On 2016-02-22 16:00, Paul Koning wrote:
On Feb 22, 2016, at 3:50 AM, Johnny Billquist <b...@softjar.se> wrote:
On 2016-02-21 16:51, Paul Koning wrote:
...
True in principle. But a number of OS used a different file structure for
DECtape than for disks -- because of the performance issues. DOS does, for
example, and RSTS copied the DOS DECtape directory structure (but not the disk
one). RT11, on the other hand, uses the same format for both, and the RT11
format works well. That's why RT runs from DECtape without any extra work,
while DOS and RSTS don't.
Interesting. I didn't know that RSTS/E used a different file system for
DECtapes. RSX do not. It's the same file system used on both large disks,
floppies and DECtapes.
The RSTS file systems uses directories that are linked lists, which means
things get really horribly slow if you can't do decent random access. That
includes the file windows (pointers to where the file blocks are). DOS ditto,
though the details differ. So for DECtape, DOS took the aproach of having each
data block contain the link to the next, rather than having those links in the
directory as it does for disks. This is why DECtape file blocks in the DOS
format are only 510 bytes long.
I see.
Well, ODS-1 have a index file, which holds the metadata for the actual
files. So you do not have a linked list there, so it works in a way that
is pretty efficient for both disks and DECtapes.
Johnny
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