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cow: make reads go at a decent speed

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Do not do two reads for each sector; load each sector of the bitmap
and use bitmap operations to process it.

Writes are still dog slow!

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Paolo Bonzini 2013-09-04 19:00:18 +02:00 committed by Stefan Hajnoczi
parent 0ca0b0d5f8
commit 276cbc7f2f
1 changed files with 32 additions and 22 deletions
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54
block/cow.c
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@ -126,18 +126,31 @@ static inline int cow_set_bit(BlockDriverState *bs, int64_t bitnum)
return 0;
}
static inline int is_bit_set(BlockDriverState *bs, int64_t bitnum)
#define BITS_PER_BITMAP_SECTOR (512 * 8)
/* Cannot use bitmap.c on big-endian machines. */
static int cow_test_bit(int64_t bitnum, const uint8_t *bitmap)
{
uint64_t offset = sizeof(struct cow_header_v2) + bitnum / 8;
uint8_t bitmap;
int ret;
return (bitmap[bitnum / 8] & (1 << (bitnum & 7))) != 0;
}
ret = bdrv_pread(bs->file, offset, &bitmap, sizeof(bitmap));
if (ret < 0) {
return ret;
static int cow_find_streak(const uint8_t *bitmap, int value, int start, int nb_sectors)
{
int streak_value = value ? 0xFF : 0;
int last = MIN(start + nb_sectors, BITS_PER_BITMAP_SECTOR);
int bitnum = start;
while (bitnum < last) {
if ((bitnum & 7) == 0 && bitmap[bitnum / 8] == streak_value) {
bitnum += 8;
continue;
}
if (cow_test_bit(bitnum, bitmap) == value) {
bitnum++;
continue;
}
break;
}
return !!(bitmap & (1 << (bitnum % 8)));
return MIN(bitnum, last) - start;
}
/* Return true if first block has been changed (ie. current version is
@ -146,23 +159,20 @@ static inline int is_bit_set(BlockDriverState *bs, int64_t bitnum)
static int coroutine_fn cow_co_is_allocated(BlockDriverState *bs,
int64_t sector_num, int nb_sectors, int *num_same)
{
int64_t bitnum = sector_num + sizeof(struct cow_header_v2) * 8;
uint64_t offset = (bitnum / 8) & -BDRV_SECTOR_SIZE;
uint8_t bitmap[BDRV_SECTOR_SIZE];
int ret;
int changed;
if (nb_sectors == 0) {
*num_same = nb_sectors;
return 0;
}
changed = is_bit_set(bs, sector_num);
if (changed < 0) {
return 0; /* XXX: how to return I/O errors? */
}
for (*num_same = 1; *num_same < nb_sectors; (*num_same)++) {
if (is_bit_set(bs, sector_num + *num_same) != changed)
break;
ret = bdrv_pread(bs->file, offset, &bitmap, sizeof(bitmap));
if (ret < 0) {
return ret;
}
bitnum &= BITS_PER_BITMAP_SECTOR - 1;
changed = cow_test_bit(bitnum, bitmap);
*num_same = cow_find_streak(bitmap, changed, bitnum, nb_sectors);
return changed;
}