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Re: File-system on tape

Hi Ted

There was a paper, I forget the locateion or details, but it
concerned a "log structured file system" that would be made to order
for  tape file system. writes at the end, maitains a log of 
space allocation, Dang! I wish I'd paid more attention to the paper.

The Sprite operating (now defunct) was associated with log structured
file systems. Not just for tapes, either.Log structured file systems
work good for disks. 

Maybe you can chase the details down. I'll keep on the look out and 
send you a note If I find it.


On Wed, 27 May 1998, Ted Harding wrote:

> On 27-May-98 Stephen Carpenter wrote:
> > On Tue, May 26, 1998 at 10:15:58PM -0600, Lazar Fleysher wrote:
> >> Is it possible to create a file system on a tape drive ( like on
> >> mainframes) and use it as a disk? I know it is very slow, but is it
> >> possible?
> > 
> > That is really very sick and twisted to even think of such a thing...
> > hmm I wonder why I never thought of it :) 
> Of course it CAN be done (and when tape was the main storage medium for larger
> archives it often was done). But only dire necessity would justify setting up a
> fullly functional file system on tape, because of the sequential access.
> For instance, you can't readily recycle freed-up space on a tape after
> "deleting" a file: OK if the next file to go in is shorter than the freed space,
> but if it's longer then either it all goes at the end (if there's room) or part
> goes in the gap and the rest at the end/next gap, which might mean several
> minutes to reposition for the next chunk of the file; and collating the logic
> required to simultaneously retrieve several files stored fragmentally in this
> way is likely to lead to mental dysfunction of the programmer ("sick and
> twisted" indeed).
> >> If not, is it possible to have several files on one tape and how to
> >> access them?
> As Steve says, tar + mt provide the basic tape resources for reading, writing
> and positioning. You could reserve a fixed size block at the beginning to store
> a file as a "pseudo directory", say with space for 1024 entries each with 256
> bytes, so you get filename, size, date and sequential position[s] on tape. You
> read this off first and use it to plan the rest of the operations. When you've
> changed the tape contents you update this "directory" and write it back to its
> old position. You could even organise fragmented storage this way ...
> Ted.
> --------------------------------------------------------------------
> E-Mail: (Ted Harding) <Ted.Harding@nessie.mcc.ac.uk>
> Date: 27-May-98                                       Time: 21:37:37
> --------------------------------------------------------------------
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