On Wed, 07/03 16:34, Paolo Bonzini wrote:
> 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!
>
> Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
> ---
> block/cow.c | 54 ++++++++++++++++++++++++++++++++----------------------
> 1 file changed, 32 insertions(+), 22 deletions(-)
>
> diff --git a/block/cow.c b/block/cow.c
> index 1cc2e89..204451e 100644
> --- a/block/cow.c
> +++ b/block/cow.c
> @@ -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)
I think type bool is better for 'value' as you don't booleanize it. And
also int64_t for start?
> +{
> + 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[512];
> + 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;
> }
>
> --
> 1.8.2.1
>
>
>
--
Fam
