On 13 Feb 2004, at 3:14pm, [EMAIL PROTECTED] 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 <[EMAIL PROTECTED]>
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