Commit 276cbc7f authored by Paolo Bonzini's avatar Paolo Bonzini Committed by Stefan Hajnoczi
Browse files

cow: make reads go at a decent speed 



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: default avatarEric Blake <eblake@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
parent 0ca0b0d5

block/cow.c

+32 −22
Original line number Diff line number Diff line
@@ -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)

{

    uint64_t offset = sizeof(struct cow_header_v2) + bitnum / 8;

    uint8_t bitmap;

    int ret;

#define BITS_PER_BITMAP_SECTOR (512 * 8)



    ret = bdrv_pread(bs->file, offset, &bitmap, sizeof(bitmap));

    if (ret < 0) {

       return ret;

/* Cannot use bitmap.c on big-endian machines.  */

static int cow_test_bit(int64_t bitnum, const uint8_t *bitmap)

{

    return (bitmap[bitnum / 8] & (1 << (bitnum & 7))) != 0;

}



    return !!(bitmap & (1 << (bitnum % 8)));

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 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;

}