An option "l2-cache-full" is introduced to automatically set the qcow2
L2 cache to a sufficient value for covering the entire image. The memory
overhead when using this option is not big (1 MB for each 8 GB of
virtual image size with the default cluster size) and it can noticeably
improve performance when using large images with frequent I/O.
Previously, for this functionality the correct L2 cache size needed to
be calculated manually or with a script, and then this size needed to be
passed to the "l2-cache-size" option. Now it is sufficient to just pass
the boolean "l2-cache-full" option.
Signed-off-by: Leonid Bloch <lbl...@janustech.com>
---
block/qcow2.c | 35 ++++++++++++++++++++++++++++-------
block/qcow2.h | 1 +
qapi/block-core.json | 8 +++++++-
qemu-options.hx | 6 +++++-
4 files changed, 41 insertions(+), 9 deletions(-)
diff --git a/block/qcow2.c b/block/qcow2.c
index ec9e6238a0..101b8b474b 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -695,6 +695,11 @@ static QemuOptsList qcow2_runtime_opts = {
.type = QEMU_OPT_SIZE,
.help = "Maximum L2 table cache size",
},
+ {
+ .name = QCOW2_OPT_L2_CACHE_FULL,
+ .type = QEMU_OPT_BOOL,
+ .help = "Create full coverage of the image with the L2 cache",
+ },
{
.name = QCOW2_OPT_L2_CACHE_ENTRY_SIZE,
.type = QEMU_OPT_SIZE,
@@ -779,10 +784,12 @@ static void read_cache_sizes(BlockDriverState *bs,
QemuOpts *opts,
BDRVQcow2State *s = bs->opaque;
uint64_t combined_cache_size;
bool l2_cache_size_set, refcount_cache_size_set, combined_cache_size_set;
+ bool l2_cache_full_set;
int min_refcount_cache = MIN_REFCOUNT_CACHE_SIZE * s->cluster_size;
combined_cache_size_set = qemu_opt_get(opts, QCOW2_OPT_CACHE_SIZE);
l2_cache_size_set = qemu_opt_get(opts, QCOW2_OPT_L2_CACHE_SIZE);
+ l2_cache_full_set = qemu_opt_get(opts, QCOW2_OPT_L2_CACHE_FULL);
refcount_cache_size_set = qemu_opt_get(opts,
QCOW2_OPT_REFCOUNT_CACHE_SIZE);
combined_cache_size = qemu_opt_get_size(opts, QCOW2_OPT_CACHE_SIZE, 0);
@@ -793,6 +800,17 @@ static void read_cache_sizes(BlockDriverState *bs,
QemuOpts *opts,
*l2_cache_entry_size = qemu_opt_get_size(
opts, QCOW2_OPT_L2_CACHE_ENTRY_SIZE, s->cluster_size);
+ uint64_t virtual_disk_size = bs->total_sectors * BDRV_SECTOR_SIZE;
+ uint64_t max_l2_cache = virtual_disk_size / (s->cluster_size / 8);
+
+ if (l2_cache_size_set && l2_cache_full_set) {
+ error_setg(errp, QCOW2_OPT_L2_CACHE_SIZE " and "
+ QCOW2_OPT_L2_CACHE_FULL " may not be set at the same time");
+ return;
+ } else if (l2_cache_full_set) {
+ *l2_cache_size = max_l2_cache;
+ }
+
if (combined_cache_size_set) {
if (l2_cache_size_set && refcount_cache_size_set) {
error_setg(errp, QCOW2_OPT_CACHE_SIZE ", " QCOW2_OPT_L2_CACHE_SIZE
@@ -800,8 +818,14 @@ static void read_cache_sizes(BlockDriverState *bs,
QemuOpts *opts,
"at the same time");
return;
} else if (*l2_cache_size > combined_cache_size) {
- error_setg(errp, QCOW2_OPT_L2_CACHE_SIZE " may not exceed "
- QCOW2_OPT_CACHE_SIZE);
+ if (l2_cache_full_set) {
+ error_setg(errp, QCOW2_OPT_CACHE_SIZE " must be greater than "
+ "the full L2 cache if " QCOW2_OPT_L2_CACHE_FULL
+ " is used");
+ } else {
+ error_setg(errp, QCOW2_OPT_L2_CACHE_SIZE " may not exceed "
+ QCOW2_OPT_CACHE_SIZE);
+ }
return;
} else if (*refcount_cache_size > combined_cache_size) {
error_setg(errp, QCOW2_OPT_REFCOUNT_CACHE_SIZE " may not exceed "
@@ -809,14 +833,11 @@ static void read_cache_sizes(BlockDriverState *bs,
QemuOpts *opts,
return;
}
- if (l2_cache_size_set) {
+ if (l2_cache_size_set || l2_cache_full_set) {
*refcount_cache_size = combined_cache_size - *l2_cache_size;
} else if (refcount_cache_size_set) {
*l2_cache_size = combined_cache_size - *refcount_cache_size;
} else {
- uint64_t virtual_disk_size = bs->total_sectors * BDRV_SECTOR_SIZE;
- uint64_t max_l2_cache = virtual_disk_size / (s->cluster_size / 8);
-
/* Assign as much memory as possible to the L2 cache, and
* use the remainder for the refcount cache */
if (combined_cache_size >= max_l2_cache + min_refcount_cache) {
@@ -829,7 +850,7 @@ static void read_cache_sizes(BlockDriverState *bs, QemuOpts
*opts,
}
}
} else {
- if (!l2_cache_size_set) {
+ if (!l2_cache_size_set && !l2_cache_full_set) {
*l2_cache_size = MAX(DEFAULT_L2_CACHE_BYTE_SIZE,
(uint64_t)DEFAULT_L2_CACHE_CLUSTERS
* s->cluster_size);
diff --git a/block/qcow2.h b/block/qcow2.h
index 81b844e936..151e014bd8 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -97,6 +97,7 @@
#define QCOW2_OPT_OVERLAP_BITMAP_DIRECTORY "overlap-check.bitmap-directory"
#define QCOW2_OPT_CACHE_SIZE "cache-size"
#define QCOW2_OPT_L2_CACHE_SIZE "l2-cache-size"
+#define QCOW2_OPT_L2_CACHE_FULL "l2-cache-full"
#define QCOW2_OPT_L2_CACHE_ENTRY_SIZE "l2-cache-entry-size"
#define QCOW2_OPT_REFCOUNT_CACHE_SIZE "refcount-cache-size"
#define QCOW2_OPT_CACHE_CLEAN_INTERVAL "cache-clean-interval"
diff --git a/qapi/block-core.json b/qapi/block-core.json
index d40d5ecc3b..c584059e23 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -2812,7 +2812,12 @@
# refcount block caches in bytes (since 2.2)
#
# @l2-cache-size: the maximum size of the L2 table cache in
-# bytes (since 2.2)
+# bytes (mutually exclusive with l2-cache-full)
+# (since 2.2)
+#
+# @l2-cache-full: make the L2 table cache large enough to cover the
+# entire image (mutually exclusive with l2-cache-size)
+# (since 3.1)
#
# @l2-cache-entry-size: the size of each entry in the L2 cache in
# bytes. It must be a power of two between 512
@@ -2840,6 +2845,7 @@
'*overlap-check': 'Qcow2OverlapChecks',
'*cache-size': 'int',
'*l2-cache-size': 'int',
+ '*l2-cache-full': 'bool',
'*l2-cache-entry-size': 'int',
'*refcount-cache-size': 'int',
'*cache-clean-interval': 'int',
diff --git a/qemu-options.hx b/qemu-options.hx
index ef0706c359..6d417cb267 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -754,7 +754,7 @@ image file)
The maximum total size of the L2 table and refcount block caches in bytes
@item l2-cache-size
-The maximum size of the L2 table cache.
+The maximum size of the L2 table cache. (Mutually exclusive with l2-cache-full)
(default: if cache-size is not defined - 1048576 bytes or 8 clusters,
whichever is larger; if cache-size is defined and is large enough to
accommodate enough L2 cache to cover the entire virtual size of the image plus
@@ -762,6 +762,10 @@ the minimal amount of refcount cache - enough to cover the
entire image;
if cache-size is defined and is not large enough - as much as possible while
leaving space for the needed refcount cache)
+@item l2-cache-full
+Make the L2 table cache large enough to cover the entire image (mutually
+exclusive with l2-cache-size) (on/off; default: off)
+
@item refcount-cache-size
The maximum size of the refcount block cache in bytes
(default: 4 times the cluster size, or if cache-size is defined and is large
--
2.14.1
