From b51da19dcd9533071033b092047b4e8a15a34018 Mon Sep 17 00:00:00 2001 Message-Id: In-Reply-To: References: From: Paolo Bonzini Date: Fri, 6 Sep 2013 18:12:27 +0200 Subject: [PATCH 07/25] 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: Eric Blake Signed-off-by: Paolo Bonzini (cherry picked from commit 276cbc7f2fc1bd3810887995dbc9cbb739c975bf) Signed-off-by: Michal Novotny --- block/cow.c | 54 ++++++++++++++++++++++++++++++++---------------------- 1 file changed, 32 insertions(+), 22 deletions(-) diff --git a/block/cow.c b/block/cow.c index 8820f84..b727428 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) +{ + 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; } -- 1.7.11.7