From d74f81f8adc504a23be3babf347b9f69e9389924 Mon Sep 17 00:00:00 2001 From: Milan Broz Date: Fri, 8 Feb 2008 02:11:27 +0000 Subject: 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 Signed-off-by: Alasdair G Kergon --- drivers/md/dm-snap.h | 50 ++++++++++++++++++++++++++++++++++++++++++++++++-- 1 file changed, 48 insertions(+), 2 deletions(-) (limited to 'drivers/md/dm-snap.h') diff --git a/drivers/md/dm-snap.h b/drivers/md/dm-snap.h index 650e0f1f51d..93bce5d4974 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; @@ -37,6 +40,49 @@ struct dm_snap_exception { chunk_t new_chunk; }; +/* + * 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). -- cgit v1.2.3