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[Qemu-devel] [PULL 19/42] cow: make reads go at a decent speed
From: |
Stefan Hajnoczi |
Subject: |
[Qemu-devel] [PULL 19/42] cow: make reads go at a decent speed |
Date: |
Fri, 6 Sep 2013 17:38:50 +0200 |
From: Paolo Bonzini <address@hidden>
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 <address@hidden>
Signed-off-by: Paolo Bonzini <address@hidden>
Signed-off-by: Stefan Hajnoczi <address@hidden>
---
block/cow.c | 54 ++++++++++++++++++++++++++++++++----------------------
1 file 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;
}
--
1.8.3.1
- [Qemu-devel] [PULL 42/42] qemu-iotests: Fixed test case 026, (continued)
- [Qemu-devel] [PULL 42/42] qemu-iotests: Fixed test case 026, Stefan Hajnoczi, 2013/09/06
- [Qemu-devel] [PULL 18/42] qmp: Documentation for BLOCK_IMAGE_CORRUPTED, Stefan Hajnoczi, 2013/09/06
- [Qemu-devel] [PULL 20/42] cow: make writes go at a less indecent speed, Stefan Hajnoczi, 2013/09/06
- [Qemu-devel] [PULL 16/42] nbd: use BlockDriverState refcnt, Stefan Hajnoczi, 2013/09/06
- [Qemu-devel] [PULL 25/42] block: remove bdrv_is_allocated_above/bdrv_co_is_allocated_above distinction, Stefan Hajnoczi, 2013/09/06
- [Qemu-devel] [PULL 26/42] block: expect errors from bdrv_co_is_allocated, Stefan Hajnoczi, 2013/09/06
- [Qemu-devel] [PULL 22/42] block: keep bs->total_sectors up to date even for growable block devices, Stefan Hajnoczi, 2013/09/06
- [Qemu-devel] [PULL 23/42] block: make bdrv_co_is_allocated static, Stefan Hajnoczi, 2013/09/06
- [Qemu-devel] [PULL 21/42] cow: do not call bdrv_co_is_allocated, Stefan Hajnoczi, 2013/09/06
- [Qemu-devel] [PULL 24/42] block: do not use ->total_sectors in bdrv_co_is_allocated, Stefan Hajnoczi, 2013/09/06
- [Qemu-devel] [PULL 19/42] cow: make reads go at a decent speed,
Stefan Hajnoczi <=
- [Qemu-devel] [PULL 17/42] block: use BDS ref for block jobs, Stefan Hajnoczi, 2013/09/06