From 7078202e55b565582fcbd831a8dd3069bdc72610 Mon Sep 17 00:00:00 2001 From: Artem Bityutskiy Date: Mon, 19 Jan 2009 19:57:27 +0200 Subject: UBIFS: document dark_wm and dead_wm better Just add more commentaries. Also some commentary fixes for lprops flags. Signed-off-by: Artem Bityutskiy --- fs/ubifs/gc.c | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) (limited to 'fs/ubifs/gc.c') 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 -- cgit v1.2.3