The glance image cache

updated: 2017-06-18 09:18
Contents

The glance image cache

The glance API server can be configured to have an optional local image cache. A local image cache stores a copy of image files, essentially enabling multiple API servers to serve the same image file, resulting in an increase in scalability due to an increased number of endpoints serving an image file.

This local image cache is transparent to the end user. The end user does not know that the glance API is streaming an image file from its local cache or from the actual backend storage system.

Managing the glance image cache

While image files are automatically placed in the image cache on successful requests to GET /images/<IMAGE_ID>, the image cache is not automatically managed.

Configuration options for the image cache

The glance cache uses two files: * One for configuring the server: glance-api.conf * Another for the utilities: glance-cache.conf

The following options are in both configuration files. These need the same values otherwise the cache will potentially run into problems.

  • image_cache_dir: This is the base directory where glance stores the cache data (Required to be set, as does not have a default).
  • image_cache_sqlite_db: Path to the sqlite file database that will be used for cache management. This is a relative path from the image_cache_dir directory (Default:cache.db).
  • image_cache_driver: The driver used for cache management. (Default:sqlite)
  • image_cache_max_size: The size when the glance-cache-pruner removes the oldest images. This reduces the bytes until under this value. (Default:10 GB)
  • image_cache_stall_time: The amount of time an incomplete image stays in the cache. After this the incomplete image will be deleted. (Default:1 day)

The following values are the ones that are specific to the glance-cache.conf and are only required for the prefetcher to run correctly.

  • admin_user: The username for an admin account, this is so it can get the image data into the cache.
  • admin_password: The password to the admin account.
  • admin_tenant_name: The tenant of the admin account.
  • auth_url: The URL used to authenticate to keystone. This will be taken from the environment variables if it exists.
  • filesystem_store_datadir: This is used if using the filesystem store, points to where the data is kept.
  • filesystem_store_datadirs: This is used to point to multiple filesystem stores.
  • registry_host: The URL to the glance registry.

Controlling the growth of the image cache

The image cache has a configurable maximum size (the image_cache_max_size configuration file option). The image_cache_max_size is an upper limit beyond which pruner, if running, starts cleaning the images cache. However, when images are successfully returned from a call to GET /images/<IMAGE_ID>, the image cache automatically writes the image file to its cache, regardless of whether the resulting write would make the image cache’s size exceed the value of image_cache_max_size. In order to keep the image cache at or below this maximum cache size, you need to run the glance-cache-pruner executable.

We recommend using cron to fire glance-cache-pruner at a regular intervals.

Cleaning the image cache

Over time, the image cache can accumulate image files that are either in a stalled or invalid state. Stalled image files are the result of an image cache write failing to complete. Invalid image files are the result of an image file not being written properly to disk.

To remove these types of files, run the glance-cache-cleaner executable.

We recommend using cron to fire glance-cache-cleaner at a semi-regular interval.

Prefetching images into the image cache

Some installations have base (sometimes called “golden”) images that are very commonly used to boot virtual machines. When spinning up a new API server, administrators may wish to prefetch these image files into the local image cache to ensure that reads of those popular image files come from a local cache.

To queue an image for prefetching, you can use one of the following methods:

  • If the cache_manage middleware is enabled in the application pipeline, call PUT /queued-images/<IMAGE_ID> to queue the image with identifier <IMAGE_ID>.

    Alternately, use the glance-cache-manage program to queue the image. This program may be run from a different host than the host containing the image cache. For example:

    $> glance-cache-manage --host=<HOST> queue-image <IMAGE_ID>

    This queues the image with identifier <IMAGE_ID> for prefetching.

Once you have queued the images you wish to prefetch, call the glance-cache-prefetcher executable. This prefetches all queued images concurrently, logging the results of the fetch for each image.

Finding images in the image cache

You can sources images in the image cache using one of the following methods:

  • If the cachemanage middleware is enabled in the application pipeline, call GET /cached-images to see a JSON-serialized list of mappings that show cached images, the number of cache hits on each image, the size of the image, and the times they were last accessed.

    Alternately, you can use the glance-cache-manage program. This program may be run from a different host than the host containing the image cache. For example:

    $> glance-cache-manage --host=<HOST> list-cached

  • You can issue the following call on \*nix systems (on the host that contains the image cache):

    $> ls -lhR $IMAGE_CACHE_DIR

    $IMAGE_CACHE_DIR is the value of the image_cache_dir configuration variable.

    Note

    The image’s cache hit is not shown using this method.

Manually removing images from the image cache

If the cachemanage middleware is enabled, you may call DELETE /cached-images/<IMAGE_ID> to remove the image file for image with identifier <IMAGE_ID> from the cache.

Alternately, you can use the glance-cache-manage program. For example:

$> glance-cache-manage --host=<HOST> delete-cached-image <IMAGE_ID>
updated: 2017-06-18 09:18
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