finding remaining space in Tape ( DAT)

[madhudeepan]

we are using DAT drive to take ignite backup,
how to identify the remaining space on that tape

[Johnson Punniyalingam]

This is difficult. I think the best way is if youre using fbackup / Backup software , it reports at the end of the backup;

total blocks written to output file /dev/rmt/2m: 54756166

[Michael Steele_2]

Hi

Ignite has two records of 2K and 10K or 2048 bytes and 10240 bytes.

A 4mm DAT tape holds 8 GB or 1024 bytes x 1024 x 1024 or 1024 MB or 1048576 KB

a) mt -f /dev/rmt/0m rew
b) mt -f /dev/rmt/0mn fsf 1 (* move forward one record past the 2K lif record *)
c) dd if=/dev/0mn of=/dev/null bs=10240

Will return with

XXXXX blocks in out

XXXXX * 10240K minus 1048576 KB minus 2K (* first LIF record *)

[Michael Steele_2]

PS WAG

[Steven Schweda]

> total blocks written to output file [...]

> XXXXX blocks in out

And if the drive does compression, then you
still know approximately nothing about the
remaining tape.

[Michael Steele_2]

Again, smartness. Do you see tape compression in the device? I'm using 0m as an example not c#t#d#BEST0m.

[Patrick Wallek]

>> I'm using 0m a

Potentially irrelevant. Some drives do compression internally no matter what you do from the OS side.

[Michael Steele_2]

Except that if compression is automatic then the total size of is also provided and then the 8 GB DAT tape will be 12 GB. So you just substitute the 12 GB.

All tape drives advertise their sizes.

[Steven Schweda]

> [...] I'm using 0m [...]

And who cares what _you_ are using?

And if a tape is written with compression, do
you think that the tape drive will read it
without compression, no matter which device
name you use?

> Again, smartness. [...]

Beats the opposite, I claim.

[Michael Steele_2]

And your personal motto says it all

"..Questions are a burden to others.."

Dude, you got some issues

[Patrick Wallek]

The other thing about tape drives that use compression is that you NEVER know exactly how well your files are compressing.

You may have an 8GB NATIVE capacity tape, but if your files are compressing REALLY REALLY well, then you might get 40 or 50 GB of data on that tape.

In my mind the whole exercise of trying to figure out how much space is left on a tape is rather pointless.

Since this post is talking about IGNITE backup, as long as the VG00 fits on the tape you are good.

If you are contemplating adding other non-OS data to the Ignite backup -- DON'T DO IT. Ignite is meant as a VG00 backup, NOTHING else.

[Steven Schweda]

> [...] you NEVER know exactly how well your
> files are compressing.

Well, "NEVER" may be a bit strong. I know
nothing, but I suspect that it's possible to
ask a compressing drive for more details
about compression and/or tape use, but I
doubt that the standard device drivers do
this. What _is_ true is that if all you know
is a byte (or block) count of the data you've
written, then, as I said before, you probably
don't know much about the tape used.


> Dude, you got some issues

Yes, I subscribe to several periodical
publications, so, for example, last month I
got several issues. But what's your point?

[Michael Steele_2]

A) "...Questions are a burden to others..."

I can think of no other bigger dicotomy than your personal motto and you being on this forum answering questions. And I think anyone would have at least 10 minutes to comment on how weird this is.

B) That your your smart arse attitude -

"...Non-psychics may need a clearer explanation of what you have and what you wish to do with it:..."

The Voices which speak to you don't seem to
speak to me.

> Ask whomever told you that [...]
(And that's whoever.)

I don't immediately see what would make this
particularly difficult. Is there more than
the obvious variability?

You're assuming more than I would.

So, you really _were_ serious. Scary.

Mine has had it in there since I installed the OS, and it wasn't my idea.

Why do you care what the prompt is? What
doesn't work?

Is there some actual problem
which you are trying to solve?

Perhaps someone who can see exactly what you
did, and exactly what happened when you did
it? (In other words, a psychic or a good
guesser.)

It might help if you showed some actual
commands and their actual output instead of
these vague descriptions and interpretations.

Perhaps if you answer some of the same
questions as in this other thread:

In short, you got more complaints about posters than answers - go blow

[OldSchool]

The OP said DAT, which *could* mean anything for DAT, DDS1-4 or 160 or???

Most recent drives compress, regardless of the device used, and the compression algorihm will compress what it can, as best it can. Since actual compression achieved can vary from one block to the next, you can't calculate the overall compression.

They (the drives) don't report actual bytes written on the media, or if they do, I don't know a tool that will retrieve that information, so you can't get an accurate account of what's been used, meaning you can't acurrately calculate what's left either.

This same question pops-up all the time on the DataProtector forum as well. As noted elsewhere, this is a rather pointless task.

[Steven Schweda]

> I can think of no [...] how weird this is.

http://en.wikipedia.org/wiki/Irony

[Insert forehead slap here.]

> In short, you got more complaints [...]

No, I _gave_ more complaints. What I _got_,
mostly, was insufficient information to make
possible reliable answers.

But I can see how this all might confuse
someone who thinks that a full tape might be
caused by a SCSI bus problem.

Am I missing something here? Is anyone
compelling anyone else to read anything in
these forums? Is there some problem here
which is crying out for a solution? Or is
the goal merely to waste time and space?