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Document ephemeral storage (tech preview in 3.10)
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| [[architecture-additional-concepts-ephemeral-storage]] | ||
| = Ephemeral Local Storage | ||
| {product-author} | ||
| {product-version} | ||
| :data-uri: | ||
| :icons: | ||
| :experimental: | ||
| :toc: macro | ||
| :toc-title: | ||
| :prewrap!: | ||
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| toc::[] | ||
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| *Note: this only applies if the ephemeral-storage tech preview in | ||
| {product-title} 3.10 has been enabled. This is disabled by default; | ||
| to enable this feature, please see | ||
| xref:../../install_config/configuring_ephemeral.adoc[Configuring for | ||
| Ephemeral Storage].* | ||
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| == Overview | ||
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| In addition to persistent storage, pods and containers may require | ||
| ephemeral (transient) local storage for their operation. The lifetime | ||
| of this ephemeral storage does not extend beyond the life of the | ||
| individual pod, and this ephemeral storage cannot be shared across | ||
| pods. | ||
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| Prior to {product-title} 3.10, ephemeral local storage was exposed to | ||
| pods via the container’s writable layer, logs directory, and EmptyDir | ||
| volumes. Pods use ephemeral local storage for scratch space, caching | ||
| and logs. There are many issues related to the lack of local storage | ||
| accounting and isolation, including: | ||
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| - Pods do not know how much local storage is available to them. | ||
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| - Pods cannot request "guaranteed" local storage. | ||
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| - Local storage is a "best effort" resource. | ||
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| - Pods can get evicted due to other pods filling up the local storage, | ||
| after which no new pods will be admitted until sufficient storage | ||
| has been reclaimed. | ||
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| Unlike persistent volumes, ephemeral storage is unstructured and | ||
| shared (the space, not the actual data) between all pods running on a | ||
| node, in addition to other uses by the system, the container runtime, | ||
| and {product-title}. The ephemeral storage framework allows pods to | ||
| specify their transient local storage needs, and {product-title} to | ||
| schedule pods where appropriate and protect the node against excessive | ||
| use of local storage. | ||
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| While the ephemeral storage framework allows administrators and | ||
| developers to better manage this local storage, it does not provide | ||
| any promises related to I/O throughput and latency. | ||
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| == Types of Ephemeral Storage | ||
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| Ephemeral local storage is always made available in the primary | ||
| partition. There are two basic ways of creating the primary | ||
| partition. | ||
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| === Root | ||
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| This partition holds the kubelet’s root directory (`/var/lib/origin` | ||
| by default) and `/var/log` directory. This partition may be shared | ||
| between user pods, OS and Kubernetes system daemons. This partition | ||
| can be consumed by pods via EmptyDir volumes, container logs, image | ||
| layers and container writable layers. Kubelet will manage shared | ||
| access and isolation of this partition. This partition is "ephemeral" | ||
| and applications cannot expect any performance SLAs (Disk IOPS for | ||
| example) from this partition. | ||
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| === Runtime | ||
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| This is an optional partition which runtimes can use for overlay | ||
| filesystems. {product-title} will attempt to identify and provide | ||
| shared access along with isolation to this partition. Container image | ||
| layers and writable layers are stored here. If the runtime partition | ||
| exists, `root` partition will not hold any image layer or other writable storage. | ||
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| _Note that when using devicemapper to provide runtime storage, | ||
| containers' copy-on-write layer is not accounted for in ephemeral | ||
| storage management. Using Overlay storage will allow this use of | ||
| ephemeral storage to be monitored. |
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