Author: mckusick
Date: Sat Mar 22 11:26:39 2014
New Revision: 263628
URL: http://svnweb.freebsd.org/changeset/base/263628
Log:
Update comment to explain search order reverted to historical order
in -r254996.
Suggested by: Pedro Giffuni <p...@freebsd.org>
MFC: 3 days
Modified:
head/sys/ufs/ffs/ffs_alloc.c
Modified: head/sys/ufs/ffs/ffs_alloc.c
==============================================================================
--- head/sys/ufs/ffs/ffs_alloc.c Sat Mar 22 10:54:02 2014
(r263627)
+++ head/sys/ufs/ffs/ffs_alloc.c Sat Mar 22 11:26:39 2014
(r263628)
@@ -1167,16 +1167,17 @@ ffs_dirpref(pip)
* We scan from our preferred cylinder group forward looking
* for a cylinder group that meets our criterion. If we get
* to the final cylinder group and do not find anything,
- * we start scanning backwards from our preferred cylinder
- * group. The ideal would be to alternate looking forward
- * and backward, but that is just too complex to code for
- * the gain it would get. The most likely place where the
- * backward scan would take effect is when we start near
- * the end of the filesystem and do not find anything from
- * where we are to the end. In that case, scanning backward
- * will likely find us a suitable cylinder group much closer
- * to our desired location than if we were to start scanning
- * forward from the beginning of the filesystem.
+ * we start scanning forwards from the beginning of the
+ * filesystem. While it might seem sensible to start scanning
+ * backwards or even to alternate looking forward and backward,
+ * this approach fails badly when the filesystem is nearly full.
+ * Specifically, we first search all the areas that have no space
+ * and finally try the one preceeding that. We repeat this on
+ * every request and in the case of the final block end up
+ * searching the entire filesystem. By jumping to the front
+ * of the filesystem, our future forward searches always look
+ * in new cylinder groups so finds every possible block after
+ * one pass over the filesystem.
*/
prefcg = ino_to_cg(fs, pip->i_number);
for (cg = prefcg; cg < fs->fs_ncg; cg++)
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