tape drive error

[bscott at ntisys.com]

On 13 Feb 2004, at 3:14pm, cfarinella at appropriatesolutions.com wrote:
> I've started getting the following error when trying to back up to tape:
> 
> "Write not multiple of tape block size"

  The tape driver in the kernel supports two modes of operation: "Fixed
block size" and "variable block size".  Some tape drives only support a
single block size (or set of block sizes), which is where fixed-block-size
mode is important.  Most recent drives (i.e., anything built in the last
couple of decades) support variable block sizes.

  In "fixed block size" mode, all I/O calls have to be a multiple of the
block size configured for the tape drive.  In "variable block size" mode,
the block size can vary from one I/O call to the next.  The block size for
each I/O call is determined by the size of the I/O call's data buffer.

  In the above, "I/O call" refers to the actual write(2) or read(2) system
calls the kernel sees.  Most of us work several layers above that, using a
utility like tar(1), cpio(1), or dump(8).  Figuring out how to translate
your utilities options for sizes (assuming it has any) to what the kernel
sees can be non-obvious.

> This has worked for a year and a half, and I haven't changed anything that
> I am aware of.

  Heh. Well, if "nothing has changed" really is true, then this is almost
certainly a hardware failure.  Check the output of "dmesg" and/or the kernel
log.  Hardware failures with tape drives will usually be accompanied by
kernel error messages.  They might not mean anything to you, but their
existence alone is a sign of trouble.

  However, I suspect something *did* change.  That happens often often
enough, for any number of reasons.

  What does the output of "mt status" tell you?  For example, from a server
we have here:

	# mt status
	SCSI 2 tape drive:
	File number=0, block number=0, partition=0.
	Tape block size 512 bytes. Density code 0x13 (DDS (61000 bpi)).
	Soft error count since last status=0
	General status bits on (41010000):
	 BOT ONLINE IM_REP_EN

The important bit here is the "Tape block size 512 bytes".  That means the
tape drive is in fixed-block-size mode.  A value of 0 (zero) indicates
variable-block-size mode.  You can set the block size using the "mt setblk"  
command.  For example, try "mt setblk 0" to switch to variable-block-size
mode.

  Note that, depending on your distribution and version, the output of "mt"  
may not be as informative.

  For more information, see the tar(1) (especially the "-b" option), st(4),
and mt(1) man pages.

  See also the following kernel files:

	drivers/scsi/README.st
	drivers/scsi/st.h

-- 
Ben Scott <bscott at ntisys.com>
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