Update the gc documentation.

Signed-off-by: Richard Scothern <richard.scothern@gmail.com>

docs/gc.md

@@ -8,21 +8,34 @@ keywords = ["registry, garbage, images, tags, repository, distribution"]
 
 # What Garbage Collection Does
 
-Garbage collection is a process that delete blobs to which no manifests refer.
+"Garbage collection deletes blobs which no manifests reference. Manifests and
-It runs in two phases. First, in the 'mark' phase, the process scans all the 
+blobs which are deleted by their digest through the Registry API will become
-manifests in the registry. From these manifests, it constructs a set of content 
+eligible for garbage collection, but the actual blobs will not be removed from
-address digests. This set is the 'mark set' and denotes the set of blobs to *not*
+storage until garbage collection is run.
-delete. Secondly, in the 'sweep' phase, the process scans all the blobs and if 
-a blob's content address digest is not in the mark set, the process will delete 
-it.
 
+# How Garbage Collection Works
+
+Garbage collection runs in two phases. First, in the 'mark' phase, the process
+scans all the manifests in the registry. From these manifests, it constructs a
+set of content address digests. This set is the 'mark set' and denotes the set
+of blobs to *not* delete. Secondly, in the 'sweep' phase, the process scans all
+the blobs and if a blob's content address digest is not in the mark set, the
+process will delete it.
+
+> **NOTE** You should ensure that the registry is in read-only mode or not running at
+> all. If you were to upload an image while garbage collection is running, there is the
+> risk that the image's layers will be mistakenly deleted, leading to a corrupted image.
+
+This type of garbage collection is known as stop-the-world garbage collection.  In
+future registry versions the intention is that garbage collection will be an 
+automated background action and this manual process will no longer apply.
 
 # How to Run
 
 You can run garbage collection by running
 
-	docker run --rm registry-image-name garbage-collect /etc/docker/registry/config.yml
+`docker run --rm registry-image-name garbage-collect /etc/docker/registry/config.yml`
+
+Additionally, garbage collection can be run in `dry-run` mode, which will print
+the progress of the mark and sweep phases without removing any data.
 
-NOTE: You should ensure that the registry itself is in read-only mode or not running at
-all. If you were to upload an image while garbage collection is running, there is the
-risk that the image's layers will be mistakenly deleted, leading to a corrupted image.

registry/garbagecollect.go

@@ -19,8 +19,7 @@ import (
 
 func emit(format string, a ...interface{}) {
 	if dryRun {
-		fmt.Printf(format, a...)
+		fmt.Printf(format+"\n", a...)
-		fmt.Println("")
 	}
 }
 
@@ -122,8 +121,8 @@ func markAndSweep(ctx context.Context, storageDriver driver.StorageDriver, regis
 	// Construct vacuum
 	vacuum := storage.NewVacuum(ctx, storageDriver)
 	for dgst := range deleteSet {
+		emit("blob eligible for deletion: %s", dgst)
 		if dryRun {
-			emit("deleting %s", dgst)
 			continue
 		}
 		err = vacuum.RemoveBlob(string(dgst))
@@ -169,7 +168,7 @@ var GCCmd = &cobra.Command{
 
 		k, err := libtrust.GenerateECP256PrivateKey()
 		if err != nil {
-			fmt.Fprintf(os.Stderr, "%s", err)
+			fmt.Fprint(os.Stderr, err)
 			os.Exit(1)
 		}