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qcow2: Order concurrent AIO requests on the same unallocated cluster

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When two AIO requests write to the same cluster, and this cluster is
unallocated, currently both requests allocate a new cluster and the second one
merges the first one when it is completed. This means an cluster allocation, a
read and a cluster deallocation which cause some overhead. If we simply let the
second request wait until the first one is done, we improve overall performance
with AIO requests (specifially, qcow2/virtio combinations).

This patch maintains a list of in-flight requests that have allocated new
clusters. A second request touching the same cluster is limited so that it
either doesn't touch the allocation of the first request (so it can have a
non-overlapping allocation) or it waits for the first request to complete.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
This commit is contained in:
Kevin Wolf 2009-08-31 16:48:49 +02:00 • committed by Anthony Liguori
parent ea80b906f4
commit f214978a42
3 changed files with 96 additions and 5 deletions
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39
block/qcow2-cluster.c
View file

@ -684,6 +684,7 @@ uint64_t qcow2_alloc_cluster_offset(BlockDriverState *bs,
int l2_index, ret;
uint64_t l2_offset, *l2_table, cluster_offset;
int nb_clusters, i = 0;
QCowL2Meta *old_alloc;
ret = get_cluster_table(bs, offset, &l2_table, &l2_offset, &l2_index);
if (ret == 0)
@ -732,6 +733,44 @@ uint64_t qcow2_alloc_cluster_offset(BlockDriverState *bs,
}
nb_clusters = i;
/*
* Check if there already is an AIO write request in flight which allocates
* the same cluster. In this case we need to wait until the previous
* request has completed and updated the L2 table accordingly.
*/
LIST_FOREACH(old_alloc, &s->cluster_allocs, next_in_flight) {
uint64_t end_offset = offset + nb_clusters * s->cluster_size;
uint64_t old_offset = old_alloc->offset;
uint64_t old_end_offset = old_alloc->offset +
old_alloc->nb_clusters * s->cluster_size;
if (end_offset < old_offset || offset > old_end_offset) {
/* No intersection */
} else {
if (offset < old_offset) {
/* Stop at the start of a running allocation */
nb_clusters = (old_offset - offset) >> s->cluster_bits;
} else {
nb_clusters = 0;
}
if (nb_clusters == 0) {
/* Set dependency and wait for a callback */
m->depends_on = old_alloc;
m->nb_clusters = 0;
*num = 0;
return 0;
}
}
}
if (!nb_clusters) {
abort();
}
LIST_INSERT_HEAD(&s->cluster_allocs, m, next_in_flight);
/* allocate a new cluster */
cluster_offset = qcow2_alloc_clusters(bs, nb_clusters * s->cluster_size);