UNIX for Dummies Questions & Answers

This isn't causing me a problem, but it is puzzling (& slightly worrying) me.

If I use tar to copy to a tape & the tape is write protected, I only get the message 'tar: tape write err' if there are a few files to copy.

eg. # tar cvf /dev/rStp4 /usr/data
will produce the messages

/usr/data/file1
/usr/data/file2
/usr/data/file3
/usr/data/file4
/usr/data/file5
tar: tape write error


however # tar cvf /dev/rStp4 temp
produces the message

a temp 1 tape block

and no error message.

Any ideas?

This is on SCO openserver 5

I don't know the answer to your question. But I will make an educated guess.

My guess is that tar is using fopen(), setvbuf(), fwrite(), and fclose() (or a very similiar set of functions) to write to the tape device.

These routines implement buffered I/O. When you call fwrite() your data is shoved into the buffer. Only if the buffer fills will a write actually occur. And fclose() will write any data left in the buffer on the way out.

Programmers will usually check the return code from a routine like fwrite(). But only a first class programmer will bother to check the return code from fclose().

So if you write enough stuff to the tape that a fwrite() needs to flush the buffer, tar detects the problem and displays an error message. But if you don't fill even one block, tar does not notice that its fclose() failed.

Remember this is only a guess. But I have seen this mistake hundreds of times.

Thanks, I thought it might be something to do with a buffer filling

It still concerns me that a backup (albeit a small one) done using tar would not give an error message. I don't think there is any way round that though since the error code ($?) is set to 0 too.

After the backup finishes, do a table of contents list on it. And extract the last file. If both of those operations work you have a fairly strong indication that the tape is good.

Yes, I suppose so or save a small file with a number (that is incremented on each backup) in it & extract that back to check. That way I can be sure that I'm not just copying back a file with the same name from an old backup.

Still, this isn't actually effecting me, all our backups are too big for this to apply. It just seemed odd.

Thanks Perderabo.