copyFile with POSIX_FADV_SEQUENTIAL (#22776)

- Continuation of https://github.com/nim-lang/Nim/pull/22769
- See
https://pubs.opengroup.org/onlinepubs/9699919799/functions/posix_fadvise.html
- The code was already there in `std/posix` since years ago. 3 line
diff.

---------

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

changelog.md

@@ -9,6 +9,7 @@
 [//]: # "Changes:"
 
 - Changed `std/osfiles.copyFile` to allow to specify `bufferSize` instead of a hardcoded one.
+- Changed `std/osfiles.copyFile` to use `POSIX_FADV_SEQUENTIAL` hints for kernel-level aggressive sequential read-aheads.
 
 [//]: # "Additions:"
 

lib/std/private/osfiles.nim

@@ -203,6 +203,7 @@ proc copyFile*(source, dest: string, options = {cfSymlinkFollow}; bufferSize = 1
   ## `-d:nimLegacyCopyFile` is used.
   ##
   ## `copyFile` allows to specify `bufferSize` to improve I/O performance.
+  ##
   ## See also:
   ## * `CopyFlag enum`_
   ## * `copyDir proc`_
@@ -243,6 +244,13 @@ proc copyFile*(source, dest: string, options = {cfSymlinkFollow}; bufferSize = 1
         if not open(d, dest, fmWrite):
           close(s)
           raiseOSError(osLastError(), dest)
+
+        # Hints for kernel-level aggressive sequential low-fragmentation read-aheads:
+        # https://pubs.opengroup.org/onlinepubs/9699919799/functions/posix_fadvise.html
+        when defined(linux) or defined(osx):
+          discard posix_fadvise(getFileHandle(d), 0.cint, 0.cint, POSIX_FADV_SEQUENTIAL)
+          discard posix_fadvise(getFileHandle(s), 0.cint, 0.cint, POSIX_FADV_SEQUENTIAL)
+
         var buf = alloc(bufferSize)
         while true:
           var bytesread = readBuffer(s, buf, bufferSize)