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authorMilan Broz <mbroz@redhat.com>2008-02-08 02:11:27 +0000
committerAlasdair G Kergon <agk@redhat.com>2008-02-08 02:11:27 +0000
commitd74f81f8adc504a23be3babf347b9f69e9389924 (patch)
tree4a687e400479ad330bb36ded54012cd8b8b84ecf /drivers/md/dm-snap.h
parent4f7f5c675fd6bacaae3c67be44de872dcff0e3b7 (diff)
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dm snapshot: combine consecutive exceptions in memory
Provided sector_t is 64 bits, reduce the in-memory footprint of the snapshot exception table by the simple method of using unused bits of the chunk number to combine consecutive entries. Signed-off-by: Milan Broz <mbroz@redhat.com> Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Diffstat (limited to 'drivers/md/dm-snap.h')
-rw-r--r--drivers/md/dm-snap.h50
1 files changed, 48 insertions, 2 deletions
diff --git a/drivers/md/dm-snap.h b/drivers/md/dm-snap.h
index 650e0f1..93bce5d 100644
--- a/drivers/md/dm-snap.h
+++ b/drivers/md/dm-snap.h
@@ -16,19 +16,22 @@
struct exception_table {
uint32_t hash_mask;
+ unsigned hash_shift;
struct list_head *table;
};
/*
* The snapshot code deals with largish chunks of the disk at a
- * time. Typically 64k - 256k.
+ * time. Typically 32k - 512k.
*/
-/* FIXME: can we get away with limiting these to a uint32_t ? */
typedef sector_t chunk_t;
/*
* An exception is used where an old chunk of data has been
* replaced by a new one.
+ * If chunk_t is 64 bits in size, the top 8 bits of new_chunk hold the number
+ * of chunks that follow contiguously. Remaining bits hold the number of the
+ * chunk within the device.
*/
struct dm_snap_exception {
struct list_head hash_list;
@@ -38,6 +41,49 @@ struct dm_snap_exception {
};
/*
+ * Funtions to manipulate consecutive chunks
+ */
+# if defined(CONFIG_LBD) || (BITS_PER_LONG == 64)
+# define DM_CHUNK_CONSECUTIVE_BITS 8
+# define DM_CHUNK_NUMBER_BITS 56
+
+static inline chunk_t dm_chunk_number(chunk_t chunk)
+{
+ return chunk & (chunk_t)((1ULL << DM_CHUNK_NUMBER_BITS) - 1ULL);
+}
+
+static inline unsigned dm_consecutive_chunk_count(struct dm_snap_exception *e)
+{
+ return e->new_chunk >> DM_CHUNK_NUMBER_BITS;
+}
+
+static inline void dm_consecutive_chunk_count_inc(struct dm_snap_exception *e)
+{
+ e->new_chunk += (1ULL << DM_CHUNK_NUMBER_BITS);
+
+ BUG_ON(!dm_consecutive_chunk_count(e));
+}
+
+# else
+# define DM_CHUNK_CONSECUTIVE_BITS 0
+
+static inline chunk_t dm_chunk_number(chunk_t chunk)
+{
+ return chunk;
+}
+
+static inline unsigned dm_consecutive_chunk_count(struct dm_snap_exception *e)
+{
+ return 0;
+}
+
+static inline void dm_consecutive_chunk_count_inc(struct dm_snap_exception *e)
+{
+}
+
+# endif
+
+/*
* Abstraction to handle the meta/layout of exception stores (the
* COW device).
*/