.. Copyright 2011 OpenStack Foundation All Rights Reserved. Licensed 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. The Glance Image Cache ====================== The Glance API server may 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 -- in other words, the end user doesn't 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/``, the image cache is not automatically managed. Here, we describe the basics of how to manage the local image cache on Glance API servers and how to automate this cache management. Configuration options for the Image Cache ----------------------------------------- The cache has a number of configuration options that are in the configuration files. - ``image_cache_dir`` This is the base directory where Glance stores the cache data. - ``image_cache_sqlite_db`` Path to the sqlite file database that will be used for cache manangement. - ``image_cache_driver`` The driver used for cache management. (Likely sqlite.) - ``image_cache_max_size`` The size when the glance-cache-pruner will remove the oldest images, to reduce the bytes until under this value. - ``image_cache_stall_time`` The amount of time an incomplete image will stay in the cache, after this the incomplete image will be deleted. 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/``, 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. The recommended practice is to use ``cron`` to fire ``glance-cache-pruner`` at a regular interval. 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, you run the ``glance-cache-cleaner`` executable. The recommended practice is to use ``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, you may call ``PUT /queued-images/`` to queue the image with identifier ```` Alternately, you can 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. Example usage:: $> glance-cache-manage --host= queue-image This will queue the image with identifier ```` for prefetching Once you have queued the images you wish to prefetch, call the ``glance-cache-prefetcher`` executable, which will prefetch all queued images concurrently, logging the results of the fetch for each image. Finding Which Images are in the Image Cache ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ You can find out which images are in the image cache using one of the following methods: * If the ``cachemanage`` middleware is enabled in the application pipeline, you may 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. Example usage:: $> glance-cache-manage --host= list-cached * You can issue the following call on \*nix systems (on the host that contains the image cache):: $> ls -lhR $IMAGE_CACHE_DIR where ``$IMAGE_CACHE_DIR`` is the value of the ``image_cache_dir`` configuration variable. Note that 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/`` to remove the image file for image with identifier ```` from the cache. Alternately, you can use the ``glance-cache-manage`` program. Example usage:: $> glance-cache-manage --host= delete-cached-image