bug#26311: dd support for POSIX_FADV_NOREUSE etc.

[Pádraig Brady]

On 30/03/17 08:42, Paul Eggert wrote:
> The 'dd' command has some complicated logic for iflag=nocache and 
> oflag=nocache, which I don't entirely understand but expect is in there 
> for good reason. I suggest that 'dd' also have simpler logic for direct 
> usage of posix_fadvise options. For example, 'dd iflag=noreuse' could 
> cause it to call fadvise (STDIN_FILENO, FADVISE_NOREUSE). I suppose 
> 'sequential' should be the default.

Yes maybe, though the low level meanings of these flags are
a bit confusing and I'm not sure how consistent they are over kernel versions,
hence why a higher level "nocache" item was chosen initially.
I have some notes on these settings at:
https://github.com/coreutils/coreutils/blob/c7bcea1b/src/sort.c#L894-L939

> Dumb question: why does dd iflag=nocache check the return value of 
> posix_fadvise? The fadvise function ignores the return value and says 
> why; do these reasons not apply to 'dd'?

Well for dd it's a request, which for other utils it's a performance advisement.
As stated in the info docs, for dd it's only significant in the special case
of dropping cache for the whole file:

  $ : | dd iflag=nocache count=0 status=none
  dd: failed to discard cache for: 'standard input': Illegal seek

cheers,
Pádraig