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IMPORTANT: No additional bug fixes or documentation updates
will be released for this version. For the latest information, see the
current release documentation.
Prevents write operations to this index while still allowing metadata
changes like deleting the index.
The current write index on a data stream cannot be split. In order to split
the current write index, the data stream must first be
rolled over so that a new write index is created
and then the previous write index can be split.
Descriptionedit
The split index API allows you to split an existing index into a new index,
where each original primary shard is split into two or more primary shards in
the new index.
The number of times the index can be split (and the number of shards that each
original shard can be split into) is determined by the
index.number_of_routing_shards setting. The number of routing shards
specifies the hashing space that is used internally to distribute documents
across shards with consistent hashing. For instance, a 5 shard index with
number_of_routing_shards set to 30 (5 x 2 x 3) could be split by a
factor of 2 or 3. In other words, it could be split as follows:
index.number_of_routing_shards is a static indexsetting. You can only set index.number_of_routing_shards at index creation
time or on a closed index.
Index creation example
The following create index API creates the
my-index-000001 index with an index.number_of_routing_shards setting of 30.
PUT /my-index-000001
"settings": {
"index": {
"number_of_routing_shards": 30
}
The index.number_of_routing_shards setting s default value depends
on the number of primary shards in the original index.
The default is designed to allow you to split
by factors of 2 up to a maximum of 1024 shards. However, the original number
of primary shards must taken into account. For instance, an index created
with 5 primary shards could be split into 10, 20, 40, 80, 160, 320, or a
maximum of 640 shards (with a single split action or multiple split actions).
If the original index contains one primary shard (or a multi-shard index has
been shrunk down to a single primary shard), then the
index may by split into an arbitrary number of shards greater than 1. The
properties of the default number of routing shards will then apply to the
newly split index.
Creates a new target index with the same definition as the source
index, but with a larger number of primary shards.
Hard-links segments from the source index into the target index. (If
the file system doesn t support hard-linking, then all segments are copied
into the new index, which is a much more time consuming process.)
Hashes all documents again, after low level files are created, to delete
documents that belong to a different shard.
Recovers the target index as though it were a closed index which
had just been re-opened.
Why doesn t Elasticsearch support incremental resharding?edit
Going from N shards to N+1 shards, aka. incremental resharding, is indeed a
feature that is supported by many key-value stores. Adding a new shard and
pushing new data to this new shard only is not an option: this would likely be
an indexing bottleneck, and figuring out which shard a document belongs to
given its _id, which is necessary for get, delete and update requests, would
become quite complex. This means that we need to rebalance existing data using
a different hashing scheme.
The most common way that key-value stores do this efficiently is by using
consistent hashing. Consistent hashing only requires 1/N-th of the keys to
be relocated when growing the number of shards from N to N+1. However
Elasticsearch s unit of storage, shards, are Lucene indices. Because of their
search-oriented data structure, taking a significant portion of a Lucene index,
be it only 5% of documents, deleting them and indexing them on another shard
typically comes with a much higher cost than with a key-value store. This cost
is kept reasonable when growing the number of shards by a multiplicative factor
as described in the above section: this allows Elasticsearch to perform the
split locally, which in-turn allows to perform the split at the index level
rather than reindexing documents that need to move, as well as using hard links
for efficient file copying.
In the case of append-only data, it is possible to get more flexibility by
creating a new index and pushing new data to it, while adding an alias that
covers both the old and the new index for read operations. Assuming that the
old and new indices have respectively M and N shards, this has no overhead
compared to searching an index that would have M+N shards.
Split an indexedit
To split my_source_index into a new index called my_target_index, issue
the following request:
POST /my_source_index/_split/my_target_index
"settings": {
"index.number_of_shards": 2
}
The above request returns immediately once the target index has been added to
the cluster state it doesn t wait for the split operation to start.
The number of primary shards in the target index must be a multiple of the
number of primary shards in the source index.
The node handling the split process must have sufficient free disk space to
accommodate a second copy of the existing index.
The _split API is similar to the create index API
and accepts settings and aliases parameters for the target index:
POST /my_source_index/_split/my_target_index
"settings": {
"index.number_of_shards": 5
"aliases": {
"my_search_indices": {}
}
The number of shards in the target index. This must be a multiple of the
number of shards in the source index.
Monitor the split processedit
The split process can be monitored with the _cat recoveryAPI, or the cluster health API can be used to wait
until all primary shards have been allocated by setting the wait_for_status
parameter to yellow.
The _split API returns as soon as the target index has been added to the
cluster state, before any shards have been allocated. At this point, all
shards are in the state unassigned. If, for any reason, the target index
can t be allocated, its primary shard will remain unassigned until it
can be allocated on that node.
Once the primary shard is allocated, it moves to state initializing, and the
split process begins. When the split operation completes, the shard will
become active. At that point, Elasticsearch will try to allocate any
replicas and may decide to relocate the primary shard to another node.
Name of the target index to create.
Index names must meet the following criteria:
Indices prior to 7.0 could contain a colon (:), but that s been deprecated and won t be supported in 7.0+
Cannot be longer than 255 bytes (note it is bytes, so multi-byte characters will count towards the 255 limit faster)
Names starting with . are deprecated, except for hidden indices and internal indices managed by plugins
(Optional, string) The number of shard copies that must be active before
proceeding with the operation. Set to all or any positive integer up
to the total number of shards in the index (number_of_replicas+1).
Default: 1, the primary shard.
See Active shards.
(Optional, time units) Specifies the period of time to wait for
a connection to the master node. If no response is received before the timeout
expires, the request fails and returns an error. Defaults to 30s.
(Optional, time units) Specifies the period of time to wait for
a response. If no response is received before the timeout expires, the request
fails and returns an error. Defaults to 30s.
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