[GitHub] [ignite] sergey-chugunov-1985 commented on a change in pull request #8465: IGNITE-13674 Documentation page added for Native Persistence Defragmentation feature

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sergey-chugunov-1985 commented on a change in pull request #8465:
URL: https://github.com/apache/ignite/pull/8465#discussion_r560157802



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File path: docs/_docs/persistence/native-persistence-defragmentation.adoc
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+// Licensed to the Apache Software Foundation (ASF) under one or more
+// contributor license agreements.  See the NOTICE file distributed with
+// this work for additional information regarding copyright ownership.
+// The ASF licenses this file to You under the Apache License, Version 2.0
+// (the "License"); you may not use this file except in compliance with
+// the License.  You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+= Persistence Defragmentation
+
+== Overview
+
+As memory management mechanism of Apache Ignite can only create or reuse pages for user data but never frees them files where Ignite persists data can only grow and never shrinks.
+
+In most use cases it doesn't cause any problems as once created page can be reused multiple times. However in certain cases it is possible that cache contains very little data but occupies large chunks of disk space because a lot of data was removed from the cache.
+
+Defragmentation is aimed to enable user to shrink data files and claim back disk space.
+
+[NOTE]
+====
+Defragmentation can only be used with historical rebalance enabled (link to historical rebalance page). If historical rebalance is disabled server node always triggers full rebalance after restart throwing away defragmented partition. Full set of data is transferred to the node from other nodes over network, depending of size of data set it may require a lot of time and may slow down the whole cluster as network capacity is important to fulfill user requests.
+====
+
+== Usage: how and when
+
+Defragmentation is costly operation in terms of disk IO so to avoid slowing down user operations it cannot be executed on regular node joined to the cluster. To execute defragmentation user needs to request it first on a particular node or set of nodes and than restart these nodes.

Review comment:
       Disagree on the suggested change: "do not execute" implies that it is possible to do the action but is not recommended. In reality executing defragmentation on a regular node is not possible at all. How can we rephrase this to catch this meaning?




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