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From 2a46f74bc62cd8affeb7516d1adf5dd855f3ed5a Mon Sep 17 00:00:00 2001
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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Fri, 18 Oct 2013 08:14:26 +0200
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Subject: [PATCH 01/81] cow: make reads go at a decent speed
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RH-Author: Paolo Bonzini <pbonzini@redhat.com>
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Message-id: <1382084091-16636-2-git-send-email-pbonzini@redhat.com>
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Patchwork-id: 54983
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O-Subject: [RHEL 7.0 qemu-kvm PATCH 01/26] cow: make reads go at a decent speed
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Bugzilla: 989646
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RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
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RH-Acked-by: Max Reitz <mreitz@redhat.com>
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RH-Acked-by: Kevin Wolf <kwolf@redhat.com>
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Do not do two reads for each sector; load each sector of the bitmap
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and use bitmap operations to process it.
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Writes are still dog slow!
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Reviewed-by: Eric Blake <eblake@redhat.com>
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Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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(cherry picked from commit 276cbc7f2fc1bd3810887995dbc9cbb739c975bf)
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---
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block/cow.c | 54 ++++++++++++++++++++++++++++++++----------------------
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1 file changed, 32 insertions(+), 22 deletions(-)
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Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
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---
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block/cow.c | 54 ++++++++++++++++++++++++++++++++----------------------
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1 files changed, 32 insertions(+), 22 deletions(-)
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diff --git a/block/cow.c b/block/cow.c
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index 9f94599..c12088b 100644
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--- a/block/cow.c
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+++ b/block/cow.c
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@@ -126,18 +126,31 @@ static inline int cow_set_bit(BlockDriverState *bs, int64_t bitnum)
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return 0;
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}
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-static inline int is_bit_set(BlockDriverState *bs, int64_t bitnum)
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+#define BITS_PER_BITMAP_SECTOR (512 * 8)
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+
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+/* Cannot use bitmap.c on big-endian machines. */
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+static int cow_test_bit(int64_t bitnum, const uint8_t *bitmap)
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{
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- uint64_t offset = sizeof(struct cow_header_v2) + bitnum / 8;
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- uint8_t bitmap;
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- int ret;
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+ return (bitmap[bitnum / 8] & (1 << (bitnum & 7))) != 0;
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+}
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- ret = bdrv_pread(bs->file, offset, &bitmap, sizeof(bitmap));
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- if (ret < 0) {
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- return ret;
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+static int cow_find_streak(const uint8_t *bitmap, int value, int start, int nb_sectors)
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+{
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+ int streak_value = value ? 0xFF : 0;
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+ int last = MIN(start + nb_sectors, BITS_PER_BITMAP_SECTOR);
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+ int bitnum = start;
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+ while (bitnum < last) {
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+ if ((bitnum & 7) == 0 && bitmap[bitnum / 8] == streak_value) {
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+ bitnum += 8;
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+ continue;
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+ }
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+ if (cow_test_bit(bitnum, bitmap) == value) {
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+ bitnum++;
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+ continue;
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+ }
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+ break;
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}
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-
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- return !!(bitmap & (1 << (bitnum % 8)));
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+ return MIN(bitnum, last) - start;
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}
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/* Return true if first block has been changed (ie. current version is
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@@ -146,23 +159,20 @@ static inline int is_bit_set(BlockDriverState *bs, int64_t bitnum)
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static int coroutine_fn cow_co_is_allocated(BlockDriverState *bs,
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int64_t sector_num, int nb_sectors, int *num_same)
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{
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+ int64_t bitnum = sector_num + sizeof(struct cow_header_v2) * 8;
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+ uint64_t offset = (bitnum / 8) & -BDRV_SECTOR_SIZE;
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+ uint8_t bitmap[BDRV_SECTOR_SIZE];
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+ int ret;
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int changed;
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- if (nb_sectors == 0) {
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- *num_same = nb_sectors;
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- return 0;
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- }
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-
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- changed = is_bit_set(bs, sector_num);
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- if (changed < 0) {
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- return 0; /* XXX: how to return I/O errors? */
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- }
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-
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- for (*num_same = 1; *num_same < nb_sectors; (*num_same)++) {
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- if (is_bit_set(bs, sector_num + *num_same) != changed)
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- break;
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+ ret = bdrv_pread(bs->file, offset, &bitmap, sizeof(bitmap));
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+ if (ret < 0) {
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+ return ret;
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}
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+ bitnum &= BITS_PER_BITMAP_SECTOR - 1;
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+ changed = cow_test_bit(bitnum, bitmap);
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+ *num_same = cow_find_streak(bitmap, changed, bitnum, nb_sectors);
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return changed;
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}
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--
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1.7.1
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