Give the refcount cache the minimum possible size by default

The L2 and refcount caches have default sizes that can be overridden using the l2-cache-size and refcount-cache-size (an additional parameter named cache-size sets the combined size of both caches).

Unless forced by one of the aforementioned parameters, QEMU will set the unspecified sizes so that the L2 cache is 4 times larger than the refcount cache.

This is based on the premise that the refcount metadata needs to be only a fourth of the L2 metadata to cover the same amount of disk space. This is incorrect for two reasons:

a) The amount of disk covered by an L2 table depends solely on the cluster size, but in the case of a refcount block it depends on the cluster size *and* the width of each refcount entry. The 4/1 ratio is only valid with 16-bit entries (the default).

b) When we talk about disk space and L2 tables we are talking about guest space (L2 tables map guest clusters to host clusters), whereas refcount blocks are used for host clusters (including L1/L2 tables and the refcount blocks themselves). On a fully populated (and uncompressed) qcow2 file, image size > virtual size so there are more refcount entries than L2 entries.

Problem (a) could be fixed by adjusting the algorithm to take into account the refcount entry width. Problem (b) could be fixed by increasing a bit the refcount cache size to account for the clusters used for qcow2 metadata.

However this patch takes a completely different approach and instead of keeping a ratio between both cache sizes it assigns as much as possible to the L2 cache and the remainder to the refcount cache.

The reason is that L2 tables are used for every single I/O request from the guest and the effect of increasing the cache is significant and clearly measurable. Refcount blocks are however only used for cluster allocation and internal snapshots and in practice are accessed sequentially in most cases, so the effect of increasing the cache is negligible (even when doing random writes from the guest).

So, make the refcount cache as small as possible unless the user explicitly asks for a larger one.

Signed-off-by Alberto Garcia <[email protected]>

Reviewed-by Eric Blake <[email protected]>

---
 block/qcow2.c              | 31 +++++++++++++++++++------------
 block/qcow2.h              |  4 ----
 tests/qemu-iotests/137.out |  2 +-
 3 files changed, 20 insertions(+), 17 deletions(-)

block/qcow2.c +19 -12

index 7472af6931..8342b0186f 100644
@@ -802,23 +802,30 @@ 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 {
-            *refcount_cache_size = combined_cache_size
-                                 / (DEFAULT_L2_REFCOUNT_SIZE_RATIO + 1);
-            *l2_cache_size = combined_cache_size - *refcount_cache_size;
+            uint64_t virtual_disk_size = bs->total_sectors * BDRV_SECTOR_SIZE;
+            uint64_t max_l2_cache = virtual_disk_size / (s->cluster_size / 8);
+            uint64_t min_refcount_cache =
+                (uint64_t) MIN_REFCOUNT_CACHE_SIZE * s->cluster_size;
+
+            /* 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;
+                *refcount_cache_size = combined_cache_size - *l2_cache_size;
+            } else {
+                *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 && !refcount_cache_size_set) {
+        if (!l2_cache_size_set) {
             *l2_cache_size = MAX(DEFAULT_L2_CACHE_BYTE_SIZE,
                                  (uint64_t)DEFAULT_L2_CACHE_CLUSTERS
                                  * s->cluster_size);
-            *refcount_cache_size = *l2_cache_size
-                                 / DEFAULT_L2_REFCOUNT_SIZE_RATIO;
-        } else if (!l2_cache_size_set) {
-            *l2_cache_size = *refcount_cache_size
-                           * DEFAULT_L2_REFCOUNT_SIZE_RATIO;
-        } else if (!refcount_cache_size_set) {
-            *refcount_cache_size = *l2_cache_size
-                                 / DEFAULT_L2_REFCOUNT_SIZE_RATIO;
+        }
+        if (!refcount_cache_size_set) {
+            *refcount_cache_size = MIN_REFCOUNT_CACHE_SIZE * s->cluster_size;
         }
     }
 

block/qcow2.h +0 -4

index ccb92a9696..cdf41055ae 100644
@@ -77,10 +77,6 @@
 #define DEFAULT_L2_CACHE_CLUSTERS 8 /* clusters */
 #define DEFAULT_L2_CACHE_BYTE_SIZE 1048576 /* bytes */
 
-/* The refblock cache needs only a fourth of the L2 cache size to cover as many
- * clusters */
-#define DEFAULT_L2_REFCOUNT_SIZE_RATIO 4
-
 #define DEFAULT_CLUSTER_SIZE 65536
 
 

tests/qemu-iotests/137.out +1 -1

index e28e1eadba..96724a6c33 100644
@@ -22,7 +22,7 @@ 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)
-L2 cache size too big
+Refcount cache size too big
 Conflicting values for qcow2 options 'overlap-check' ('constant') and 'overlap-check.template' ('all')
 Unsupported value 'blubb' for qcow2 option 'overlap-check'. Allowed are any of the following: none, constant, cached, all
 Unsupported value 'blubb' for qcow2 option 'overlap-check'. Allowed are any of the following: none, constant, cached, all

Quoted 24 lines · Alberto Garcia

The L2 and refcount caches have default sizes that can be overridden using the l2-cache-size and refcount-cache-size (an additional parameter named cache-size sets the combined size of both caches).

Unless forced by one of the aforementioned parameters, QEMU will set the unspecified sizes so that the L2 cache is 4 times larger than the refcount cache.

This is based on the premise that the refcount metadata needs to be only a fourth of the L2 metadata to cover the same amount of disk space. This is incorrect for two reasons:

a) The amount of disk covered by an L2 table depends solely on the cluster size, but in the case of a refcount block it depends on the cluster size *and* the width of each refcount entry. The 4/1 ratio is only valid with 16-bit entries (the default).

b) When we talk about disk space and L2 tables we are talking about guest space (L2 tables map guest clusters to host clusters), whereas refcount blocks are used for host clusters (including L1/L2 tables and the refcount blocks themselves). On a fully populated (and uncompressed) qcow2 file, image size > virtual size so there are more refcount entries than L2 entries.

Quoted 4 lines

Problem (a) could be fixed by adjusting the algorithm to take into account the refcount entry width. Problem (b) could be fixed by increasing a bit the refcount cache size to account for the clusters used for qcow2 metadata.

Yeah, well, but I wouldn't give too much thought to (b). In case of internal snapshots, you won't write to them, so those clusters don't matter. So, yes, you get a bit of overhead for the metadata, but come on, that's not really much.

Quoted 3 lines

However this patch takes a completely different approach and instead of keeping a ratio between both cache sizes it assigns as much as possible to the L2 cache and the remainder to the refcount cache.

Seems reasonable in a sense, but I'm wary still. I especially don't like that it just changes the default when nothing has been specified.

I can see that we want to limit the refcount cache size, so if you want to give really much RAM to the L2 tables, then you don't want a 25 % overhead for the refcount cache, but do we really need to limit it to the minimum size?

OTOH, four clusters doesn't seem extremely limited... Did you do your benchmark on an HDD?

Quoted 30 lines · Max Reitz

Quoted 24 lines · Alberto Garcia

The L2 and refcount caches have default sizes that can be overridden using the l2-cache-size and refcount-cache-size (an additional parameter named cache-size sets the combined size of both caches).

Unless forced by one of the aforementioned parameters, QEMU will set the unspecified sizes so that the L2 cache is 4 times larger than the refcount cache.

This is based on the premise that the refcount metadata needs to be only a fourth of the L2 metadata to cover the same amount of disk space. This is incorrect for two reasons:

a) The amount of disk covered by an L2 table depends solely on the cluster size, but in the case of a refcount block it depends on the cluster size *and* the width of each refcount entry. The 4/1 ratio is only valid with 16-bit entries (the default).

b) When we talk about disk space and L2 tables we are talking about guest space (L2 tables map guest clusters to host clusters), whereas refcount blocks are used for host clusters (including L1/L2 tables and the refcount blocks themselves). On a fully populated (and uncompressed) qcow2 file, image size > virtual size so there are more refcount entries than L2 entries.

Quoted 4 lines

Problem (a) could be fixed by adjusting the algorithm to take into account the refcount entry width. Problem (b) could be fixed by increasing a bit the refcount cache size to account for the clusters used for qcow2 metadata.

Quoted 4 lines

Yeah, well, but I wouldn't give too much thought to (b). In case of internal snapshots, you won't write to them, so those clusters don't matter. So, yes, you get a bit of overhead for the metadata, but come on, that's not really much.

With that text I just wanted to point out that the calculation mixes host clusters and guest clusters. I agree that the ratio we're using is a good approximation (apart from the refcount width issue).

Quoted 7 lines

Quoted 3 lines

However this patch takes a completely different approach and instead of keeping a ratio between both cache sizes it assigns as much as possible to the L2 cache and the remainder to the refcount cache.

Seems reasonable in a sense, but I'm wary still. I especially don't like that it just changes the default when nothing has been specified.

Quoted 7 lines

I can see that we want to limit the refcount cache size, so if you want to give really much RAM to the L2 tables, then you don't want a 25 % overhead for the refcount cache, but do we really need to limit it to the minimum size?

OTOH, four clusters doesn't seem extremely limited... Did you do your benchmark on an HDD?

I tried, I see no difference, and I don't see why there would be any.

Refcount entries are used to reference host clusters, and host clusters are always(*) allocated sequentially, so caching refcount blocks doesn't do much. You're always loading the same refcount block until it's full, then you move on to the next one. As I mentioned in a previous e-mail, having a very large refcount cache would even be detrimental because it would make cache hits slower (noticeable under tmpfs).

(*) when a cluster gets deallocated (e.g. when the refcount table needs to grow) then there's a hole in the file and QEMU starts looking for free clusters again from the beginning of the file. In that case old refcount blocks are loaded again, but the cache wouldn't help here either (unless we would have all refcount blocks cached).

Quoted 7 lines

Refcount entries are used to reference host clusters, and host clusters are always(*) allocated sequentially, so caching refcount blocks doesn't do much. You're always loading the same refcount block until it's full, then you move on to the next one. As I mentioned in a previous e-mail, having a very large refcount cache would even be detrimental because it would make cache hits slower (noticeable under tmpfs).

Ah, right. I'm not sure whether I want to ask you whether you have tested internal snapshots. I suppose it can be detrimental for them because when taking or deleting a snapshot you need to update the refcounts of a whole bunch of clusters that might be spread randomly across the image. But I suspect that it would be rather time-consuming to produce an image with such a configuration; and I suppose if people want to make heavy use of internal snapshots they can adapt the cache size themselves?

The L2 and refcount caches have default sizes that can be overridden using the l2-cache-size and refcount-cache-size (an additional parameter named cache-size sets the combined size of both caches).

Unless forced by one of the aforementioned parameters, QEMU will set the unspecified sizes so that the L2 cache is 4 times larger than the refcount cache.

This is based on the premise that the refcount metadata needs to be only a fourth of the L2 metadata to cover the same amount of disk space. This is incorrect for two reasons:

a) The amount of disk covered by an L2 table depends solely on the cluster size, but in the case of a refcount block it depends on the cluster size *and* the width of each refcount entry. The 4/1 ratio is only valid with 16-bit entries (the default).

b) When we talk about disk space and L2 tables we are talking about guest space (L2 tables map guest clusters to host clusters), whereas refcount blocks are used for host clusters (including L1/L2 tables and the refcount blocks themselves). On a fully populated (and uncompressed) qcow2 file, image size > virtual size so there are more refcount entries than L2 entries.

Problem (a) could be fixed by adjusting the algorithm to take into account the refcount entry width. Problem (b) could be fixed by increasing a bit the refcount cache size to account for the clusters used for qcow2 metadata.

However this patch takes a completely different approach and instead of keeping a ratio between both cache sizes it assigns as much as possible to the L2 cache and the remainder to the refcount cache.

The reason is that L2 tables are used for every single I/O request from the guest and the effect of increasing the cache is significant and clearly measurable. Refcount blocks are however only used for cluster allocation and internal snapshots and in practice are accessed sequentially in most cases, so the effect of increasing the cache is negligible (even when doing random writes from the guest).

So, make the refcount cache as small as possible unless the user explicitly asks for a larger one.

Signed-off-by Alberto Garcia <[email protected]>

Reviewed-by Eric Blake <[email protected]>

---
 block/qcow2.c              | 31 +++++++++++++++++++------------
 block/qcow2.h              |  4 ----
 tests/qemu-iotests/137.out |  2 +-
 3 files changed, 20 insertions(+), 17 deletions(-)

We have just reduced the refcount cache size to the minimum unless the user explicitly requests a larger one, so we have to update the documentation to reflect this change.

Signed-off-by Alberto Garcia <[email protected]>

Reviewed-by Eric Blake <[email protected]>

---
 docs/qcow2-cache.txt | 31 ++++++++++++++-----------------
 1 file changed, 14 insertions(+), 17 deletions(-)

docs/qcow2-cache.txt +7 -8

index 170191a242..c640d45d06 100644
@@ -116,31 +116,28 @@ There are three options available, and all of them take bytes:
 "refcount-cache-size":   maximum size of the refcount block cache
 "cache-size":            maximum size of both caches combined
 
-There are two things that need to be taken into account:
+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).
 
- - If you only set one of the options above, QEMU will automatically
-   adjust the others so that the L2 cache is 4 times bigger than the
-   refcount cache.
+ - 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).
 
-This means that these options are equivalent:
+ - The default (and minimum) refcount cache size is 4 clusters.
 
-   -drive file=hd.qcow2,l2-cache-size=2097152
-   -drive file=hd.qcow2,refcount-cache-size=524288
-   -drive file=hd.qcow2,cache-size=2621440
+ - If only "cache-size" is specified then QEMU will assign as much
+   memory as possible to the L2 cache before increasing the refcount
+   cache size.

docs/qcow2-cache.txt +3 -4

-It's also worth mentioning that there's no strict need for both caches
-to cover the same amount of disk space. The refcount cache is used
-much less often than the L2 cache, so it's perfectly reasonable to
-keep it small.
+Before QEMU 2.12 the refcount cache had a default size of 1/4 of the
+L2 cache size. This resulted in unnecessarily large caches, so now the
+refcount cache is as small as possible unless overridden by the user.
 
 
 Using smaller cache entries