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-rw-r--r--block/qcow2-cluster.c32
-rw-r--r--block/qcow2.c18
-rw-r--r--block/qcow2.h3
-rw-r--r--qapi/block-core.json12
-rw-r--r--qemu-options.hx12
5 files changed, 73 insertions, 4 deletions
diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 39cda7f907..2e76de027c 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1925,6 +1925,10 @@ static int discard_in_l2_slice(BlockDriverState *bs, uint64_t offset,
uint64_t new_l2_bitmap = old_l2_bitmap;
QCow2ClusterType cluster_type =
qcow2_get_cluster_type(bs, old_l2_entry);
+ bool keep_reference = (cluster_type != QCOW2_CLUSTER_COMPRESSED) &&
+ !full_discard &&
+ (s->discard_no_unref &&
+ type == QCOW2_DISCARD_REQUEST);
/*
* If full_discard is true, the cluster should not read back as zeroes,
@@ -1943,10 +1947,22 @@ static int discard_in_l2_slice(BlockDriverState *bs, uint64_t offset,
new_l2_entry = new_l2_bitmap = 0;
} else if (bs->backing || qcow2_cluster_is_allocated(cluster_type)) {
if (has_subclusters(s)) {
- new_l2_entry = 0;
+ if (keep_reference) {
+ new_l2_entry = old_l2_entry;
+ } else {
+ new_l2_entry = 0;
+ }
new_l2_bitmap = QCOW_L2_BITMAP_ALL_ZEROES;
} else {
- new_l2_entry = s->qcow_version >= 3 ? QCOW_OFLAG_ZERO : 0;
+ if (s->qcow_version >= 3) {
+ if (keep_reference) {
+ new_l2_entry |= QCOW_OFLAG_ZERO;
+ } else {
+ new_l2_entry = QCOW_OFLAG_ZERO;
+ }
+ } else {
+ new_l2_entry = 0;
+ }
}
}
@@ -1960,8 +1976,16 @@ static int discard_in_l2_slice(BlockDriverState *bs, uint64_t offset,
if (has_subclusters(s)) {
set_l2_bitmap(s, l2_slice, l2_index + i, new_l2_bitmap);
}
- /* Then decrease the refcount */
- qcow2_free_any_cluster(bs, old_l2_entry, type);
+ if (!keep_reference) {
+ /* Then decrease the refcount */
+ qcow2_free_any_cluster(bs, old_l2_entry, type);
+ } else if (s->discard_passthrough[type] &&
+ (cluster_type == QCOW2_CLUSTER_NORMAL ||
+ cluster_type == QCOW2_CLUSTER_ZERO_ALLOC)) {
+ /* If we keep the reference, pass on the discard still */
+ bdrv_pdiscard(s->data_file, old_l2_entry & L2E_OFFSET_MASK,
+ s->cluster_size);
+ }
}
qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
diff --git a/block/qcow2.c b/block/qcow2.c
index 7f3948360d..e23edd48c2 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -682,6 +682,7 @@ static const char *const mutable_opts[] = {
QCOW2_OPT_DISCARD_REQUEST,
QCOW2_OPT_DISCARD_SNAPSHOT,
QCOW2_OPT_DISCARD_OTHER,
+ QCOW2_OPT_DISCARD_NO_UNREF,
QCOW2_OPT_OVERLAP,
QCOW2_OPT_OVERLAP_TEMPLATE,
QCOW2_OPT_OVERLAP_MAIN_HEADER,
@@ -727,6 +728,11 @@ static QemuOptsList qcow2_runtime_opts = {
.help = "Generate discard requests when other clusters are freed",
},
{
+ .name = QCOW2_OPT_DISCARD_NO_UNREF,
+ .type = QEMU_OPT_BOOL,
+ .help = "Do not unreference discarded clusters",
+ },
+ {
.name = QCOW2_OPT_OVERLAP,
.type = QEMU_OPT_STRING,
.help = "Selects which overlap checks to perform from a range of "
@@ -969,6 +975,7 @@ typedef struct Qcow2ReopenState {
bool use_lazy_refcounts;
int overlap_check;
bool discard_passthrough[QCOW2_DISCARD_MAX];
+ bool discard_no_unref;
uint64_t cache_clean_interval;
QCryptoBlockOpenOptions *crypto_opts; /* Disk encryption runtime options */
} Qcow2ReopenState;
@@ -1140,6 +1147,15 @@ static int qcow2_update_options_prepare(BlockDriverState *bs,
r->discard_passthrough[QCOW2_DISCARD_OTHER] =
qemu_opt_get_bool(opts, QCOW2_OPT_DISCARD_OTHER, false);
+ r->discard_no_unref = qemu_opt_get_bool(opts, QCOW2_OPT_DISCARD_NO_UNREF,
+ false);
+ if (r->discard_no_unref && s->qcow_version < 3) {
+ error_setg(errp,
+ "discard-no-unref is only supported since qcow2 version 3");
+ ret = -EINVAL;
+ goto fail;
+ }
+
switch (s->crypt_method_header) {
case QCOW_CRYPT_NONE:
if (encryptfmt) {
@@ -1220,6 +1236,8 @@ static void qcow2_update_options_commit(BlockDriverState *bs,
s->discard_passthrough[i] = r->discard_passthrough[i];
}
+ s->discard_no_unref = r->discard_no_unref;
+
if (s->cache_clean_interval != r->cache_clean_interval) {
cache_clean_timer_del(bs);
s->cache_clean_interval = r->cache_clean_interval;
diff --git a/block/qcow2.h b/block/qcow2.h
index 4f67eb912a..ea9adb5706 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -133,6 +133,7 @@
#define QCOW2_OPT_DISCARD_REQUEST "pass-discard-request"
#define QCOW2_OPT_DISCARD_SNAPSHOT "pass-discard-snapshot"
#define QCOW2_OPT_DISCARD_OTHER "pass-discard-other"
+#define QCOW2_OPT_DISCARD_NO_UNREF "discard-no-unref"
#define QCOW2_OPT_OVERLAP "overlap-check"
#define QCOW2_OPT_OVERLAP_TEMPLATE "overlap-check.template"
#define QCOW2_OPT_OVERLAP_MAIN_HEADER "overlap-check.main-header"
@@ -385,6 +386,8 @@ typedef struct BDRVQcow2State {
bool discard_passthrough[QCOW2_DISCARD_MAX];
+ bool discard_no_unref;
+
int overlap_check; /* bitmask of Qcow2MetadataOverlap values */
bool signaled_corruption;
diff --git a/qapi/block-core.json b/qapi/block-core.json
index 4bf89171c6..5dd5f7e4b0 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -3478,6 +3478,17 @@
# @pass-discard-other: whether discard requests for the data source
# should be issued on other occasions where a cluster gets freed
#
+# @discard-no-unref: when enabled, discards from the guest will not cause
+# cluster allocations to be relinquished. This prevents qcow2 fragmentation
+# that would be caused by such discards. Besides potential
+# performance degradation, such fragmentation can lead to increased
+# allocation of clusters past the end of the image file,
+# resulting in image files whose file length can grow much larger
+# than their guest disk size would suggest.
+# If image file length is of concern (e.g. when storing qcow2
+# images directly on block devices), you should consider enabling
+# this option. (since 8.1)
+#
# @overlap-check: which overlap checks to perform for writes to the
# image, defaults to 'cached' (since 2.2)
#
@@ -3516,6 +3527,7 @@
'*pass-discard-request': 'bool',
'*pass-discard-snapshot': 'bool',
'*pass-discard-other': 'bool',
+ '*discard-no-unref': 'bool',
'*overlap-check': 'Qcow2OverlapChecks',
'*cache-size': 'int',
'*l2-cache-size': 'int',
diff --git a/qemu-options.hx b/qemu-options.hx
index b37eb9662b..b57489d7ca 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -1431,6 +1431,18 @@ SRST
issued on other occasions where a cluster gets freed
(on/off; default: off)
+ ``discard-no-unref``
+ When enabled, discards from the guest will not cause cluster
+ allocations to be relinquished. This prevents qcow2 fragmentation
+ that would be caused by such discards. Besides potential
+ performance degradation, such fragmentation can lead to increased
+ allocation of clusters past the end of the image file,
+ resulting in image files whose file length can grow much larger
+ than their guest disk size would suggest.
+ If image file length is of concern (e.g. when storing qcow2
+ images directly on block devices), you should consider enabling
+ this option.
+
``overlap-check``
Which overlap checks to perform for writes to the image
(none/constant/cached/all; default: cached). For details or
generated by cgit v1.2.3 (git 2.25.1) at 2024-07-22 18:25:47 +0000