use largish logical block sizes specified to tar,dd, whatever!   Seems to work.

By the way,  I assume that the ECC correction on the tapes is done on a
physical block by block basis - if so really large physical tape blocks might
be a potential for problems if a tape is a bit 'iffy' !

Tim
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