cp: improve fallback when fiemap is not supported

[Dominique Martinet]

Hi,

A bit of background first: we use LUSTRE at CEA, and unfortunately it
does not support fiemap well enough (working on fixing that as well, but
this will likely take some time)
We also have many, many sparse files with no data. These are empty
shells of real files that can be read from tapes transparently on access
through HSM (hierarchical storage management), but as far as admins as
concerned we just want to manipulate the empty data.

For these files, tar has a heuristic that does a stat() and if st_blocks
is 0 it means there is no extent, and does not read any data.
I'd like to add the same heuristic to cp if fiemap failed.


Here's a suggested patch that does this. I could not find any rule
regarding code IP but feel free to consider this public domain or
whatever is best for coreutils. (I understand it's not useable as is
anyway, would likely need to place the function somewhere else, or
at least make it static; possible rename; add comments; etc. This is
more about testing water.)

----------
diff --git a/src/extent-scan.c b/src/extent-scan.c
index bbb686a..e5388a7 100644
--- a/src/extent-scan.c
+++ b/src/extent-scan.c
@@ -81,6 +81,20 @@ extent_scan_init (int src_fd, struct extent_scan *scan)
 # ifndef FS_IOC_FIEMAP
 #  define FS_IOC_FIEMAP _IOWR ('f', 11, struct fiemap)
 # endif
+
+bool
+zero_block (struct extent_scan *scan)
+{
+  struct stat buf;
+  int rc;
+
+  rc = fstat(scan->fd, &buf);
+  if (rc != 0)
+    return false;
+
+  return buf.st_blocks == 0;
+}
+
 /* Call ioctl(2) with FS_IOC_FIEMAP (available in linux 2.6.27) to
    obtain a map of file extents excluding holes.  */
 extern bool
@@ -112,7 +126,14 @@ extent_scan_read (struct extent_scan *scan)
       if (ioctl (scan->fd, FS_IOC_FIEMAP, fiemap) < 0)
         {
           if (scan->scan_start == 0)
-            scan->initial_scan_failed = true;
+            {
+              if (zero_block(scan))
+                {
+                  scan->hit_final_extent = true;
+                  return true;
+                }
+              scan->initial_scan_failed = true;
+            }
           return false;
         }
 
------------

Thank you,
-- 
Dominique Martinet