Writing to tape from C system calls

Writing to tape from C system calls

[Ujval Lodha]

Hi

I'm new to tape devices.

My application requires me to seek to a certain block on the tape device and
write data on it (besides being able to retrieve data by reading).

So far, I have no trouble reading from the tape.

When I write to it at any block that block becomes the last block on the
device. I am unable to seek to any block after that on the tape. I have
tried :

    a. Using mt to seek and dd to write
    b. Using open / ioctl / write from C

Both with the same results. I am using a Tandberg SDLT for my tests with the
linux kernel st drivers.

Is this normal? Is it not possible to use the tape device - and modify data
at any block by seeking to it and writing?

Is there some source of information where I can read about programming tape
devices from C? So far, I havent been able to find one.

Please advise.
Regards,
Ujval Lodha

RE: Writing to tape from C system calls

[Tony Battersby]

> When I write to it at any block that block becomes the last
> block on the device. I am unable to seek to any block after
> that on the tape.

> Is this normal? Is it not possible to use the tape device -
> and modify data at any block by seeking to it and writing?

It is completely normal.  All modern tape drives work this way.  Unlike
a disk drive, a block address on tape does not correspond to a
predefined location on the medium.  The tape drive (re)formats the
medium while it writes, which invalidates anything past the last written
point on tape.  So, you can seek/read/write the tape from the beginning
of the tape up to just after the last written block, but the tape medium
after that point is not considered to be formatted and cannot be used
(again) until it is reached while writing.

As you observed, writing to tape in the middle of valid data will cause
the contents of the tape to be truncated at the write point.  That's the
way the technology works.  There is no way to update a block in the
middle of the valid data without losing everything that comes after it.
The only way around this is to use multiple partitions, but not all tape
drives support partitions, and changing partitions takes time.

> Is there some source of information where I can read about
> programming tape devices from C? So far, I havent been able
> to find one.

man st

But that's pretty low-level, of course.

Anthony J. Battersby
Cybernetics

Re: Writing to tape from C system calls

[Ujval Lodha]

Thanks for your reply. That clears up my understanding.

I've already read the man page. That is how I got so far, in fact. It seems
to require some understanding of the functionality of tape drives. The
description of some of the options seems very cryptic to me - as I dont know
much of the jargon associated with tape drives. Any material that would help
understand tape devices and tape as a medium - filemarks, setmarks, and the
order of operations one is expected to perform.

Also, I wonder if it absolutely not possible to use the tape in the manner I
intended. I am using the tape without compression - and I dont mind making
'sub-optimal' use of the capacity of the tape.

Thanks again.

Regards,
Ujval


----- Original Message ----- 
From: "Tony Battersby" <tonyb@cybernetics.com>
To: "'Ujval Lodha'" <ujval@gdc-tech.com>; <linux-scsi@vger.kernel.org>
Sent: Friday, December 12, 2003 11:14 PM
Subject: RE: Writing to tape from C system calls


> > When I write to it at any block that block becomes the last
> > block on the device. I am unable to seek to any block after
> > that on the tape.
>
> > Is this normal? Is it not possible to use the tape device -
> > and modify data at any block by seeking to it and writing?
>
> It is completely normal.  All modern tape drives work this way.  Unlike
> a disk drive, a block address on tape does not correspond to a
> predefined location on the medium.  The tape drive (re)formats the
> medium while it writes, which invalidates anything past the last written
> point on tape.  So, you can seek/read/write the tape from the beginning
> of the tape up to just after the last written block, but the tape medium
> after that point is not considered to be formatted and cannot be used
> (again) until it is reached while writing.
>
> As you observed, writing to tape in the middle of valid data will cause
> the contents of the tape to be truncated at the write point.  That's the
> way the technology works.  There is no way to update a block in the
> middle of the valid data without losing everything that comes after it.
> The only way around this is to use multiple partitions, but not all tape
> drives support partitions, and changing partitions takes time.
>
> > Is there some source of information where I can read about
> > programming tape devices from C? So far, I havent been able
> > to find one.
>
> man st
>
> But that's pretty low-level, of course.
>
> Anthony J. Battersby
> Cybernetics
>

RE: Writing to tape from C system calls

[Tony Battersby]

> Any material that would help understand tape devices and
> tape as a medium - filemarks, setmarks, and the order of
> operations one is expected to perform.

Try the "SCSI-3 Stream Commands (SSC)" and "SCSI Stream Commands - 2
(SSC-2)" specifications available at http://www.t10.org/drafts.htm.
They give a nice general overview of tape technology in the opening
section.

Anthony J. Battersby
Cybernetics