[Qemu-devel] [PATCH v2 0/5] qcow2: Make the L2 cache cover the whole image by default

[Qemu-devel] [PATCH v2 0/5] qcow2: Make the L2 cache cover the whole image by default

[Leonid Bloch]

This series makes the qcow2 L2 cache cover the entire image by default.
The importance of this change is in noticeable performance improvement,
especially with heavy random I/O. The memory overhead is not big in most
cases, as only 1 MB of cache for every 8 GB of image size is used.
For cases with very large images and/or small cluster sizes, or systems
with limited RAM resources, there is an upper limit on the default L2
cache: 32 MB. To modify this limit one can use the already existing
'l2-cache-size' and 'cache-size' options. Moreover, this fixes the
behavior of 'l2-cache-size', as it was documented as the *maximum* L2
cache size, but in practice behaved as an absolute size.

To compensate the memory overhead which may be increased following this
behavior, the default cache-clean-interval is set to 30 seconds by
default (was disabled by default before).

The L2 cache is also resized accordingly, by default, if the image is
resized.

Additionally, few minor changes are made (refactoring and documentation
fixes).

Differences from v1:
* .gitignore modification patch removed (unneeded).
* The grammar fix in conflicting cache sizing patch removed (merged).
* The update total_sectors when resizing patch squashed with the
  resizing patch.
* L2 cache is now capped at 32 MB.
* The default cache-clean-interval is set to 30 seconds.

Leonid Bloch (5):
  qcow2: Options' documentation fixes
  qcow2: Make the default L2 cache sufficient to cover the entire image
  qcow2: Resize the cache upon image resizing
  qcow2: Set the default cache-clean-interval to 30 seconds
  qcow2: Explicit number replaced by a constant

 block/qcow2.c              | 47 +++++++++++++++++++-------------------
 block/qcow2.h              |  5 ++--
 docs/qcow2-cache.txt       | 30 ++++++++++++++----------
 qapi/block-core.json       |  3 ++-
 qemu-options.hx            | 11 +++++----
 tests/qemu-iotests/137     |  1 -
 tests/qemu-iotests/137.out |  1 -
 7 files changed, 53 insertions(+), 45 deletions(-)

-- 
2.17.1

[Qemu-devel] [PATCH v2 1/5] qcow2: Options' documentation fixes

[Leonid Bloch]

Signed-off-by: Leonid Bloch <lbloch@janustech.com>
---
 docs/qcow2-cache.txt | 3 +++
 qemu-options.hx      | 9 ++++++---
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
index 8a09a5cc5f..5bf2a8ad29 100644
--- a/docs/qcow2-cache.txt
+++ b/docs/qcow2-cache.txt
@@ -130,6 +130,9 @@ There are a few things that need to be taken into account:
    memory as possible to the L2 cache before increasing the refcount
    cache size.
 
+ - At most two of "l2-cache-size", "refcount-cache-size", and "cache-size"
+   can be set simultaneously.
+
 Unlike L2 tables, refcount blocks are not used during normal I/O but
 only during allocations and internal snapshots. In most cases they are
 accessed sequentially (even during random guest I/O) so increasing the
diff --git a/qemu-options.hx b/qemu-options.hx
index b1bf0f485f..f6804758d3 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -752,15 +752,18 @@ image file)
 
 @item cache-size
 The maximum total size of the L2 table and refcount block caches in bytes
-(default: 1048576 bytes or 8 clusters, whichever is larger)
+(default: the sum of l2-cache-size and refcount-cache-size)
 
 @item l2-cache-size
 The maximum size of the L2 table cache in bytes
-(default: 4/5 of the total cache size)
+(default: if cache-size is not defined - 1048576 bytes or 8 clusters, whichever
+is larger; otherwise, as large as possible or needed within the cache-size,
+while permitting the requested or the minimal refcount cache size)
 
 @item refcount-cache-size
 The maximum size of the refcount block cache in bytes
-(default: 1/5 of the total cache size)
+(default: 4 times the cluster size; or if cache-size is specified, the part of
+it which is not used for the L2 cache)
 
 @item cache-clean-interval
 Clean unused entries in the L2 and refcount caches. The interval is in seconds.
-- 
2.17.1

[Qemu-devel] [PATCH v2 2/5] qcow2: Make the default L2 cache sufficient to cover the entire image

[Leonid Bloch]

Sufficient L2 cache can noticeably improve the performance when using
large images with frequent I/O. The memory overhead is not significant
in most cases, as the cache size is only 1 MB for each 8 GB of virtual
image size (with the default cluster size of 64 KB). For cases with very
large images and/or small cluster sizes, there is an upper limit on the
default L2 cache: 32 MB. To modify this limit one can use the already
existing 'l2-cache-size' option. This option was previously documented
as the *maximum* L2 cache size, and this patch makes it behave as such,
instead of a constant size. Also, the existing option 'cache-size' can
limit the sum of both L2 and refcount caches, as previously.

Signed-off-by: Leonid Bloch <lbloch@janustech.com>
---
 block/qcow2.c              | 33 +++++++++++++--------------------
 block/qcow2.h              |  4 +---
 docs/qcow2-cache.txt       | 24 ++++++++++++++----------
 qemu-options.hx            |  6 +++---
 tests/qemu-iotests/137     |  1 -
 tests/qemu-iotests/137.out |  1 -
 6 files changed, 31 insertions(+), 38 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index ec9e6238a0..98cb96aaca 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -777,29 +777,35 @@ static void read_cache_sizes(BlockDriverState *bs, QemuOpts *opts,
                              uint64_t *refcount_cache_size, Error **errp)
 {
     BDRVQcow2State *s = bs->opaque;
-    uint64_t combined_cache_size;
+    uint64_t combined_cache_size, l2_cache_max_setting;
     bool l2_cache_size_set, refcount_cache_size_set, combined_cache_size_set;
-    int min_refcount_cache = MIN_REFCOUNT_CACHE_SIZE * s->cluster_size;
+    uint64_t 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);
     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);
-    *l2_cache_size = qemu_opt_get_size(opts, QCOW2_OPT_L2_CACHE_SIZE, 0);
+    l2_cache_max_setting = qemu_opt_get_size(opts, QCOW2_OPT_L2_CACHE_SIZE,
+                                             DEFAULT_L2_CACHE_MAX_SIZE);
     *refcount_cache_size = qemu_opt_get_size(opts,
                                              QCOW2_OPT_REFCOUNT_CACHE_SIZE, 0);
 
     *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);
+    *l2_cache_size = MIN(max_l2_cache, l2_cache_max_setting);
+
     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
                        " and " QCOW2_OPT_REFCOUNT_CACHE_SIZE " may not be set "
                        "at the same time");
             return;
-        } else if (*l2_cache_size > combined_cache_size) {
+        } else if (l2_cache_size_set &&
+                   (l2_cache_max_setting > combined_cache_size)) {
             error_setg(errp, QCOW2_OPT_L2_CACHE_SIZE " may not exceed "
                        QCOW2_OPT_CACHE_SIZE);
             return;
@@ -814,29 +820,16 @@ static void read_cache_sizes(BlockDriverState *bs, QemuOpts *opts,
         } 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) {
-                *l2_cache_size = max_l2_cache;
+            if (combined_cache_size >= *l2_cache_size + min_refcount_cache) {
                 *refcount_cache_size = combined_cache_size - *l2_cache_size;
             } else {
-                *refcount_cache_size =
-                    MIN(combined_cache_size, min_refcount_cache);
+                *refcount_cache_size = MIN(combined_cache_size,
+                                           min_refcount_cache);
                 *l2_cache_size = combined_cache_size - *refcount_cache_size;
             }
         }
-    } else {
-        if (!l2_cache_size_set) {
-            *l2_cache_size = MAX(DEFAULT_L2_CACHE_BYTE_SIZE,
-                                 (uint64_t)DEFAULT_L2_CACHE_CLUSTERS
-                                 * s->cluster_size);
-        }
-        if (!refcount_cache_size_set) {
-            *refcount_cache_size = min_refcount_cache;
-        }
     }
 
     if (*l2_cache_entry_size < (1 << MIN_CLUSTER_BITS) ||
diff --git a/block/qcow2.h b/block/qcow2.h
index 81b844e936..d77a31d932 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -73,9 +73,7 @@
 /* Must be at least 4 to cover all cases of refcount table growth */
 #define MIN_REFCOUNT_CACHE_SIZE 4 /* clusters */
 
-/* Whichever is more */
-#define DEFAULT_L2_CACHE_CLUSTERS 8 /* clusters */
-#define DEFAULT_L2_CACHE_BYTE_SIZE 1048576 /* bytes */
+#define DEFAULT_L2_CACHE_MAX_SIZE 0x2000000U /* bytes */
 
 #define DEFAULT_CLUSTER_SIZE 65536
 
diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
index 5bf2a8ad29..c7625cdeb3 100644
--- a/docs/qcow2-cache.txt
+++ b/docs/qcow2-cache.txt
@@ -97,12 +97,14 @@ need:
    l2_cache_size = disk_size_GB * 131072
    refcount_cache_size = disk_size_GB * 32768
 
-QEMU has a default L2 cache of 1MB (1048576 bytes) and a refcount
-cache of 256KB (262144 bytes), so using the formulas we've just seen
-we have
+QEMU will use a default L2 cache sufficient to cover the entire virtual
+size of an image, which with the default cluster size will result in 1 MB
+of cache for every 8 GB of virtual image size:
 
-   1048576 / 131072 = 8 GB of virtual disk covered by that cache
-    262144 /  32768 = 8 GB
+   65536 / 8 = 8192 = 8 GB / 1 MB
+
+A default refcount cache is 4 times the cluster size, which defaults to
+256 KB (262144 bytes).
 
 
 How to configure the cache sizes
@@ -121,8 +123,11 @@ There are a few things that need to be taken into account:
  - Both caches must have a size that is a multiple of the cluster size
    (or the cache entry size: see "Using smaller cache sizes" below).
 
- - The default L2 cache size is 8 clusters or 1MB (whichever is more),
-   and the minimum is 2 clusters (or 2 cache entries, see below).
+ - The default L2 cache size will cover the entire virtual size of an
+   image, but is capped at 32 MB (enough for image sizes of up to 256 GB
+   with the default cluster size). This maximum value can be reduced or
+   enlarged using the "l2-cache-size" option. The minimum is 2 clusters
+   (or 2 cache entries, see below).
 
  - The default (and minimum) refcount cache size is 4 clusters.
 
@@ -180,9 +185,8 @@ Some things to take into account:
    always uses the cluster size as the entry size.
 
  - If the L2 cache is big enough to hold all of the image's L2 tables
-   (as explained in the "Choosing the right cache sizes" section
-   earlier in this document) then none of this is necessary and you
-   can omit the "l2-cache-entry-size" parameter altogether.
+   (the default behavior) then none of this is necessary and you can
+   omit the "l2-cache-entry-size" parameter altogether.
 
 
 Reducing the memory usage
diff --git a/qemu-options.hx b/qemu-options.hx
index f6804758d3..d6e15b2f06 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -756,9 +756,9 @@ 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 in bytes
-(default: if cache-size is not defined - 1048576 bytes or 8 clusters, whichever
-is larger; otherwise, as large as possible or needed within the cache-size,
-while permitting the requested or the minimal refcount cache size)
+(default: if cache-size is not defined - 32M; otherwise, as large as possible
+or needed within the cache-size, while permitting the requested or the minimal
+refcount cache size)
 
 @item refcount-cache-size
 The maximum size of the refcount block cache in bytes
diff --git a/tests/qemu-iotests/137 b/tests/qemu-iotests/137
index 87965625d8..e3fb078588 100755
--- a/tests/qemu-iotests/137
+++ b/tests/qemu-iotests/137
@@ -109,7 +109,6 @@ $QEMU_IO \
     -c "reopen -o cache-size=1M,l2-cache-size=64k,refcount-cache-size=64k" \
     -c "reopen -o cache-size=1M,l2-cache-size=2M" \
     -c "reopen -o cache-size=1M,refcount-cache-size=2M" \
-    -c "reopen -o l2-cache-size=256T" \
     -c "reopen -o l2-cache-entry-size=33k" \
     -c "reopen -o l2-cache-entry-size=128k" \
     -c "reopen -o refcount-cache-size=256T" \
diff --git a/tests/qemu-iotests/137.out b/tests/qemu-iotests/137.out
index 6a2ffc71fd..70f245ae7a 100644
--- a/tests/qemu-iotests/137.out
+++ b/tests/qemu-iotests/137.out
@@ -19,7 +19,6 @@ Parameter 'lazy-refcounts' expects 'on' or 'off'
 cache-size, l2-cache-size and refcount-cache-size may not be set at the same time
 l2-cache-size may not exceed cache-size
 refcount-cache-size may not exceed cache-size
-L2 cache size too big
 L2 cache entry size must be a power of two between 512 and the cluster size (65536)
 L2 cache entry size must be a power of two between 512 and the cluster size (65536)
 Refcount cache size too big
-- 
2.17.1

[Qemu-devel] [PATCH v2 3/5] qcow2: Resize the cache upon image resizing

[Leonid Bloch]

The caches are now recalculated upon image resizing. This is done
because the new default behavior of assigning a sufficient L2 cache to
cover the entire image implies that the cache will still be sufficient
after an image resizing.

Signed-off-by: Leonid Bloch <lbloch@janustech.com>
---
 block/qcow2.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/block/qcow2.c b/block/qcow2.c
index 98cb96aaca..f60cb92169 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -3639,6 +3639,8 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
         }
     }
 
+    bs->total_sectors = offset / BDRV_SECTOR_SIZE;
+
     /* write updated header.size */
     offset = cpu_to_be64(offset);
     ret = bdrv_pwrite_sync(bs->file, offsetof(QCowHeader, size),
@@ -3649,6 +3651,12 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
     }
 
     s->l1_vm_state_index = new_l1_size;
+    /* Update cache sizes */
+    QDict *options = qdict_clone_shallow(bs->options);
+    ret = qcow2_update_options(bs, options, s->flags, errp);
+    if (ret < 0) {
+        goto fail;
+    }
     ret = 0;
 fail:
     qemu_co_mutex_unlock(&s->lock);
-- 
2.17.1

[Qemu-devel] [PATCH v2 4/5] qcow2: Set the default cache-clean-interval to 30 seconds

[Leonid Bloch]

The default cache-clean-interval is set to 30 seconds, in order to lower
the overhead of the qcow2 caches.

Signed-off-by: Leonid Bloch <lbloch@janustech.com>
---
 block/qcow2.c        | 2 +-
 block/qcow2.h        | 1 +
 docs/qcow2-cache.txt | 3 +--
 qapi/block-core.json | 3 ++-
 qemu-options.hx      | 2 +-
 5 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index f60cb92169..453a6377ac 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -941,7 +941,7 @@ static int qcow2_update_options_prepare(BlockDriverState *bs,
     /* New interval for cache cleanup timer */
     r->cache_clean_interval =
         qemu_opt_get_number(opts, QCOW2_OPT_CACHE_CLEAN_INTERVAL,
-                            s->cache_clean_interval);
+                            DEFAULT_CACHE_CLEAN_INTERVAL);
 #ifndef CONFIG_LINUX
     if (r->cache_clean_interval != 0) {
         error_setg(errp, QCOW2_OPT_CACHE_CLEAN_INTERVAL
diff --git a/block/qcow2.h b/block/qcow2.h
index d77a31d932..96a2808685 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -77,6 +77,7 @@
 
 #define DEFAULT_CLUSTER_SIZE 65536
 
+#define DEFAULT_CACHE_CLEAN_INTERVAL 30  /* seconds */
 
 #define QCOW2_OPT_LAZY_REFCOUNTS "lazy-refcounts"
 #define QCOW2_OPT_DISCARD_REQUEST "pass-discard-request"
diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
index c7625cdeb3..1e6711a63a 100644
--- a/docs/qcow2-cache.txt
+++ b/docs/qcow2-cache.txt
@@ -202,8 +202,7 @@ This example removes all unused cache entries every 15 minutes:
 
    -drive file=hd.qcow2,cache-clean-interval=900
 
-If unset, the default value for this parameter is 0 and it disables
-this feature.
+If unset, the default value for this parameter is 30.
 
 Note that this functionality currently relies on the MADV_DONTNEED
 argument for madvise() to actually free the memory. This is a
diff --git a/qapi/block-core.json b/qapi/block-core.json
index 5b9084a394..4d8fca9128 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -2830,7 +2830,8 @@
 #
 # @cache-clean-interval:  clean unused entries in the L2 and refcount
 #                         caches. The interval is in seconds. The default value
-#                         is 0 and it disables this feature (since 2.5)
+#                         is 30. (since 2.5)
+#
 # @encrypt:               Image decryption options. Mandatory for
 #                         encrypted images, except when doing a metadata-only
 #                         probe of the image. (since 2.10)
diff --git a/qemu-options.hx b/qemu-options.hx
index d6e15b2f06..0cda3d3c3a 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -767,7 +767,7 @@ it which is not used for the L2 cache)
 
 @item cache-clean-interval
 Clean unused entries in the L2 and refcount caches. The interval is in seconds.
-The default value is 0 and it disables this feature.
+The default value is 30.
 
 @item pass-discard-request
 Whether discard requests to the qcow2 device should be forwarded to the data
-- 
2.17.1

[Qemu-devel] [PATCH v2 5/5] qcow2: Explicit number replaced by a constant

[Leonid Bloch]

Signed-off-by: Leonid Bloch <lbloch@janustech.com>
---
 block/qcow2.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index 453a6377ac..a54e20402b 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1321,7 +1321,7 @@ static int coroutine_fn qcow2_do_open(BlockDriverState *bs, QDict *options,
     /* 2^(s->refcount_order - 3) is the refcount width in bytes */
     s->refcount_block_bits = s->cluster_bits - (s->refcount_order - 3);
     s->refcount_block_size = 1 << s->refcount_block_bits;
-    bs->total_sectors = header.size / 512;
+    bs->total_sectors = header.size / BDRV_SECTOR_SIZE;
     s->csize_shift = (62 - (s->cluster_bits - 8));
     s->csize_mask = (1 << (s->cluster_bits - 8)) - 1;
     s->cluster_offset_mask = (1LL << s->csize_shift) - 1;
@@ -3446,7 +3446,7 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
         goto fail;
     }
 
-    old_length = bs->total_sectors * 512;
+    old_length = bs->total_sectors * BDRV_SECTOR_SIZE;
     new_l1_size = size_to_l1(s, offset);
 
     if (offset < old_length) {
-- 
2.17.1

Re: [Qemu-devel] [PATCH v2 4/5] qcow2: Set the default cache-clean-interval to 30 seconds

[Eric Blake]

On 08/07/2018 03:21 PM, Leonid Bloch wrote:
> The default cache-clean-interval is set to 30 seconds, in order to lower
> the overhead of the qcow2 caches.
> 
> Signed-off-by: Leonid Bloch <lbloch@janustech.com>
> ---

> +++ b/qapi/block-core.json
> @@ -2830,7 +2830,8 @@
>   #
>   # @cache-clean-interval:  clean unused entries in the L2 and refcount
>   #                         caches. The interval is in seconds. The default value
> -#                         is 0 and it disables this feature (since 2.5)
> +#                         is 30. (since 2.5)
> +#
>   # @encrypt:               Image decryption options. Mandatory for
>   #                         encrypted images, except when doing a metadata-only
>   #                         probe of the image. (since 2.10)
> diff --git a/qemu-options.hx b/qemu-options.hx
> index d6e15b2f06..0cda3d3c3a 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -767,7 +767,7 @@ it which is not used for the L2 cache)
>   
>   @item cache-clean-interval
>   Clean unused entries in the L2 and refcount caches. The interval is in seconds.
> -The default value is 0 and it disables this feature.
> +The default value is 30.

May be worth wording as:

The default value is 30, while setting it to 0 disables this feature.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

Re: [Qemu-devel] [PATCH v2 4/5] qcow2: Set the default cache-clean-interval to 30 seconds

[Leonid Bloch]

On 08/07/2018 11:34 PM, Eric Blake wrote:
> On 08/07/2018 03:21 PM, Leonid Bloch wrote:
>> The default cache-clean-interval is set to 30 seconds, in order to lower
>> the overhead of the qcow2 caches.
>>
>> Signed-off-by: Leonid Bloch <lbloch@janustech.com>
>> ---
> 
>> +++ b/qapi/block-core.json
>> @@ -2830,7 +2830,8 @@
>>   #
>>   # @cache-clean-interval:  clean unused entries in the L2 and refcount
>>   #                         caches. The interval is in seconds. The 
>> default value
>> -#                         is 0 and it disables this feature (since 2.5)
>> +#                         is 30. (since 2.5)
>> +#
>>   # @encrypt:               Image decryption options. Mandatory for
>>   #                         encrypted images, except when doing a 
>> metadata-only
>>   #                         probe of the image. (since 2.10)
>> diff --git a/qemu-options.hx b/qemu-options.hx
>> index d6e15b2f06..0cda3d3c3a 100644
>> --- a/qemu-options.hx
>> +++ b/qemu-options.hx
>> @@ -767,7 +767,7 @@ it which is not used for the L2 cache)
>>   @item cache-clean-interval
>>   Clean unused entries in the L2 and refcount caches. The interval is 
>> in seconds.
>> -The default value is 0 and it disables this feature.
>> +The default value is 30.
> 
> May be worth wording as:
> 
> The default value is 30, while setting it to 0 disables this feature.
> 

Definitely! Thanks! Will fix.