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authorArtem Bityutskiy <Artem.Bityutskiy@nokia.com>2009-01-19 19:57:27 +0200
committerArtem Bityutskiy <Artem.Bityutskiy@nokia.com>2009-01-20 10:10:47 +0200
commit7078202e55b565582fcbd831a8dd3069bdc72610 (patch)
tree0f8cabaa23a05210cc1b95a97d7cab490f38ebca /fs/ubifs/gc.c
parenta50412e3f8ce95d7ed558370d7dde5171fd04283 (diff)
downloadlinux-7078202e55b565582fcbd831a8dd3069bdc72610.tar.bz2
UBIFS: document dark_wm and dead_wm better
Just add more commentaries. Also some commentary fixes for lprops flags. Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Diffstat (limited to 'fs/ubifs/gc.c')
-rw-r--r--fs/ubifs/gc.c20
1 files changed, 20 insertions, 0 deletions
diff --git a/fs/ubifs/gc.c b/fs/ubifs/gc.c
index 9832f9abe28e..b2e5f1133377 100644
--- a/fs/ubifs/gc.c
+++ b/fs/ubifs/gc.c
@@ -31,6 +31,26 @@
* to be reused. Garbage collection will cause the number of dirty index nodes
* to grow, however sufficient space is reserved for the index to ensure the
* commit will never run out of space.
+ *
+ * Notes about dead watermark. At current UBIFS implementation we assume that
+ * LEBs which have less than @c->dead_wm bytes of free + dirty space are full
+ * and not worth garbage-collecting. The dead watermark is one min. I/O unit
+ * size, or min. UBIFS node size, depending on what is greater. Indeed, UBIFS
+ * Garbage Collector has to synchronize the GC head's write buffer before
+ * returning, so this is about wasting one min. I/O unit. However, UBIFS GC can
+ * actually reclaim even very small pieces of dirty space by garbage collecting
+ * enough dirty LEBs, but we do not bother doing this at this implementation.
+ *
+ * Notes about dark watermark. The results of GC work depends on how big are
+ * the UBIFS nodes GC deals with. Large nodes make GC waste more space. Indeed,
+ * if GC move data from LEB A to LEB B and nodes in LEB A are large, GC would
+ * have to waste large pieces of free space at the end of LEB B, because nodes
+ * from LEB A would not fit. And the worst situation is when all nodes are of
+ * maximum size. So dark watermark is the amount of free + dirty space in LEB
+ * which are guaranteed to be reclaimable. If LEB has less space, the GC migh
+ * be unable to reclaim it. So, LEBs with free + dirty greater than dark
+ * watermark are "good" LEBs from GC's point of few. The other LEBs are not so
+ * good, and GC takes extra care when moving them.
*/
#include <linux/pagemap.h>