Commit 7078202e authored by Artem Bityutskiy's avatar Artem Bityutskiy
Browse files

UBIFS: document dark_wm and dead_wm better 



Just add more commentaries. Also some commentary fixes for
lprops flags.

Signed-off-by: default avatarArtem Bityutskiy <Artem.Bityutskiy@nokia.com>
parent a50412e3

fs/ubifs/gc.c

+20 −0
Original line number Diff line number Diff line
@@ -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>

fs/ubifs/super.c

+2 −9
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@@ -573,15 +573,8 @@ static int init_constants_early(struct ubifs_info *c) 
	c->ranges[UBIFS_IDX_NODE].max_len = INT_MAX;



	/*

	 * Initialize dead and dark LEB space watermarks.

	 *

	 * Dead space is the space which cannot be used. Its watermark is

	 * equivalent to min. I/O unit or minimum node size if it is greater

	 * then min. I/O unit.

	 *

	 * Dark space is the space which might be used, or might not, depending

	 * on which node should be written to the LEB. Its watermark is

	 * equivalent to maximum UBIFS node size.

	 * Initialize dead and dark LEB space watermarks. See gc.c for comments

	 * about these values.

	 */

	c->dead_wm = ALIGN(MIN_WRITE_SZ, c->min_io_size);

	c->dark_wm = ALIGN(UBIFS_MAX_NODE_SZ, c->min_io_size);

fs/ubifs/ubifs.h

+2 −2
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@@ -426,9 +426,9 @@ struct ubifs_unclean_leb { 
 * LEB properties flags.

 *

 * LPROPS_UNCAT: not categorized

 * LPROPS_DIRTY: dirty > 0, not index

 * LPROPS_DIRTY: dirty > free, dirty >= @c->dead_wm, not index

 * LPROPS_DIRTY_IDX: dirty + free > @c->min_idx_node_sze and index

 * LPROPS_FREE: free > 0, not empty, not index

 * LPROPS_FREE: free > 0, dirty < @c->dead_wm, not empty, not index

 * LPROPS_HEAP_CNT: number of heaps used for storing categorized LEBs

 * LPROPS_EMPTY: LEB is empty, not taken

 * LPROPS_FREEABLE: free + dirty == leb_size, not index, not taken