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Docs: Add garbage collection page (#25715)
* add garbage collection page * finish client; add resources section * finish server section; task driver section * add front matter description * fix typos * Address Tim's feedback
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Aimee Ukasick
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website/content/docs/operations/garbage-collection.mdx
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website/content/docs/operations/garbage-collection.mdx
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---
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layout: docs
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page_title: Garbage collection
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description: |-
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Nomad garbage collects Access Control List (ACL) tokens, allocations, deployments, encryption root keys, evaluations, jobs, nodes, plugins, and Container Storage Interface (CSI) volumes. Learn about server-side and client-side garbage collection processes, including configuration and triggers.
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---
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# Garbage collection
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Nomad garbage collection is not the same as garbage collection in a programming
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language, but the motivation behind its design is similar: garbage collection
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frees up memory allocated for objects that the schedular no longer references or
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needs. Nomad only garbage collects objects that are in a terminal state and only
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after a delay to allow inspection or debugging.
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Nomad runs garbage collection processes on servers and on client nodes. You may
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also manually trigger garbage collection on the server.
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Nomad garbage collects the following objects:
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- [ACL token](#configuration)
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- [Allocation](#client-side-garbage-collection)
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- [CSI Plugin](#configuration)
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- [Deployment](#configuration)
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- [Encryption root key](#configuration)
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- [Evaluation](#configuration)
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- [Job](#configuration)
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- [Node](#configuration)
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- [Volume](#configuration)
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## Cascading garbage collection
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Nomad's scheduled garbage collection processes generally handle each resource
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type independently. However, there is an implicit cascading relationship because
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of how objects reference each other. In practice, when Nomad garbage collects a
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higher-level object, Nomad also removes the object's associated sub-objects to
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prevent orphaned objects.
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For example, garbage collecting a job also causes Nomad to drop all of that
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job's remaining evaluations, deployments, and allocation records from the state.
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Nomad garbage collects those objects, either as part of the job garbage
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collection process or by each object's own garbage collection processes running
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immediately after. Nomad's scheduled garbage collection processes only garbage
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collect objects after they are terminal for at least the specified time
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threshold and no longer needed for future scheduling decisions. Note that when
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you force garbage collection by running the `nomad system gc` command, Nomad
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ignores the specified time threshold.
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## Server-side garbage collection
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The Nomad server leader starts periodic garbage collection processes that clean
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objects marked for garbage collection from memory. Nomad automatically marks
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some objects, like evaluations, for garbage collection. Alternatively, you may
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manually mark jobs for garbage collection by running `nomad system gc`, which
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runs the garbage collection process.
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### Configuration
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These settings govern garbage collection behavior on the server nodes. You may
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review the intervals in the [`config.go`
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class](https://github.com/hashicorp/nomad/blob/b11619010e1c83488e14e2785569e515b2769062/nomad/config.go#L564)
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for objects without a configurable interval setting.
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| Object | Interval | Threshold |
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|---|---|---|
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| **ACL token** | 5 minutes | [`acl_token_gc_threshold`](/nomad/docs/configuration/server#acl_token_gc_threshold)<br/>Default: 1 hour |
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| **CSI Plugin** | 5 minutes | [`csi_plugin_gc_threshold`](/nomad/docs/configuration/server#csi_plugin_gc_threshold)<br/>Default: 1 hour |
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| **Deployment** | 5 minutes | [`deployment_gc_threshold`](/nomad/docs/configuration/server#deployment_gc_threshold)<br/>Default: 1 hour |
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| **Encryption root key** | [`root_key_gc_interval`](/nomad/docs/configuration/server#root_key_gc_interval)<br/>Default: 10 minutes | [`root_key_gc_threshold`](/nomad/docs/configuration/server#root_key_gc_threshold)<br/>Default: 1 hour |
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| **Evaluation** | 5 minutes | [`eval_gc_threshold`](/nomad/docs/configuration/server#eval_gc_threshold) <br/>Default: 1 hour |
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| **Evaluation, batch** | 5 minutes | [`batch_eval_gc_threshold`](/nomad/docs/configuration/server#batch_eval_gc_threshold)<br/>Default: 24 hours |
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| **Job** | [`job_gc_interval`](/nomad/docs/configuration/server#job_gc_interval)<br/>Default: 5 minutes | [`job_gc_threshold`](/nomad/docs/configuration/server#job_gc_threshold)<br/>Default: 4 hours |
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| **Node** | 5 minutes | [`node_gc_threshold`](/nomad/docs/configuration/server#node_gc_threshold)<br/>Default: 24 hours |
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| **Volume** | [`csi_volume_claim_gc_interval`](/nomad/docs/configuration/server#csi_volume_claim_gc_interval)<br/>Default: 5 minutes| [`csi_volume_claim_gc_threshold`](/nomad/docs/configuration/server#csi_volume_claim_gc_threshold)<br/>Default: 1 hour |
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### Triggers
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The server garbage collection processes wake up at configured intervals to scan
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for any expired or terminal objects to permanently delete, provided the object's
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time in a terminal state exceeds its garbage collection threshold. For example,
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a job's default garbage collection threshold is four hours, so the job must be
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in a terminal state for at least four hours before the garbage collection
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process permanently deletes the job and its dependent objects.
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When you force garbage collection by manually running the `nomad system gc`
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command, you are telling the garbage collection process to ignore thresholds and
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immediately purge all terminal objects on all servers and clients.
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## Client-side garbage collection
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On each client node, Nomad must clean up resources from terminated allocations
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to free disk and memory on the machine.
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### Configuration
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These settings govern allocation garbage collection behavior on each client node.
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| Parameter | Default | Description |
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| -------- | ------- | ------------- |
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| [`gc_interval`](/nomad/docs/configuration/client#gc_interval) | 1 minute | Interval at which Nomad attempts to garbage collect terminal allocation directories |
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| [ `gc_disk_usage_threshold` ](/nomad/docs/configuration/client#gc_disk_usage_threshold) | 80 | Disk usage percent which Nomad tries to maintain by garbage collecting terminal allocations |
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| [ `gc_inode_usage_threshold` ](/nomad/docs/configuration/client#gc_inode_usage_threshold) | 70 | Inode usage percent which Nomad tries to maintain by garbage collecting terminal allocations |
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| [ `gc_max_allocs` ](/nomad/docs/configuration/client#gc_max_allocs) | 50 | Maximum number of allocations which a client will track before triggering a garbage collection of terminal allocations |
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| [ `gc_parallel_destroys ` ](/nomad/docs/configuration/client#gc_parallel_destroys) | 2 | Maximum number of parallel destroys allowed by the garbage collector |
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Refer to the [client block in agent configuration
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reference](/nomad/docs/configuration/client) for complete parameter descriptions
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and examples.
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Note that there is no time-based retention setting for allocations. Unlike jobs
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or evaluations, you cannot specify a time to keep allocations alive before
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garbage collection. As soon as an allocation is terminal, it becomes eligible
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for cleanup if the configured thresholds demand it.
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### Triggers
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Nomad's client runs allocation garbage collection based on these triggers:
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- Scheduled interval
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The garbage collection process launches a ticker based on the configured
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`gc_interval`. On each tick, the garbage collection process checks to see if it needs to remove terminal allocations.
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- Terminal state
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When an allocation transitions to a terminal state, Nomad marks
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the allocation for garbage collection and then signals the garbage collection
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process to run immediately.
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- Allocation placement
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Nomad may preemptively run garbage collection to make room for new
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allocations. The client garbage collects older, terminal allocations if adding new allocations would exceed the `gc_max_allocs` limit.
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- Forced garbage collection
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When you force garbage collection by running the `nomad system gc` command,
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the garbage collection process removes all terminal objects on all servers and
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clients, ignoring thresholds.
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Nomad does not continuously monitor disk or inode usage to trigger garbage
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collection. Instead, Nomad only checks disk and inode thresholds when one of the
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aforementioned triggers invokes the garbage collection process. The
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`gc_inode_usage_threshold` and `gc_disk_usage_threshold` values do not trigger
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garbage collection; rather, those values influence how the garbage collector
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behaves during a collection run.
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### Allocation selection
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When the garbage collection process runs, Nomad destroys as many finished
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allocations as needed to meet the resource thresholds. The client maintains a
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priority queue of terminal allocations ordered by the time they were marked
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finished, oldest first.
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The process repeatedly evicts allocations from the queue until the conditions
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are back within configured limits. Specifically, the garbage collection loop
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checks, in order:
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1. If disk usage exceeds `gc_disk_usage_threshold` value
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1. If inode usage exceeds `gc_inode_usage_threshold` value
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1. If the count of allocations exceeds `gc_max_allocs` value
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If any one of these conditions is true, the garbage collector selects the oldest
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finished allocation for removal.
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After deleting one allocation, the loop re-checks the metrics and continues
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removing the next-oldest allocation until all thresholds are satisfied or
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until there are no more terminal allocs. This means in a single run, the
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garbage collection removes multiple allocations back-to-back if the node was
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far over the limits. The evictions happen in termination-time order, which is
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oldest completed allocations first.
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If node's usage and allocation count are under the limits, a normal garbage
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collection cycle does not remove any allocations. In other words, periodic and
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event-driven garbage collection does not delete allocations just because they
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are finished. There has to be pressure or a limit reached. The exception is when
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an administrative command or server-side removal triggers client-side garbage
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collection. Aside from that forced scenario, the default behavior is
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threshold-driven: Nomad leaves allocations on disk until it needs to reclaim
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those allocations due to space, inode, or count limits being hit.
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### Task driver resources garbage collection
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Most task drivers do not have their own garbage collection process. When an
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allocation is terminal, the client garbage collection process communicates with
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the task driver to ensure the task's resources have been cleaned up. Note that
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the Docker task driver periodically cleans up its own resources. Refer to the
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[Docker task driver plugin
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options](https://developer.hashicorp.com/nomad/docs/drivers/docker#gc) for
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details.
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When a task has configured restart attempts and the task fails, the Nomad client
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attempts an in-place task restart within the same allocation. The task driver
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starts a new process or container for the task. If the task continues to fail
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and exceeds the configured restart attempts, Nomad terminates the task and marks
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the allocation as terminal. The task driver then cleans up its resources, such
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as a Docker container or cgroups. When the garbage collection process runs, it
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makes sure that the task driver cleanup is done before deleting the allocation.
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If a task driver fails to clean up properly, Nomad logs errors but continues the
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garbage collection process. Task driver cleanup failure issues can influence
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when the allocation truly frees up. For instance, if volumes are not detached,
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disk space might not be fully reclaimed until fixed.
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## Resources
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- [Nomad's internal garbage collection and optimization discovery during the
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Nomad Bench project blog post](https://www.hashicorp.com/en/blog/nomad-garbage-collection-optimization-discovery-during-nomad-bench)
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- Configuration
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- [client Block in Agent Configuration](/nomad/docs/configuration/client)
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- [server Block in Agent Configuration](/nomad/docs/configuration/server)
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- [the `nomad system gc` command reference](/nomad/docs/commands/system/gc)
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- [System HTTP API Force GC](/nomad/api-docs/system#force-gc)
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@@ -2393,6 +2393,10 @@
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{
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"title": "IPv6 Support",
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"path": "operations/ipv6-support"
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},
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{
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"title": "Garbage Collection",
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"path": "operations/garbage-collection"
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}
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]
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},
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