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qcow2: Reduce number of I/O requests 

If the first part of a write request is allocated, but the second isn't
and it can be allocated so that the resulting area is contiguous, handle
it at once. This is a common case for sequential writes.

After this patch, alloc_cluster_offset() only checks if the clusters are
already allocated or how many new clusters can be allocated contigouosly.
The actual cluster allocation is split off into a new function
do_alloc_cluster_offset().

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
This commit is contained in:
Kevin Wolf 2012-03-02 14:10:54 +01:00
parent 256900b16b
commit 250196f19c
3 changed files with 169 additions and 78 deletions
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245
block/qcow2-cluster.c
View File

@ -589,7 +589,7 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
BDRVQcowState *s = bs->opaque;
int i, j = 0, l2_index, ret;
uint64_t *old_cluster, start_sect, l2_offset, *l2_table;
uint64_t cluster_offset = m->cluster_offset;
uint64_t cluster_offset = m->alloc_offset;
bool cow = false;
trace_qcow2_cluster_link_l2(qemu_coroutine_self(), m->nb_clusters);
@ -711,13 +711,95 @@ static int count_cow_clusters(BDRVQcowState *s, int nb_clusters,
return i;
}
/*
* Allocates new clusters for the given guest_offset.
*
* At most *nb_clusters are allocated, and on return *nb_clusters is updated to
* contain the number of clusters that have been allocated and are contiguous
* in the image file.
*
* If *host_offset is non-zero, it specifies the offset in the image file at
* which the new clusters must start. *nb_clusters can be 0 on return in this
* case if the cluster at host_offset is already in use. If *host_offset is
* zero, the clusters can be allocated anywhere in the image file.
*
* *host_offset is updated to contain the offset into the image file at which
* the first allocated cluster starts.
*
* Return 0 on success and -errno in error cases. -EAGAIN means that the
* function has been waiting for another request and the allocation must be
* restarted, but the whole request should not be failed.
*/
static int do_alloc_cluster_offset(BlockDriverState *bs, uint64_t guest_offset,
uint64_t *host_offset, unsigned int *nb_clusters, uint64_t *l2_table)
{
BDRVQcowState *s = bs->opaque;
int64_t cluster_offset;
QCowL2Meta *old_alloc;
trace_qcow2_do_alloc_clusters_offset(qemu_coroutine_self(), guest_offset,
*host_offset, *nb_clusters);
/*
* 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.
*/
QLIST_FOREACH(old_alloc, &s->cluster_allocs, next_in_flight) {
uint64_t start = guest_offset >> s->cluster_bits;
uint64_t end = start + *nb_clusters;
uint64_t old_start = old_alloc->offset >> s->cluster_bits;
uint64_t old_end = old_start + old_alloc->nb_clusters;
if (end < old_start || start > old_end) {
/* No intersection */
} else {
if (start < old_start) {
/* Stop at the start of a running allocation */
*nb_clusters = old_start - start;
} else {
*nb_clusters = 0;
}
if (*nb_clusters == 0) {
/* Wait for the dependency to complete. We need to recheck
* the free/allocated clusters when we continue. */
qemu_co_mutex_unlock(&s->lock);
qemu_co_queue_wait(&old_alloc->dependent_requests);
qemu_co_mutex_lock(&s->lock);
return -EAGAIN;
}
}
}
if (!*nb_clusters) {
abort();
}
/* Allocate new clusters */
trace_qcow2_cluster_alloc_phys(qemu_coroutine_self());
if (*host_offset == 0) {
cluster_offset = qcow2_alloc_clusters(bs, *nb_clusters * s->cluster_size);
} else {
cluster_offset = *host_offset;
*nb_clusters = qcow2_alloc_clusters_at(bs, cluster_offset, *nb_clusters);
}
if (cluster_offset < 0) {
return cluster_offset;
}
*host_offset = cluster_offset;
return 0;
}
/*
* alloc_cluster_offset
*
* For a given offset of the disk image, return cluster offset in qcow2 file.
* If the offset is not found, allocate a new cluster.
* For a given offset on the virtual disk, find the cluster offset in qcow2
* file. If the offset is not found, allocate a new cluster.
*
* If the cluster was already allocated, m->nb_clusters is set to 0,
* If the cluster was already allocated, m->nb_clusters is set to 0 and
* other fields in m are meaningless.
*
* If the cluster is newly allocated, m->nb_clusters is set to the number of
@ -734,119 +816,126 @@ int qcow2_alloc_cluster_offset(BlockDriverState *bs, uint64_t offset,
int n_start, int n_end, int *num, QCowL2Meta *m)
{
BDRVQcowState *s = bs->opaque;
int l2_index, ret;
int l2_index, ret, sectors;
uint64_t l2_offset, *l2_table;
int64_t cluster_offset;
unsigned int nb_clusters, i = 0;
QCowL2Meta *old_alloc;
unsigned int nb_clusters, keep_clusters;
uint64_t cluster_offset;
trace_qcow2_alloc_clusters_offset(qemu_coroutine_self(), offset,
n_start, n_end);
/* Find L2 entry for the first involved cluster */
ret = get_cluster_table(bs, offset, &l2_table, &l2_offset, &l2_index);
if (ret < 0) {
return ret;
}
/*
* Calculate the number of clusters to look for. We stop at L2 table
* boundaries to keep things simple.
*/
again:
nb_clusters = size_to_clusters(s, n_end << 9);
nb_clusters = MIN(nb_clusters, s->l2_size - l2_index);
nb_clusters = MIN(size_to_clusters(s, n_end << BDRV_SECTOR_BITS),
s->l2_size - l2_index);
cluster_offset = be64_to_cpu(l2_table[l2_index]);
/* We keep all QCOW_OFLAG_COPIED clusters */
/*
* Check how many clusters are already allocated and don't need COW, and how
* many need a new allocation.
*/
if (cluster_offset & QCOW_OFLAG_COPIED) {
nb_clusters = count_contiguous_clusters(nb_clusters, s->cluster_size,
&l2_table[l2_index], 0, 0);
/* We keep all QCOW_OFLAG_COPIED clusters */
keep_clusters = count_contiguous_clusters(nb_clusters, s->cluster_size,
&l2_table[l2_index], 0, 0);
assert(keep_clusters <= nb_clusters);
nb_clusters -= keep_clusters;
} else {
/* For the moment, overwrite compressed clusters one by one */
if (cluster_offset & QCOW_OFLAG_COMPRESSED) {
nb_clusters = 1;
} else {
nb_clusters = count_cow_clusters(s, nb_clusters, l2_table, l2_index);
}
cluster_offset &= ~QCOW_OFLAG_COPIED;
m->nb_clusters = 0;
goto out;
keep_clusters = 0;
cluster_offset = 0;
}
/* for the moment, multiple compressed clusters are not managed */
cluster_offset &= ~QCOW_OFLAG_COPIED;
if (cluster_offset & QCOW_OFLAG_COMPRESSED)
nb_clusters = 1;
/* If there is something left to allocate, do that now */
*m = (QCowL2Meta) {
.cluster_offset = cluster_offset,
.nb_clusters = 0,
};
qemu_co_queue_init(&m->dependent_requests);
/* how many available clusters ? */
if (nb_clusters > 0) {
uint64_t alloc_offset;
uint64_t alloc_cluster_offset;
uint64_t keep_bytes = keep_clusters * s->cluster_size;
i = count_cow_clusters(s, nb_clusters, l2_table, l2_index);
assert(i <= nb_clusters);
nb_clusters = i;
/* Calculate start and size of allocation */
alloc_offset = offset + keep_bytes;
/*
* 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.
*/
QLIST_FOREACH(old_alloc, &s->cluster_allocs, next_in_flight) {
uint64_t start = offset >> s->cluster_bits;
uint64_t end = start + nb_clusters;
uint64_t old_start = old_alloc->offset >> s->cluster_bits;
uint64_t old_end = old_start + old_alloc->nb_clusters;
if (end < old_start || start > old_end) {
/* No intersection */
if (keep_clusters == 0) {
alloc_cluster_offset = 0;
} else {
if (start < old_start) {
/* Stop at the start of a running allocation */
nb_clusters = old_start - start;
} else {
nb_clusters = 0;
}
alloc_cluster_offset = cluster_offset + keep_bytes;
}
if (nb_clusters == 0) {
/* Wait for the dependency to complete. We need to recheck
* the free/allocated clusters when we continue. */
qemu_co_mutex_unlock(&s->lock);
qemu_co_queue_wait(&old_alloc->dependent_requests);
qemu_co_mutex_lock(&s->lock);
goto again;
}
/* Allocate, if necessary at a given offset in the image file */
ret = do_alloc_cluster_offset(bs, alloc_offset, &alloc_cluster_offset,
&nb_clusters, l2_table);
if (ret == -EAGAIN) {
goto again;
} else if (ret < 0) {
goto fail;
}
/* save info needed for meta data update */
if (nb_clusters > 0) {
int requested_sectors = n_end - keep_clusters * s->cluster_sectors;
int avail_sectors = (keep_clusters + nb_clusters)
<< (s->cluster_bits - BDRV_SECTOR_BITS);
*m = (QCowL2Meta) {
.cluster_offset = keep_clusters == 0 ?
alloc_cluster_offset : cluster_offset,
.alloc_offset = alloc_cluster_offset,
.offset = alloc_offset,
.n_start = keep_clusters == 0 ? n_start : 0,
.nb_clusters = nb_clusters,
.nb_available = MIN(requested_sectors, avail_sectors),
};
qemu_co_queue_init(&m->dependent_requests);
QLIST_INSERT_HEAD(&s->cluster_allocs, m, next_in_flight);
}
}
if (!nb_clusters) {
abort();
}
/* save info needed for meta data update */
m->offset = offset;
m->n_start = n_start;
m->nb_clusters = nb_clusters;
QLIST_INSERT_HEAD(&s->cluster_allocs, m, next_in_flight);
/* allocate a new cluster */
trace_qcow2_cluster_alloc_phys(qemu_coroutine_self());
cluster_offset = qcow2_alloc_clusters(bs, nb_clusters * s->cluster_size);
if (cluster_offset < 0) {
ret = cluster_offset;
goto fail;
}
out:
/* Some cleanup work */
ret = qcow2_cache_put(bs, s->l2_table_cache, (void**) &l2_table);
if (ret < 0) {
goto fail_put;
}
m->nb_available = MIN(nb_clusters << (s->cluster_bits - 9), n_end);
m->cluster_offset = cluster_offset;
sectors = (keep_clusters + nb_clusters) << (s->cluster_bits - 9);
if (sectors > n_end) {
sectors = n_end;
}
*num = m->nb_available - n_start;
assert(sectors > n_start);
*num = sectors - n_start;
return 0;
fail:
qcow2_cache_put(bs, s->l2_table_cache, (void**) &l2_table);
fail_put:
QLIST_REMOVE(m, next_in_flight);
if (nb_clusters > 0) {
QLIST_REMOVE(m, next_in_flight);
}
return ret;
}

1
block/qcow2.h
View File

@ -155,6 +155,7 @@ typedef struct QCowL2Meta
{
uint64_t offset;
uint64_t cluster_offset;
uint64_t alloc_offset;
int n_start;
int nb_available;
int nb_clusters;

1
trace-events
View File

@ -320,6 +320,7 @@ qcow2_writev_done_part(void *co, int cur_nr_sectors) "co %p cur_nr_sectors %d"
qcow2_writev_data(void *co, uint64_t offset) "co %p offset %" PRIx64
qcow2_alloc_clusters_offset(void *co, uint64_t offset, int n_start, int n_end) "co %p offet %" PRIx64 " n_start %d n_end %d"
qcow2_do_alloc_clusters_offset(void *co, uint64_t guest_offset, uint64_t host_offset, int nb_clusters) "co %p guest_offet %" PRIx64 " host_offset %" PRIx64 " nb_clusters %d"
qcow2_cluster_alloc_phys(void *co) "co %p"
qcow2_cluster_link_l2(void *co, int nb_clusters) "co %p nb_clusters %d"