Merge pull request #183 from Desiki-high/doc-gc

docs: add garbage-collection.md for gc

README.md

@@ -148,4 +148,5 @@ We can specify the driver name by modifying `converter.driver` in the configurat
 
 ## Documentation
 
-* [API](./docs/api.md)
+* [API](./docs/api.md)
+* [Garbage Collection](./docs/garbage-collection.md)

docs/garbage-collection.md

@@ -0,0 +1,30 @@
+# Garbage Collection
+
+`Acceleration Service` has a garbage collector capable of removing resources
+which are not used for a long time or unfrequently. The garbage collector consists
+of leaseManager and leaseCache. The `acceld` has responsible for
+ensuring that all resources(blobs) which are stored locally are held by
+[leases](https://github.com/containerd/containerd/blob/main/docs/garbage-collection.md#what-is-a-lease)
+at all times, else they will be cleared.
+
+## What is leaseManager and leaseCache ?
+
+`Acceleration Service` uses `leaseManager` and `leaseCache` to manager all leases
+which are persisted in boltdb. The `leaseManager` will creat a new lease for the
+new blob and delete leases of the blobs which should be cleared away.
+The `leaseCache` will sync with the leases which are persisted in boltdb and provide
+the lease that should be cleared in LFRU(LFU first, LRU second) order. The `acceld` will
+init `leaseCache` when daemon started.
+
+## Garbage Collection Policy
+
+Garbage Collection can be triggered in two ways. Firstly, gc will be triggered after each
+task completed. Secondly, gc has scheduled tasks every hourly since dameon started. GC clears
+the blobs until local storage size below 80% of the threshold. Specially, scheduled gc tasks
+clear the blobs until local storage size below 40% of the threshold.
+
+## Garbage Collection Configuration
+
+| Configuration | Example |Description |
+|---|---|---|
+| `gcpolicy.threshold` | "100MB" |size threshold that triggers GC, the `gc` will start reclaim blobs if exceeds the size. |