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Update garbage-collection.md (#4317)

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Wang Jie 2017-08-30 06:43:24 +08:00 committed by Misty Stanley-Jones
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@ -7,27 +7,17 @@ title: Garbage collection
As of v2.4.0 a garbage collector command is included within the registry binary. As of v2.4.0 a garbage collector command is included within the registry binary.
This document describes what this command does and how and why it should be used. This document describes what this command does and how and why it should be used.
## What is Garbage Collection? ## About garbage collection
From [wikipedia](https://en.wikipedia.org/wiki/Garbage_collection_(computer_science)):
"In computer science, garbage collection (GC) is a form of automatic memory management. The
garbage collector, or just collector, attempts to reclaim garbage, or memory occupied by
objects that are no longer in use by the program."
In the context of the Docker registry, garbage collection is the process of In the context of the Docker registry, garbage collection is the process of
removing blobs from the filesystem which are no longer referenced by a removing blobs from the filesystem when they are no longer referenced by a
manifest. Blobs can include both layers and manifests. manifest. Blobs can include both layers and manifests.
Registry data can occupy considerable amounts of disk space. In addition,
garbage collection can be a security consideration, when it is desirable to ensure
that certain layers no longer exist on the filesystem.
## Why Garbage Collection? ## Garbage collection in practice
Registry data can occupy considerable amounts of disk space and freeing up
this disk space is an oft-requested feature. Additionally for reasons of security it
can be desirable to ensure that certain layers no longer exist on the filesystem.
## Garbage Collection in the Registry
Filesystem layers are stored by their content address in the Registry. This Filesystem layers are stored by their content address in the Registry. This
has many advantages, one of which is that data is stored once and referred to by manifests. has many advantages, one of which is that data is stored once and referred to by manifests.
@ -72,7 +62,7 @@ collection. Layer `a` had one reference removed but will not be garbage
collected as it is still referenced by manifest `A`. The blob representing collected as it is still referenced by manifest `A`. The blob representing
manifest `B` will also be eligible for garbage collection. manifest `B` will also be eligible for garbage collection.
After garbage collection has been run manifest `A` and its blobs remain. After garbage collection has been run, manifest `A` and its blobs remain.
``` ```
A -----> a A -----> a
@ -80,7 +70,7 @@ A -----> a
``` ```
## How Garbage Collection works ### More details about garbage collection
Garbage collection runs in two phases. First, in the 'mark' phase, the process 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 scans all the manifests in the registry. From these manifests, it constructs a
@ -90,7 +80,7 @@ the blobs and if a blob's content address digest is not in the mark set, the
process will delete it. process will delete it.
> **NOTE**: You should ensure that the registry is in read-only mode or not running at > **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 > 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. > risk that the image's layers will be mistakenly deleted, leading to a corrupted image.
@ -100,7 +90,7 @@ action and this manual process will no longer apply.
# Running garbage collection ## Run garbage collection
Garbage collection can be run as follows Garbage collection can be run as follows