SCSI tape question

Chuck Guzis cclist at
Fri Dec 12 10:54:04 CST 2014

On 12/12/2014 04:18 AM, Peter Corlett wrote:

> My copy of the *draft* standard states "If the logical unit encounters a
> filemark during a READ command, CHECK CONDITION status shall be returned and
> the filemark and valid bits shall be set to one in the sense data" but perhaps
> the final standard decided to be different for perverse reasons.  Draft copies
> of questionable provenance is what everybody's working from though, because the
> final versions are $$$.

Well, this is just odd--if I do my read not as variable length, but 
rather using specified block size and the "fixed" bit (bit 0 of byte 1 
in the READ command), I do indeed get the filemark indication.

I'll have a look at the *nix tape driver code, but AFAIK, the standard 
tape handlers for Linux and Unix depend on the block length being 
explicitly specified in a MODE SELECT command.  When doing 
variable-length reads, this block length is 0.

And yes, I can read past double filemarks and get to the previously 
written data.  Eventually, a blank media or EOM condition arises and 
calls a halt to things.  Recently, I ran into a case where the 
double-filemark was an accident--a zero-length file was written, so the 
disrespect for double filemarks is necessary.

I've got an old X3T9 spec here, as well as a few vendors' manuals and 
the strange non-filemark behavior isn't detailed--however, it's simple 
enough to translate this into the expected "filemark-sensed" code.

It's not a stumble, just curiosity.


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