[Libguestfs] [PATCH libnbd v2 1/2] copy: Set POSIX_FADV_SEQUENTIAL flag on files.

[Richard W.M. Jones]

On Linux this doubles the readahead.  Even though we are not strictly
speaking going to read or write the file sequentially (only mostly
sequentially) it may provide some minor benefit.
---
 configure.ac    | 3 +++
 copy/file-ops.c | 7 +++++++
 2 files changed, 10 insertions(+)

diff --git a/configure.ac b/configure.ac
index 5723912..fb72953 100644
--- a/configure.ac
+++ b/configure.ac
@@ -98,6 +98,9 @@ AC_CHECK_HEADERS([\
 
 AC_CHECK_HEADERS([linux/vm_sockets.h], [], [], [#include <sys/socket.h>])
 
+dnl posix_fadvise helps to optimise linear reads and writes (optional).
+AC_CHECK_FUNCS([posix_fadvise])
+
 dnl Check for strerrordesc_np (optional, glibc only).
 dnl Prefer this over sys_errlist.
 dnl https://lists.fedoraproject.org/archives/list/glibc@lists.fedoraproject.org/thread/WJHGG2OO7ABNAYICGA5WQZ2Q34Q2FEHU/
diff --git a/copy/file-ops.c b/copy/file-ops.c
index 1d7e6a6..73cbdcb 100644
--- a/copy/file-ops.c
+++ b/copy/file-ops.c
@@ -116,6 +116,13 @@ file_create (const char *name, int fd, off_t st_size, bool is_block)
     rwf->can_fallocate = true;
   }
 
+  /* Set the POSIX_FADV_SEQUENTIAL flag on the file descriptor, but
+   * don't fail.
+   */
+#if defined (HAVE_POSIX_FADVISE) && defined (POSIX_FADV_SEQUENTIAL)
+  posix_fadvise (fd, 0, 0, POSIX_FADV_SEQUENTIAL);
+#endif
+
   return &rwf->rw;
 }
 
-- 
2.29.0.rc2