514 byte block i/o buffers in v6?

514 byte block i/o buffers in v6?

Post by Roger Hurwi » Sun, 21 Jul 1991 04:01:16



    I am currently taking a UNIX internals class, where we
    study the source code from version 6.  Recently, we were looking at
    the code for block i/o, and it was noticed that buffers were
    declared as:

    char buffers[NBUF][514];
                      ~~~~~
    Since the disk sectors are 512 bytes, and the buffers are treated
    as if they were 512 bytes, does anyone know why they were declared
    as 514 bytes?

 
 
 

514 byte block i/o buffers in v6?

Post by Rick Stevens » Mon, 22 Jul 1991 22:53:57



>    I am currently taking a UNIX internals class, where we
>    study the source code from version 6.  Recently, we were looking at
>    the code for block i/o, and it was noticed that buffers were
>    declared as:

>    char buffers[NBUF][514];
>                  ~~~~~
>    Since the disk sectors are 512 bytes, and the buffers are treated
>    as if they were 512 bytes, does anyone know why they were declared
>    as 514 bytes?

Version 7 buffers were declared to have a length of BSIZE+BSLOP bytes,
where BSIZE was 512 and BSLOP was normally 0. There was a comment
in param.h where BSLOP was defined which said that it could be 0
unless you had a TIU/Spider. I don't know what a TIU/Spider was,
but it sounds like it had a habit of overrunning the 512 bytes
by just a little.

Rick.

 
 
 

1. FreeBSD File System 512 byte-blocks --> 1024 byte-blocks

I was installing FreeBSD 2.1 several times trying to get things right
and I noticed that on my last install, the df command would return file
system block sizes of 512 bytes. All of my previous installs were of
1024 byte blocks. I always used the auto feature during the
partition. Does anyone know why it changed? And is it possible to change
the block size back to 1024bytes w/o reinstalling FreeBSD? ...what
gives?

Any suggestions would be appreciated.

Hook Hua

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