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[PATCH] qcow2: Reduce write_zeroes size in handle_alloc_space()
From: |
Kevin Wolf |
Subject: |
[PATCH] qcow2: Reduce write_zeroes size in handle_alloc_space() |
Date: |
Tue, 9 Jun 2020 16:08:59 +0200 |
Since commit c8bb23cbdbe, handle_alloc_space() is called for newly
allocated clusters to efficiently initialise the COW areas with zeros if
necessary. It skips the whole operation if both start_cow nor end_cow
are empty. However, it requests zeroing the whole request size (possibly
multiple megabytes) even if only one end of the request actually needs
this.
This patch reduces the write_zeroes request size in this case so that we
don't unnecessarily zero-initialise a region that we're going to
overwrite immediately.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
block/qcow2.c | 16 +++++++++++-----
1 file changed, 11 insertions(+), 5 deletions(-)
diff --git a/block/qcow2.c b/block/qcow2.c
index 0cd2e6757e..77742877fb 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2403,6 +2403,8 @@ static int handle_alloc_space(BlockDriverState *bs,
QCowL2Meta *l2meta)
}
for (m = l2meta; m != NULL; m = m->next) {
+ uint64_t start = m->alloc_offset;
+ uint64_t len = m->nb_clusters * s->cluster_size;
int ret;
if (!m->cow_start.nb_bytes && !m->cow_end.nb_bytes) {
@@ -2413,21 +2415,25 @@ static int handle_alloc_space(BlockDriverState *bs,
QCowL2Meta *l2meta)
continue;
}
+ if (!m->cow_start.nb_bytes) {
+ start += m->cow_end.offset;
+ len -= m->cow_end.offset;
+ } else if (!m->cow_end.nb_bytes) {
+ len = m->cow_start.nb_bytes;
+ }
+
/*
* instead of writing zero COW buffers,
* efficiently zero out the whole clusters
*/
- ret = qcow2_pre_write_overlap_check(bs, 0, m->alloc_offset,
- m->nb_clusters * s->cluster_size,
- true);
+ ret = qcow2_pre_write_overlap_check(bs, 0, start, len, true);
if (ret < 0) {
return ret;
}
BLKDBG_EVENT(bs->file, BLKDBG_CLUSTER_ALLOC_SPACE);
- ret = bdrv_co_pwrite_zeroes(s->data_file, m->alloc_offset,
- m->nb_clusters * s->cluster_size,
+ ret = bdrv_co_pwrite_zeroes(s->data_file, start, len,
BDRV_REQ_NO_FALLBACK);
if (ret < 0) {
if (ret != -ENOTSUP && ret != -EAGAIN) {
--
2.25.4
- [PATCH] qcow2: Reduce write_zeroes size in handle_alloc_space(),
Kevin Wolf <=
Re: [PATCH] qcow2: Reduce write_zeroes size in handle_alloc_space(), Eric Blake, 2020/06/09