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

[Arne Vajhøj]

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.

Arne