[[searchable-snapshots]] == {search-snaps-cap} {search-snaps-cap} let you use <> to search infrequently accessed and read-only data in a very cost-effective fashion. The <> and <> data tiers use {search-snaps} to reduce your storage and operating costs. {search-snaps-cap} eliminate the need for <>, potentially halving the local storage needed to search your data. {search-snaps-cap} rely on the same snapshot mechanism you already use for backups and have minimal impact on your snapshot repository storage costs. [discrete] [[using-searchable-snapshots]] === Using {search-snaps} Searching a {search-snap} index is the same as searching any other index. By default, {search-snap} indices have no replicas. The underlying snapshot provides resilience and the query volume is expected to be low enough that a single shard copy will be sufficient. However, if you need to support a higher query volume, you can add replicas by adjusting the `index.number_of_replicas` index setting. If a node fails and {search-snap} shards need to be recovered elsewhere, there is a brief window of time while {es} allocates the shards to other nodes where the cluster health will not be `green`. Searches that hit these shards may fail or return partial results until the shards are reallocated to healthy nodes. You typically manage {search-snaps} through {ilm-init}. The <> action automatically converts a regular index into a {search-snap} index when it reaches the `cold` or `frozen` phase. You can also make indices in existing snapshots searchable by manually mounting them using the <> API. To mount an index from a snapshot that contains multiple indices, we recommend creating a <> of the snapshot that contains only the index you want to search, and mounting the clone. You should not delete a snapshot if it has any mounted indices, so creating a clone enables you to manage the lifecycle of the backup snapshot independently of any {search-snaps}. If you use {ilm-init} to manage your {search-snaps} then it will automatically look after cloning the snapshot as needed. You can control the allocation of the shards of {search-snap} indices using the same mechanisms as for regular indices. For example, you could use <> to restrict {search-snap} shards to a subset of your nodes. The speed of recovery of a {search-snap} index is limited by the repository setting `max_restore_bytes_per_sec` and the node setting `indices.recovery.max_bytes_per_sec` just like a normal restore operation. By default `max_restore_bytes_per_sec` is unlimited, but the default for `indices.recovery.max_bytes_per_sec` depends on the configuration of the node. See <>. We recommend that you <> indices to a single segment per shard before taking a snapshot that will be mounted as a {search-snap} index. Each read from a snapshot repository takes time and costs money, and the fewer segments there are the fewer reads are needed to restore the snapshot or to respond to a search. [TIP] ==== {search-snaps-cap} are ideal for managing a large archive of historical data. Historical information is typically searched less frequently than recent data and therefore may not need replicas for their performance benefits. For more complex or time-consuming searches, you can use <> with {search-snaps}. ==== [[searchable-snapshots-repository-types]] // tag::searchable-snapshot-repo-types[] Use any of the following repository types with searchable snapshots: * {plugins}/repository-s3.html[AWS S3] * {plugins}/repository-gcs.html[Google Cloud Storage] * {plugins}/repository-azure.html[Azure Blob Storage] * {plugins}/repository-hdfs.html[Hadoop Distributed File Store (HDFS)] * <> such as NFS You can also use alternative implementations of these repository types, for instance {plugins}/repository-s3-client.html#repository-s3-compatible-services[Minio], as long as they are fully compatible. Use the <> API to analyze your repository's suitability for use with searchable snapshots. // end::searchable-snapshot-repo-types[] [discrete] [[how-searchable-snapshots-work]] === How {search-snaps} work When an index is mounted from a snapshot, {es} allocates its shards to data nodes within the cluster. The data nodes then automatically retrieve the relevant shard data from the repository onto local storage, based on the <> specified. If possible, searches use data from local storage. If the data is not available locally, {es} downloads the data that it needs from the snapshot repository. If a node holding one of these shards fails, {es} automatically allocates the affected shards on another node, and that node restores the relevant shard data from the repository. No replicas are needed, and no complicated monitoring or orchestration is necessary to restore lost shards. Although searchable snapshot indices have no replicas by default, you may add replicas to these indices by adjusting `index.number_of_replicas`. Replicas of {search-snap} shards are recovered by copying data from the snapshot repository, just like primaries of {search-snap} shards. In contrast, replicas of regular indices are restored by copying data from the primary. [discrete] [[searchable-snapshot-mount-storage-options]] ==== Mount options To search a snapshot, you must first mount it locally as an index. Usually {ilm-init} will do this automatically, but you can also call the <> API yourself. There are two options for mounting a snapshot, each with different performance characteristics and local storage footprints: [[full-copy]] Full copy:: Loads a full copy of the snapshotted index's shards onto node-local storage within the cluster. This is the default mount option. {ilm-init} uses this option by default in the `hot` and `cold` phases. + Search performance for a full-copy searchable snapshot index is normally comparable to a regular index, since there is minimal need to access the snapshot repository. While recovery is ongoing, search performance may be slower than with a regular index because a search may need some data that has not yet been retrieved into the local copy. If that happens, {es} will eagerly retrieve the data needed to complete the search in parallel with the ongoing recovery. [[shared-cache]] Shared cache:: + experimental::[] + Uses a local cache containing only recently searched parts of the snapshotted index's data. {ilm-init} uses this option by default in the `frozen` phase and corresponding frozen tier. + If a search requires data that is not in the cache, {es} fetches the missing data from the snapshot repository. Searches that require these fetches are slower, but the fetched data is stored in the cache so that similar searches can be served more quickly in future. {es} will evict infrequently used data from the cache to free up space. + Although slower than a full local copy or a regular index, a shared-cache searchable snapshot index still returns search results quickly, even for large data sets, because the layout of data in the repository is heavily optimized for search. Many searches will need to retrieve only a small subset of the total shard data before returning results. To mount a searchable snapshot index with the shared cache mount option, you must configure the `xpack.searchable.snapshot.shared_cache.size` setting to reserve space for the cache on one or more nodes. Indices mounted with the shared cache mount option are only allocated to nodes that have this setting configured. [[searchable-snapshots-shared-cache]] `xpack.searchable.snapshot.shared_cache.size`:: (<>, <>) The size of the space reserved for the shared cache. Defaults to `0b`, meaning that the node has no shared cache. You can configure the setting in `elasticsearch.yml`: [source,yaml] ---- xpack.searchable.snapshot.shared_cache.size: 4TB ---- IMPORTANT: You can only configure this setting on nodes with the <> role. You can set `xpack.searchable.snapshot.shared_cache.size` to any size between a couple of gigabytes up to 90% of available disk space. We only recommend larger sizes if you use the node exclusively on a frozen tier or for searchable snapshots. [discrete] [[back-up-restore-searchable-snapshots]] === Back up and restore {search-snaps} You can use <> to back up a cluster containing {search-snap} indices. When you restore a snapshot containing {search-snap} indices, these indices are restored as {search-snap} indices again. Before you restore a snapshot containing a {search-snap} index, you must first <> containing the original index snapshot. When restored, the {search-snap} index mounts the original index snapshot from its original repository. If wanted, you can use separate repositories for regular snapshots and {search-snaps}. A snapshot of a {search-snap} index contains only a small amount of metadata which identifies its original index snapshot. It does not contain any data from the original index. The restore of a backup will fail to restore any {search-snap} indices whose original index snapshot is unavailable. [discrete] [[searchable-snapshots-reliability]] === Reliability of {search-snaps} The sole copy of the data in a {search-snap} index is the underlying snapshot, stored in the repository. If the repository fails or corrupts the contents of the snapshot then the data is lost. Although {es} may have made copies of the data onto local storage, these copies may be incomplete and cannot be used to recover any data after a repository failure. You must make sure that your repository is reliable and protects against corruption of your data while it is at rest in the repository. The blob storage offered by all major public cloud providers typically offers very good protection against data loss or corruption. If you manage your own repository storage then you are responsible for its reliability. [discrete] [[searchable-snapshots-frozen-tier-on-cloud]] === Configure a frozen tier on {ess} The frozen data tier is not yet available on {ess-trial}[{ess}]. However, you can configure another tier to use <>. This effectively recreates a frozen tier in your deployment. See <>.