Re: [rdiff-backup-users] Tar replacement - format proposal

[John Goerzen]

On Mon, Sep 29, 2003 at 03:20:48PM -0400, Greg Freemyer wrote:
> Commercial backup software often writes "file marks" on the tape every
> once in a while.  (Linux can do this with the mt command.  Also tape
> file marks have nothing to do with traditional on disk files, but are a
> unique tape concept.)  

FWIW, on Linux, writing a file mark happens automatically when you close()
the fd referring to the tape device.  I believe this behavior is common with
other operting systems as well.

> Then tape drives have the ability to move between file marks extremely
> quickly.  Apparently they are physically very large and easy for the
> tape drive to register even at very high speed.

True; however, it also complicates things because it's a misuse of what the
file mark is really for.  It's really for just that -- marking EOF and
skipping to the desired file on a tape.  Many tape drive also have the
ability to quite rapidly seek to a particular block anywhere on the tape,
whether it's at a file mark or not.

> As you say, tar's inability to take advantage of this type of tape
> handling causes it to be extremely slow for partial restores.

Indeed.