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authorPaolo Bonzini <pbonzini@redhat.com>2013-09-04 19:00:18 +0200
committerStefan Hajnoczi <stefanha@redhat.com>2013-09-06 15:25:08 +0200
commit276cbc7f2fc1bd3810887995dbc9cbb739c975bf (patch)
tree1c6cbb1d11063a1e53f1047e4d01d9ec27f993de /block/cow.c
parent0ca0b0d5f8a87dbc6daa5095771d036d0e6dc5b4 (diff)
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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 <eblake@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Diffstat (limited to 'block/cow.c')
-rw-r--r--block/cow.c54
1 files changed, 32 insertions, 22 deletions
diff --git a/block/cow.c b/block/cow.c
index 767639c..4512806 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;
}