build: garbage collection page

_data/toc.yaml

@@ -1521,24 +1521,28 @@ manuals:
               title: Kubernetes driver
             - path: /build/building/drivers/remote/
               title: Remote driver
-        - path: /build/building/cache/
-          title: Optimizing builds with cache
-        - sectiontitle: Cache backends
+        - sectiontitle: Cache
           section:
-            - path: /build/building/cache/backends/
-              title: Overview
-            - path: /build/building/cache/backends/inline/
-              title: Inline
-            - path: /build/building/cache/backends/local/
-              title: Local
-            - path: /build/building/cache/backends/registry/
-              title: Registry
-            - path: /build/building/cache/backends/gha/
-              title: GitHub Actions
-            - path: /build/building/cache/backends/azblob/
-              title: Azure Blob Storage
-            - path: /build/building/cache/backends/s3/
-              title: Amazon S3
+            - path: /build/building/cache/
+              title: Optimizing builds with cache
+            - path: /build/building/cache/garbage-collection/
+              title: Garbage collection
+            - sectiontitle: Cache backends
+              section:
+                - path: /build/building/cache/backends/
+                  title: Overview
+                - path: /build/building/cache/backends/inline/
+                  title: Inline
+                - path: /build/building/cache/backends/local/
+                  title: Local
+                - path: /build/building/cache/backends/registry/
+                  title: Registry
+                - path: /build/building/cache/backends/gha/
+                  title: GitHub Actions
+                - path: /build/building/cache/backends/azblob/
+                  title: Azure Blob Storage
+                - path: /build/building/cache/backends/s3/
+                  title: Amazon S3
         - path: /build/building/multi-stage/
           title: Multi-stage builds
         - path: /build/building/multi-platform/

build/building/cache/garbage-collection.md

@@ -0,0 +1,90 @@
+---
+title: Garbage collection
+keywords: build, buildx, buildkit, garbage collection, prune
+---
+
+While [`docker build prune`](../../../engine/reference/commandline/builder_prune.md)
+or [`docker buildx prune`](../../../engine/reference/commandline/buildx_prune.md)
+commands run at once, garbage collection runs periodically and follows an
+ordered list of prune policies.
+
+Garbage collection runs in the BuildKit daemon. The daemon clears the build
+cache when the cache size becomes too big, or when the cache age expires. The
+following sections describe how you can configure both the size and age
+parameters by defining garbage collection policies.
+
+## Configuration
+
+Depending on the [driver](../drivers/index.md) used by your builder instance,
+the garbage collection will use a different configuration file.
+
+If you're using the [`docker` driver](../drivers/docker.md), garbage collection
+can be configured in the [Docker Daemon configuration](../../../engine/reference/commandline/dockerd.md#daemon-configuration-file).
+file:
+
+```json
+{
+  "builder": {
+    "gc": {
+      "enabled": true,
+      "defaultKeepStorage": "10GB",
+      "policy": [
+          {"keepStorage": "10GB", "filter": ["unused-for=2200h"]},
+          {"keepStorage": "50GB", "filter": ["unused-for=3300h"]},
+          {"keepStorage": "100GB", "all": true}
+      ]
+    }
+  }
+}
+```
+
+For other drivers, garbage collection can be configured using the
+[BuildKit configuration](../../buildkit/toml-configuration.md) file:
+
+```toml
+[worker.oci]
+  gc = true
+  gckeepstorage = 10000
+  [[worker.oci.gcpolicy]]
+    keepBytes = 512000000
+    keepDuration = 172800
+    filters = [ "type==source.local", "type==exec.cachemount", "type==source.git.checkout"]
+  [[worker.oci.gcpolicy]]
+    all = true
+    keepBytes = 1024000000
+```
+
+## Default policies
+
+Default garbage collection policies are applied to all builders if not
+already set:
+
+```
+GC Policy rule#0:
+        All:            false
+        Filters:        type==source.local,type==exec.cachemount,type==source.git.checkout
+        Keep Duration:  48h0m0s
+        Keep Bytes:     512MB
+GC Policy rule#1:
+        All:            false
+        Keep Duration:  1440h0m0s
+        Keep Bytes:     26GB
+GC Policy rule#2:
+        All:            false
+        Keep Bytes:     26GB
+GC Policy rule#3:
+        All:            true
+        Keep Bytes:     26GB
+```
+
+* `rule#0`: if build cache uses more than 512MB delete the most easily
+  reproducible data after it has not been used for 2 days.
+* `rule#1`: remove any data not used for 60 days.
+* `rule#2`: keep the unshared build cache under cap.
+* `rule#3`: if previous policies were insufficient start deleting internal data
+  to keep build cache under cap.
+
+> **Note**
+>
+> "Keep bytes" defaults to 10% of the size of the disk. If the disk size cannot
+> be determined, it defaults to 2GB.

build/building/cache/index.md

@@ -289,4 +289,5 @@ of continuing.)
 
 For more information on using cache to do efficient builds, see:
 
+- [Garbage collection](garbage-collection.md)
 - [Cache storage backends](./backends/index.md)