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qcow2: Merge the writing of the COW regions with the guest data

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If the guest tries to write data that results on the allocation of a
new cluster, instead of writing the guest data first and then the data
from the COW regions, write everything together using one single I/O
operation.

This can improve the write performance by 25% or more, depending on
several factors such as the media type, the cluster size and the I/O
request size.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Alberto Garcia 2017-06-19 16:40:08 +03:00 committed by Kevin Wolf
parent 86b862c431
commit ee22a9d869
3 changed files with 91 additions and 20 deletions
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40
block/qcow2-cluster.c
View File

@ -776,6 +776,7 @@ static int perform_cow(BlockDriverState *bs, QCowL2Meta *m)
assert(start->nb_bytes <= UINT_MAX - end->nb_bytes);
assert(start->nb_bytes + end->nb_bytes <= UINT_MAX - data_bytes);
assert(start->offset + start->nb_bytes <= end->offset);
assert(!m->data_qiov || m->data_qiov->size == data_bytes);
if (start->nb_bytes == 0 && end->nb_bytes == 0) {
return 0;
@ -807,7 +808,7 @@ static int perform_cow(BlockDriverState *bs, QCowL2Meta *m)
/* The part of the buffer where the end region is located */
end_buffer = start_buffer + buffer_size - end->nb_bytes;
qemu_iovec_init(&qiov, 1);
qemu_iovec_init(&qiov, 2 + (m->data_qiov ? m->data_qiov->niov : 0));
qemu_co_mutex_unlock(&s->lock);
/* First we read the existing data from both COW regions. We
@ -842,17 +843,36 @@ static int perform_cow(BlockDriverState *bs, QCowL2Meta *m)
}
}
/* And now we can write everything */
qemu_iovec_reset(&qiov);
qemu_iovec_add(&qiov, start_buffer, start->nb_bytes);
ret = do_perform_cow_write(bs, m->alloc_offset, start->offset, &qiov);
if (ret < 0) {
goto fail;
/* And now we can write everything. If we have the guest data we
* can write everything in one single operation */
if (m->data_qiov) {
qemu_iovec_reset(&qiov);
if (start->nb_bytes) {
qemu_iovec_add(&qiov, start_buffer, start->nb_bytes);
}
qemu_iovec_concat(&qiov, m->data_qiov, 0, data_bytes);
if (end->nb_bytes) {
qemu_iovec_add(&qiov, end_buffer, end->nb_bytes);
}
/* NOTE: we have a write_aio blkdebug event here followed by
* a cow_write one in do_perform_cow_write(), but there's only
* one single I/O operation */
BLKDBG_EVENT(bs->file, BLKDBG_WRITE_AIO);
ret = do_perform_cow_write(bs, m->alloc_offset, start->offset, &qiov);
} else {
/* If there's no guest data then write both COW regions separately */
qemu_iovec_reset(&qiov);
qemu_iovec_add(&qiov, start_buffer, start->nb_bytes);
ret = do_perform_cow_write(bs, m->alloc_offset, start->offset, &qiov);
if (ret < 0) {
goto fail;
}
qemu_iovec_reset(&qiov);
qemu_iovec_add(&qiov, end_buffer, end->nb_bytes);
ret = do_perform_cow_write(bs, m->alloc_offset, end->offset, &qiov);
}
qemu_iovec_reset(&qiov);
qemu_iovec_add(&qiov, end_buffer, end->nb_bytes);
ret = do_perform_cow_write(bs, m->alloc_offset, end->offset, &qiov);
fail:
qemu_co_mutex_lock(&s->lock);

64
block/qcow2.c
View File

@ -1575,6 +1575,44 @@ fail:
return ret;
}
/* Check if it's possible to merge a write request with the writing of
* the data from the COW regions */
static bool merge_cow(uint64_t offset, unsigned bytes,
QEMUIOVector *hd_qiov, QCowL2Meta *l2meta)
{
QCowL2Meta *m;
for (m = l2meta; m != NULL; m = m->next) {
/* If both COW regions are empty then there's nothing to merge */
if (m->cow_start.nb_bytes == 0 && m->cow_end.nb_bytes == 0) {
continue;
}
/* The data (middle) region must be immediately after the
* start region */
if (l2meta_cow_start(m) + m->cow_start.nb_bytes != offset) {
continue;
}
/* The end region must be immediately after the data (middle)
* region */
if (m->offset + m->cow_end.offset != offset + bytes) {
continue;
}
/* Make sure that adding both COW regions to the QEMUIOVector
* does not exceed IOV_MAX */
if (hd_qiov->niov > IOV_MAX - 2) {
continue;
}
m->data_qiov = hd_qiov;
return true;
}
return false;
}
static coroutine_fn int qcow2_co_pwritev(BlockDriverState *bs, uint64_t offset,
uint64_t bytes, QEMUIOVector *qiov,
int flags)
@ -1657,16 +1695,22 @@ static coroutine_fn int qcow2_co_pwritev(BlockDriverState *bs, uint64_t offset,
goto fail;
}
qemu_co_mutex_unlock(&s->lock);
BLKDBG_EVENT(bs->file, BLKDBG_WRITE_AIO);
trace_qcow2_writev_data(qemu_coroutine_self(),
cluster_offset + offset_in_cluster);
ret = bdrv_co_pwritev(bs->file,
cluster_offset + offset_in_cluster,
cur_bytes, &hd_qiov, 0);
qemu_co_mutex_lock(&s->lock);
if (ret < 0) {
goto fail;
/* If we need to do COW, check if it's possible to merge the
* writing of the guest data together with that of the COW regions.
* If it's not possible (or not necessary) then write the
* guest data now. */
if (!merge_cow(offset, cur_bytes, &hd_qiov, l2meta)) {
qemu_co_mutex_unlock(&s->lock);
BLKDBG_EVENT(bs->file, BLKDBG_WRITE_AIO);
trace_qcow2_writev_data(qemu_coroutine_self(),
cluster_offset + offset_in_cluster);
ret = bdrv_co_pwritev(bs->file,
cluster_offset + offset_in_cluster,
cur_bytes, &hd_qiov, 0);
qemu_co_mutex_lock(&s->lock);
if (ret < 0) {
goto fail;
}
}
while (l2meta != NULL) {

7
block/qcow2.h
View File

@ -343,6 +343,13 @@ typedef struct QCowL2Meta
*/
Qcow2COWRegion cow_end;
/**
* The I/O vector with the data from the actual guest write request.
* If non-NULL, this is meant to be merged together with the data
* from @cow_start and @cow_end into one single write operation.
*/
QEMUIOVector *data_qiov;
/** Pointer to next L2Meta of the same write request */
struct QCowL2Meta *next;