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https://issues.apache.org/jira/browse/IGNITE-13674?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Sergey Chugunov updated IGNITE-13674:
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Docs Text:
Defragmentation
Introduction
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.
Important: 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.
How to use it
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.
To request defragmentation use the following command: <specific command>
After restart node with requested defragmentation will enter special mode
called maintenance mode. Node in maintenance doesn't join the rest of the
cluster but stays isolated until defragmentation is completed (or cancelled by
explicit user request). After that user has to restart the node one more time:
it will exit maintenance mode and returns back to normal operations (joins the
cluster and starts to serve regular workload).
Important: as nodes in maintenance don't participate in serving usual workload,
it is not recommended to execute defragmentation on several nodes at once as it
reduces number of backups thus increasing the risk of partition loss.
When node executes defragmentation it is possible to retrieve operation status
or cancel it fully or partially using the following commands available in
control utility:
<command for status>
<command for cancel>.
For more information about commands refer to their help.
Important: to reduce disk space requirements during defragmentation caches are
defragmented one by one (if defragmentation of more than one cache was
requested). To calculate additional space required find the cache that occupies
the most disk space. The same amount of disk space is required for
defragmentation at max.
Conclusion
In most situations defragmentation isn't needed as existing memory management
mechanism effectively reuses memory left after data deletion. But in rare cases
it may be necessary to employ it to free up disk space.
Its usage requires taking nodes out of normal operations so it careful planning
is needed.
> Document Persistent store defragmentation
> -----------------------------------------
>
> Key: IGNITE-13674
> URL: https://issues.apache.org/jira/browse/IGNITE-13674
> Project: Ignite
> Issue Type: Sub-task
> Reporter: Sergey Chugunov
> Assignee: Sergey Chugunov
> Priority: Major
> Labels: IEP-47
> Original Estimate: 48h
> Remaining Estimate: 48h
>
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