Author: frm
Date: Wed Nov 30 11:24:12 2016
New Revision: 1772015
URL: http://svn.apache.org/viewvc?rev=1772015&view=rev
Log:
OAK-5167 - Describe the role of offline garbage collection
Modified:
jackrabbit/oak/trunk/oak-doc/src/site/markdown/nodestore/segment/overview.md
Modified:
jackrabbit/oak/trunk/oak-doc/src/site/markdown/nodestore/segment/overview.md
URL:
http://svn.apache.org/viewvc/jackrabbit/oak/trunk/oak-doc/src/site/markdown/nodestore/segment/overview.md?rev=1772015&r1=1772014&r2=1772015&view=diff
==============================================================================
---
jackrabbit/oak/trunk/oak-doc/src/site/markdown/nodestore/segment/overview.md
(original)
+++
jackrabbit/oak/trunk/oak-doc/src/site/markdown/nodestore/segment/overview.md
Wed Nov 30 11:24:12 2016
@@ -22,6 +22,7 @@ Oak Segment Tar is an implementation of
* [Garbage Collection](#garbage-collection)
* [Generational Garbage Collection](#generational-garbage-collection)
* [Estimation, Compaction and Cleanup](#estimation-compaction-cleanup)
+ * [Offline Garbage Collection](#offline-garbage-collection)
* [Online Garbage Collection](#online-garbage-collection)
* [Monitoring the log](#monitoring-the-log)
* [When did garbage collection
start?](#when-did-garbage-collection-start)
@@ -88,6 +89,25 @@ Once relevant data is safe in the new ge
This phase locates outdated pieces of data from one of the oldest generations
and removes it from the system.
This is the only phase where data is actually deleted and disk space is
finally freed.
+### <a name="offline-garbage-collection"/> Offline Garbage Collection
+
+Offline garbage collection is the procedure followed by Oak Segment Tar to
execute garbage collection by taking exclusive control of the repository.
+
+Offline garbage collection runs as a standalone Java tool manually or
semi-automatically started from the command line.
+The way offline garbage collection works is simpler than the online version.
+It is assumed that a human operator is in charge of deciding when offline
compaction is needed.
+In such a case, the human operator has to take offline - hence the name - the
system using the repository and start the compaction utility from the command
line.
+
+Since offline garbage collection requires human intervention to run, the
estimation phase is not executed at all.
+The human operator who decides to run offline garbage collection does so
because he or she decided that the garbage in the repository is exceeding some
arbitrary threshold.
+Since the decision comes from a human operator, offline garbage collection is
not in charge of implementing heuristics to decide if and when garbage
collection should be run.
+The offline garbage collection process consist of the compaction and cleanup
phases only.
+
+The main drawback of offline garbage collection is that the process has to
take exclusive control of the repository.
+Nevertheless, this is also a strength.
+Having exclusive access to the repository, offline garbage collection is
usually faster and more effective of its online counterpart.
+Because of this, offline garbage collection is (and will always be) an
important tool in repository management.
+
### <a name="online-garbage-collection"/> Online Garbage Collection
Online garbage collection is the procedure followed by Oak Segment Tar to
execute garbage collection on a running system.
