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dm snapshot: combine consecutive exceptions in memory
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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 <[email protected]>
Signed-off-by: Alasdair G Kergon <[email protected]>
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@kergon
Milan Broz authored and kergon committed Feb 8, 2008
1 parent 4f7f5c6 commit d74f81f
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Showing 2 changed files with 116 additions and 16 deletions.
82 changes: 68 additions & 14 deletions drivers/md/dm-snap.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -213,11 +213,15 @@ static void unregister_snapshot(struct dm_snapshot *s)

/*
* Implementation of the exception hash tables.
* The lowest hash_shift bits of the chunk number are ignored, allowing
* some consecutive chunks to be grouped together.
*/
static int init_exception_table(struct exception_table *et, uint32_t size)
static int init_exception_table(struct exception_table *et, uint32_t size,
unsigned hash_shift)
{
unsigned int i;

et->hash_shift = hash_shift;
et->hash_mask = size - 1;
et->table = dm_vcalloc(size, sizeof(struct list_head));
if (!et->table)
Expand Down Expand Up @@ -248,7 +252,7 @@ static void exit_exception_table(struct exception_table *et, struct kmem_cache *

static uint32_t exception_hash(struct exception_table *et, chunk_t chunk)
{
return chunk & et->hash_mask;
return (chunk >> et->hash_shift) & et->hash_mask;
}

static void insert_exception(struct exception_table *eh,
Expand All @@ -275,7 +279,8 @@ static struct dm_snap_exception *lookup_exception(struct exception_table *et,

slot = &et->table[exception_hash(et, chunk)];
list_for_each_entry (e, slot, hash_list)
if (e->old_chunk == chunk)
if (chunk >= e->old_chunk &&
chunk <= e->old_chunk + dm_consecutive_chunk_count(e))
return e;

return NULL;
Expand Down Expand Up @@ -307,6 +312,49 @@ static void free_pending_exception(struct dm_snap_pending_exception *pe)
mempool_free(pe, pending_pool);
}

static void insert_completed_exception(struct dm_snapshot *s,
struct dm_snap_exception *new_e)
{
struct exception_table *eh = &s->complete;
struct list_head *l;
struct dm_snap_exception *e = NULL;

l = &eh->table[exception_hash(eh, new_e->old_chunk)];

/* Add immediately if this table doesn't support consecutive chunks */
if (!eh->hash_shift)
goto out;

/* List is ordered by old_chunk */
list_for_each_entry_reverse(e, l, hash_list) {
/* Insert after an existing chunk? */
if (new_e->old_chunk == (e->old_chunk +
dm_consecutive_chunk_count(e) + 1) &&
new_e->new_chunk == (dm_chunk_number(e->new_chunk) +
dm_consecutive_chunk_count(e) + 1)) {
dm_consecutive_chunk_count_inc(e);
free_exception(new_e);
return;
}

/* Insert before an existing chunk? */
if (new_e->old_chunk == (e->old_chunk - 1) &&
new_e->new_chunk == (dm_chunk_number(e->new_chunk) - 1)) {
dm_consecutive_chunk_count_inc(e);
e->old_chunk--;
e->new_chunk--;
free_exception(new_e);
return;
}

if (new_e->old_chunk > e->old_chunk)
break;
}

out:
list_add(&new_e->hash_list, e ? &e->hash_list : l);
}

int dm_add_exception(struct dm_snapshot *s, chunk_t old, chunk_t new)
{
struct dm_snap_exception *e;
Expand All @@ -316,8 +364,12 @@ int dm_add_exception(struct dm_snapshot *s, chunk_t old, chunk_t new)
return -ENOMEM;

e->old_chunk = old;

/* Consecutive_count is implicitly initialised to zero */
e->new_chunk = new;
insert_exception(&s->complete, e);

insert_completed_exception(s, e);

return 0;
}

Expand Down Expand Up @@ -352,7 +404,8 @@ static int init_hash_tables(struct dm_snapshot *s)
hash_size = min(hash_size, max_buckets);

hash_size = rounddown_pow_of_two(hash_size);
if (init_exception_table(&s->complete, hash_size))
if (init_exception_table(&s->complete, hash_size,
DM_CHUNK_CONSECUTIVE_BITS))
return -ENOMEM;

/*
Expand All @@ -363,7 +416,7 @@ static int init_hash_tables(struct dm_snapshot *s)
if (hash_size < 64)
hash_size = 64;

if (init_exception_table(&s->pending, hash_size)) {
if (init_exception_table(&s->pending, hash_size, 0)) {
exit_exception_table(&s->complete, exception_cache);
return -ENOMEM;
}
Expand Down Expand Up @@ -722,7 +775,7 @@ static void pending_complete(struct dm_snap_pending_exception *pe, int success)
* Add a proper exception, and remove the
* in-flight exception from the list.
*/
insert_exception(&s->complete, e);
insert_completed_exception(s, e);

out:
remove_exception(&pe->e);
Expand Down Expand Up @@ -856,11 +909,12 @@ __find_pending_exception(struct dm_snapshot *s, struct bio *bio)
}

static void remap_exception(struct dm_snapshot *s, struct dm_snap_exception *e,
struct bio *bio)
struct bio *bio, chunk_t chunk)
{
bio->bi_bdev = s->cow->bdev;
bio->bi_sector = chunk_to_sector(s, e->new_chunk) +
(bio->bi_sector & s->chunk_mask);
bio->bi_sector = chunk_to_sector(s, dm_chunk_number(e->new_chunk) +
(chunk - e->old_chunk)) +
(bio->bi_sector & s->chunk_mask);
}

static int snapshot_map(struct dm_target *ti, struct bio *bio,
Expand Down Expand Up @@ -891,7 +945,7 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio,
/* If the block is already remapped - use that, else remap it */
e = lookup_exception(&s->complete, chunk);
if (e) {
remap_exception(s, e, bio);
remap_exception(s, e, bio, chunk);
goto out_unlock;
}

Expand All @@ -908,7 +962,7 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio,
goto out_unlock;
}

remap_exception(s, &pe->e, bio);
remap_exception(s, &pe->e, bio, chunk);
bio_list_add(&pe->snapshot_bios, bio);

r = DM_MAPIO_SUBMITTED;
Expand Down Expand Up @@ -1196,7 +1250,7 @@ static int origin_status(struct dm_target *ti, status_type_t type, char *result,

static struct target_type origin_target = {
.name = "snapshot-origin",
.version = {1, 5, 0},
.version = {1, 6, 0},
.module = THIS_MODULE,
.ctr = origin_ctr,
.dtr = origin_dtr,
Expand All @@ -1207,7 +1261,7 @@ static struct target_type origin_target = {

static struct target_type snapshot_target = {
.name = "snapshot",
.version = {1, 5, 0},
.version = {1, 6, 0},
.module = THIS_MODULE,
.ctr = snapshot_ctr,
.dtr = snapshot_dtr,
Expand Down
50 changes: 48 additions & 2 deletions drivers/md/dm-snap.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand All @@ -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).
Expand Down

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