[Info-vax] File I/O BandWidth Versus Disk I/O Bandwidth

[Arne Vajhøj]

On 1/14/2024 7:22 PM, Arne Vajhøj wrote:
> On 1/14/2024 7:10 PM, Lawrence D'Oliveiro wrote:
>> Back then, other OSes (like VMS) did not try to hide from applications 
>> the
>> fact that file space allocations were done in units of sectors (or some
>> multiple thereof). Whereas Unix pioneered the idea that, if an 
>> application
>> wrote 975 bytes to a file, then it will only read back 975 bytes, not 
>> 1024
>> bytes (or some even larger amount).
> 
> If the VMS application use language RTL IO or RMS then then it
> also write N bytes and read N bytes.
> 
> (there is a small difference in that the number of bytes on
> disk may be different from N depending on API and the
> record format of the file!)
> 
> It is only if using SYS$QIO(W) or SYS$IO_PERFORM(W) to do IO
> that the view of the file becomes X allocated blocks where the code
> needs to deal with FAT$L_EFBLK and FAT$W_FFBYTE. And explicit
> extend file when needed.

To clarify. Applications do not *need* to know or deal with
it, but they still *can* if they want to.

Heck - even DCL can.

scare.com:

$ set ver
$ type 'p1'
$ eof = f$file(p1, "eof")
$ ffb = f$file(p1, "ffb")
$ set file/attr=(ebk=0,ffb=0) 'p1'
$ type 'p1'
$ set file/attr=(ebk='eof',ffb='ffb') 'p1'
$ type 'p1'
$ exit

and:

$ @scare something.ext

:-)

Arne