Hi,

don't know where to post this, in software or in hardware? Because the questions will have a little bit of both.

Ok, first, let's start with USB TapeDrives. How well are they supported in Linux? Are there any problems, like drivers etc? Or is it better to use standard "build-me-in" tapedrive?

Now onto tar. I've used tar many times...for files, but never on a tapedrive.
How to use it?
tar -x /dev/{tapedrive} files ?
How to append new stuff to the tape?
tar -Ax /dev/{tapedrive} files ?

On man page:
What exactly does it meen differences between archive and file system?

Again on man page:
Does this mean that you can not delete certain files from the tape archive?

Then I'd need to pull out, file by file from the tape, check it with the new version of the file(programatically) and add new differences from the newer file back to the tape archive. How to achieve this?

Regards,
Thomas

You can find answers about using the tape drive here.

Google should be of help in solving the problem of how to delete files from the tape.

The first thing to get clear on is that a tape drive is a sequential device, not a random access device like a disk drive. What that means is that you write it from beginning to end and read it from beginning to end. You can do things like fast forward and backward to file marks, and you can rewind. But, in particular, when writing to the tape, you write to the end of the data on the tape. If you write to an intermediate point on the tape, then that becomes the end. You have essentially wiped all data beyond that point. So, . . .

While tar refers to tape files and has the ability to manipulate and change those files, it cannot do those things to a file on a tape. You would have to read the file off the tape, manipulate it, and then write it back out to the tape. In so doing you would either append it to the tape or overwrite what's already on the tape. That's not typically what people use tapes for.

`man mt` would give you the sorts of operations you can do on a tape. The previous link given by bigrigdriver gives you how to put it all together. Note, however, that the device name you use for the tape drive affects how it behaves. /dev/st0 is a rewind device, which means it will rewind after each operation. If you write a file to it, and then in a separate command write another file to it, it will rewind in between and end up overwriting the file. Use /dev/nst0 to avoid that behavior. The "n" refers to the non-rewinding device. Same tape drive, just different driver behavior.

So if I get this right, even if I'm doing recursive backups only of changed files to a tape-drive, every way I turn it, I'll end up with 2 file versions on that tape? (except if I read all and re-write all data on that tape, which I would rather avoid)
How about data recovery then? Can I hunt it down by date-time of writing the data on the tape, or will I just have to guess, which file is newer?

If you are doing incremental to the tape, you can assume the later file on the tape is newer. You could track what is on the tape by keeping a catalog file on disk. Gnutar will generate that for you from the tar file. This is basically what programs like Amanda do for you automatically. When you do an amrecover, for example, it is looking at the catalog files while you navigate through to what you want to recover. When you ask it to go ahead and do it, it tells you what tape to load and reads the appropriate records. Depending on what you want, Amanda may be overkill for you, but you can find out more about it here http://wiki.zmanda.com/index.php/Main_Page, if you are interested.

Thank you both for lots of information. I'm sure it will be useful for me.
As for Amanda...good to know that it exists, and I'm sure I'll learn much from it, but it's not an option for me at the moment.

one thing is tape drives are VERY outdated
and have been since the mid 1990s or earlyer and external harddisks are so cheap and reliable that it would be better to just get one of those

They may be outdated, but still many systems use TapeDrives. And for testing purposes, I'd scouted a USB TapeDrive. As for hard disks, they are way less reliable than magnetic tapes. An electric surge can damage the disk, while the electric surge can at worst damage the tapedrive and the current tape inserted, as for the other tapes, they stay intact, so you may loose maybe a week of backups, if you're not backing up enormous amounts of data.
No surge protecting hardware will protect you from a direct lightning strike.

That's a very opinionated response. A lot depends on your environment and requirements. I have several departments that have fairly new, latest technology, tape libraries. We run a 6 week cycle of nightly tapes that handle backups for substantial networks of servers. They are also used for periodic archival tapes. That is in addition to having lab computers backed up to the server's hard drives, and having ZFS running nightly snapshots for the semester on multi-terabyte raid arrays (raidz2 [which is similar to raid 6] with a hot spare) for classroom and student lab data.