dd interface to posix_fadvise

[Paul Eggert]

Pádraig Brady <address@hidden> writes:

> I was thinking it would be useful to
> add an option to dd to tell it to
> instruct the OS not to cache the data.

Yes, that's handy, but doesn't "dd iflag=direct" already do that for you?

Hmm, I see that Linus doesn't like O_DIRECT
<http://kerneltrap.org/node/7563>.  Perhaps we should also support the
method that Linus prefers.  Can you fill us in as to why that is?

> Note, posix_fadvise(POSIX_FADV_DONTNEED) on linux,
> invalidates the cache for a file rather than a process....
> This is not what is usually required, and
> I'm working on the kernel guys to get them
> to change this.

Yes, thanks, that sounds right.

But dd always accesses the file sequentially.  Wouldn't
POSIX_FADV_SEQUENTIAL be better?  or perhaps POSIX_FADV_NOREUSE?  And
if not, why not?

I just tried the following naive patch on my Debian stable host and dd
immediately dumped core inside posix_fadvise.  This is not a good
sign.  It sounds like, despite Linus's wishes, posix_fadvise is not
really ready for prime_time.


diff --git a/src/dd.c b/src/dd.c
index 27a4a08..824c928 100644
--- a/src/dd.c
+++ b/src/dd.c
@@ -68,6 +68,10 @@ static void process_signals (void);
 # define fdatasync(fd) (errno = ENOSYS, -1)
 #endif
 
+#ifndef POSIX_FADV_SEQUENTIAL
+# define posix_fadvise(fd, offset, len, advice) ENOSYS
+#endif
+
 #define max(a, b) ((a) > (b) ? (a) : (b))
 #define output_char(c)                         \
   do                                           \
@@ -1734,6 +1738,9 @@ main (int argc, char **argv)
 #endif
     }
 
+  posix_fadvise (STDIN_FILENO, 0, 0, POSIX_FADV_SEQUENTIAL);
+  posix_fadvise (STDOUT_FILENO, 0, 0, POSIX_FADV_SEQUENTIAL);
+
   install_signal_handlers ();
 
   start_time = gethrxtime ();