Blame SOURCES/kvm-qcow2-Assign-the-L2-cache-relatively-to-the-image-si.patch

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From 85dd7b43885d64406c5ea9af9daefe89a6c7475f Mon Sep 17 00:00:00 2001
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From: Kevin Wolf <kwolf@redhat.com>
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Date: Thu, 6 Dec 2018 17:12:34 +0000
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Subject: [PATCH 09/15] qcow2: Assign the L2 cache relatively to the image size
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RH-Author: Kevin Wolf <kwolf@redhat.com>
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Message-id: <20181206171240.5674-10-kwolf@redhat.com>
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Patchwork-id: 83289
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O-Subject: [RHEL-8.0 qemu-kvm PATCH 09/15] qcow2: Assign the L2 cache relatively to the image size
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Bugzilla: 1656507
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RH-Acked-by: Max Reitz <mreitz@redhat.com>
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RH-Acked-by: John Snow <jsnow@redhat.com>
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RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
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From: Leonid Bloch <lbloch@janustech.com>
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Sufficient L2 cache can noticeably improve the performance when using
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large images with frequent I/O.
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Previously, unless 'cache-size' was specified and was large enough, the
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L2 cache was set to a certain size without taking the virtual image size
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into account.
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Now, the L2 cache assignment is aware of the virtual size of the image,
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and will cover the entire image, unless the cache size needed for that is
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larger than a certain maximum. This maximum is set to 1 MB by default
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(enough to cover an 8 GB image with the default cluster size) but can
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be increased or decreased using the 'l2-cache-size' option. This option
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was previously documented as the *maximum* L2 cache size, and this patch
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makes it behave as such, instead of as a constant size. Also, the
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existing option 'cache-size' can limit the sum of both L2 and refcount
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caches, as previously.
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Signed-off-by: Leonid Bloch <lbloch@janustech.com>
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Reviewed-by: Alberto Garcia <berto@igalia.com>
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Reviewed-by: Kevin Wolf <kwolf@redhat.com>
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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(cherry picked from commit b749562d9822d14ef69c9eaa5f85903010b86c30)
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>
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---
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 block/qcow2.c              | 21 +++++++++------------
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 block/qcow2.h              |  4 +---
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 docs/qcow2-cache.txt       | 15 ++++++++++-----
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 qemu-options.hx            |  6 +++---
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 tests/qemu-iotests/137     |  8 +++++++-
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 tests/qemu-iotests/137.out |  4 +++-
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 6 files changed, 33 insertions(+), 25 deletions(-)
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diff --git a/block/qcow2.c b/block/qcow2.c
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index f3b2860..fc6bddd 100644
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--- a/block/qcow2.c
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+++ b/block/qcow2.c
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@@ -773,29 +773,35 @@ static void read_cache_sizes(BlockDriverState *bs, QemuOpts *opts,
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                              uint64_t *refcount_cache_size, Error **errp)
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 {
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     BDRVQcow2State *s = bs->opaque;
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-    uint64_t combined_cache_size;
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+    uint64_t combined_cache_size, l2_cache_max_setting;
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     bool l2_cache_size_set, refcount_cache_size_set, combined_cache_size_set;
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     int min_refcount_cache = MIN_REFCOUNT_CACHE_SIZE * s->cluster_size;
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+    uint64_t virtual_disk_size = bs->total_sectors * BDRV_SECTOR_SIZE;
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+    uint64_t max_l2_cache = virtual_disk_size / (s->cluster_size / 8);
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     combined_cache_size_set = qemu_opt_get(opts, QCOW2_OPT_CACHE_SIZE);
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     l2_cache_size_set = qemu_opt_get(opts, QCOW2_OPT_L2_CACHE_SIZE);
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     refcount_cache_size_set = qemu_opt_get(opts, QCOW2_OPT_REFCOUNT_CACHE_SIZE);
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     combined_cache_size = qemu_opt_get_size(opts, QCOW2_OPT_CACHE_SIZE, 0);
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-    *l2_cache_size = qemu_opt_get_size(opts, QCOW2_OPT_L2_CACHE_SIZE, 0);
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+    l2_cache_max_setting = qemu_opt_get_size(opts, QCOW2_OPT_L2_CACHE_SIZE,
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+                                             DEFAULT_L2_CACHE_MAX_SIZE);
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     *refcount_cache_size = qemu_opt_get_size(opts,
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                                              QCOW2_OPT_REFCOUNT_CACHE_SIZE, 0);
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     *l2_cache_entry_size = qemu_opt_get_size(
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         opts, QCOW2_OPT_L2_CACHE_ENTRY_SIZE, s->cluster_size);
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+    *l2_cache_size = MIN(max_l2_cache, l2_cache_max_setting);
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+
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     if (combined_cache_size_set) {
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         if (l2_cache_size_set && refcount_cache_size_set) {
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             error_setg(errp, QCOW2_OPT_CACHE_SIZE ", " QCOW2_OPT_L2_CACHE_SIZE
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                        " and " QCOW2_OPT_REFCOUNT_CACHE_SIZE " may not be set "
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                        "the same time");
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             return;
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-        } else if (*l2_cache_size > combined_cache_size) {
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+        } else if (l2_cache_size_set &&
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+                   (l2_cache_max_setting > combined_cache_size)) {
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             error_setg(errp, QCOW2_OPT_L2_CACHE_SIZE " may not exceed "
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                        QCOW2_OPT_CACHE_SIZE);
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             return;
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@@ -810,9 +816,6 @@ static void read_cache_sizes(BlockDriverState *bs, QemuOpts *opts,
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         } else if (refcount_cache_size_set) {
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             *l2_cache_size = combined_cache_size - *refcount_cache_size;
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         } else {
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-            uint64_t virtual_disk_size = bs->total_sectors * BDRV_SECTOR_SIZE;
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-            uint64_t max_l2_cache = virtual_disk_size / (s->cluster_size / 8);
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-
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             /* Assign as much memory as possible to the L2 cache, and
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              * use the remainder for the refcount cache */
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             if (combined_cache_size >= max_l2_cache + min_refcount_cache) {
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@@ -824,12 +827,6 @@ static void read_cache_sizes(BlockDriverState *bs, QemuOpts *opts,
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                 *l2_cache_size = combined_cache_size - *refcount_cache_size;
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             }
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         }
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-    } else {
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-        if (!l2_cache_size_set) {
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-            *l2_cache_size = MAX(DEFAULT_L2_CACHE_SIZE,
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-                                 (uint64_t)DEFAULT_L2_CACHE_CLUSTERS
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-                                 * s->cluster_size);
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-        }
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     }
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     /* l2_cache_size and refcount_cache_size are ensured to have at least
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      * their minimum values in qcow2_update_options_prepare() */
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diff --git a/block/qcow2.h b/block/qcow2.h
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index f73a48a..d0dd4a2 100644
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--- a/block/qcow2.h
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+++ b/block/qcow2.h
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@@ -74,9 +74,7 @@
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 /* Must be at least 4 to cover all cases of refcount table growth */
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 #define MIN_REFCOUNT_CACHE_SIZE 4 /* clusters */
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-/* Whichever is more */
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-#define DEFAULT_L2_CACHE_CLUSTERS 8 /* clusters */
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-#define DEFAULT_L2_CACHE_SIZE S_1MiB
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+#define DEFAULT_L2_CACHE_MAX_SIZE S_1MiB
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 #define DEFAULT_CLUSTER_SIZE S_64KiB
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diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
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index 7e28b41..750447e 100644
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--- a/docs/qcow2-cache.txt
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+++ b/docs/qcow2-cache.txt
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@@ -125,8 +125,12 @@ There are a few things that need to be taken into account:
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  - Both caches must have a size that is a multiple of the cluster size
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    (or the cache entry size: see "Using smaller cache sizes" below).
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- - The default L2 cache size is 8 clusters or 1MB (whichever is more),
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-   and the minimum is 2 clusters (or 2 cache entries, see below).
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+ - The maximum L2 cache size is 1 MB by default (enough for full coverage
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+   of 8 GB images, with the default cluster size). This value can be
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+   modified using the "l2-cache-size" option. QEMU will not use more memory
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+   than needed to hold all of the image's L2 tables, regardless of this max.
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+   value. The minimal L2 cache size is 2 clusters (or 2 cache entries, see
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+   below).
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  - The default (and minimum) refcount cache size is 4 clusters.
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@@ -184,9 +188,10 @@ Some things to take into account:
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    always uses the cluster size as the entry size.
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  - If the L2 cache is big enough to hold all of the image's L2 tables
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-   (as explained in the "Choosing the right cache sizes" section
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-   earlier in this document) then none of this is necessary and you
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-   can omit the "l2-cache-entry-size" parameter altogether.
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+   (as explained in the "Choosing the right cache sizes" and "How to
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+   configure the cache sizes" sections in this document) then none of
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+   this is necessary and you can omit the "l2-cache-entry-size"
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+   parameter altogether.
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 Reducing the memory usage
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diff --git a/qemu-options.hx b/qemu-options.hx
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index 5e15d6f..5e3bd74 100644
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--- a/qemu-options.hx
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+++ b/qemu-options.hx
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@@ -756,9 +756,9 @@ The maximum total size of the L2 table and refcount block caches in bytes
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 @item l2-cache-size
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 The maximum size of the L2 table cache in bytes
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-(default: if cache-size is not defined - 1048576 bytes or 8 clusters, whichever
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-is larger; otherwise, as large as possible or needed within the cache-size,
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-while permitting the requested or the minimal refcount cache size)
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+(default: if cache-size is not specified - 1M; otherwise, as large as possible
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+within the cache-size, while permitting the requested or the minimal refcount
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+cache size)
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 @item refcount-cache-size
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 The maximum size of the refcount block cache in bytes
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diff --git a/tests/qemu-iotests/137 b/tests/qemu-iotests/137
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index 8796562..19e8597 100755
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--- a/tests/qemu-iotests/137
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+++ b/tests/qemu-iotests/137
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@@ -109,7 +109,6 @@ $QEMU_IO \
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     -c "reopen -o cache-size=1M,l2-cache-size=64k,refcount-cache-size=64k" \
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     -c "reopen -o cache-size=1M,l2-cache-size=2M" \
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     -c "reopen -o cache-size=1M,refcount-cache-size=2M" \
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-    -c "reopen -o l2-cache-size=256T" \
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     -c "reopen -o l2-cache-entry-size=33k" \
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     -c "reopen -o l2-cache-entry-size=128k" \
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     -c "reopen -o refcount-cache-size=256T" \
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@@ -119,6 +118,13 @@ $QEMU_IO \
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     -c "reopen -o cache-clean-interval=-1" \
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     "$TEST_IMG" | _filter_qemu_io
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+IMGOPTS="cluster_size=256k" _make_test_img 32P
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+$QEMU_IO \
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+    -c "reopen -o l2-cache-entry-size=512,l2-cache-size=1T" \
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+    "$TEST_IMG" | _filter_qemu_io
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+
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+_make_test_img 64M
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+
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 echo
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 echo === Test transaction semantics ===
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 echo
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diff --git a/tests/qemu-iotests/137.out b/tests/qemu-iotests/137.out
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index 96724a6..afcc000 100644
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--- a/tests/qemu-iotests/137.out
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+++ b/tests/qemu-iotests/137.out
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@@ -19,7 +19,6 @@ Parameter 'lazy-refcounts' expects 'on' or 'off'
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 cache-size, l2-cache-size and refcount-cache-size may not be set the same time
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 l2-cache-size may not exceed cache-size
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 refcount-cache-size may not exceed cache-size
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-L2 cache size too big
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 L2 cache entry size must be a power of two between 512 and the cluster size (65536)
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 L2 cache entry size must be a power of two between 512 and the cluster size (65536)
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 Refcount cache size too big
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@@ -27,6 +26,9 @@ Conflicting values for qcow2 options 'overlap-check' ('constant') and 'overlap-c
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 Unsupported value 'blubb' for qcow2 option 'overlap-check'. Allowed are any of the following: none, constant, cached, all
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 Unsupported value 'blubb' for qcow2 option 'overlap-check'. Allowed are any of the following: none, constant, cached, all
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 Cache clean interval too big
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+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=36028797018963968
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+L2 cache size too big
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+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864
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 === Test transaction semantics ===
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-- 
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1.8.3.1
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