>> Back
in the day, we used to move copies around on magtape,
bypassing
>> all sorts of stream-of-bytes issues.
Today, of course, most
things
>
expect streams of bytes, so that's how most things are presented.
On Apr 4, 2009, at 12:53 PM, Richard wrote:
How does magtape avoid the stream-of-bytes issue?
Magtape has blocks.
What exactly is a block?
Is it defined as a sequence of bits or as a sequence of bytes?
If its just a sequence of bytes that define a block, I'm not sure I
understand how blocks avoid the stream-of-bytes issue.
I think the more important point is that tapes also have file
attributes, similar to what RMS has. So when you need valid attributes
-- as you do on VMS -- then transferring the file via ANSI labeled tapes
will do that, while transferring via FTP will not.
But even if you don't need attributes, which is true on some other
operating systems, block boundaries may matter. So a block is some
number of bytes WITH a boundary. By contrast, TCP transfers bytes with
no boundaries. This is why there are so many application layer
protocols that have to do extra work to put boundaries back in, to make
up for this omission in TCP. (It's pretty much unique to TCP; every
other transport layer protocol I can think of it transfers packets, not
byte streams, so it does provide the notion of blocks.)
paul
