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Shard a Collection Using a Hashed Shard Key¶
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New in version 2.4.
Hashed shard keys use a hashed index of a field as the shard key to partition data across your sharded cluster.
For suggestions on choosing the right field as your hashed shard key, see Hashed Shard Keys. For limitations on hashed indexes, see Create a Hashed Index.
Note
If chunk migrations are in progress while creating a hashed shard key collection, the initial chunk distribution may be uneven until the balancer automatically balances the collection.
Shard the Collection¶
To shard a collection using a hashed shard key, use an operation in the mongo that resembles the following:
sh.shardCollection( "records.active", { a: "hashed" } )
This operation shards the active collection in the records database, using a hash of the a field as the shard key.
Specify the Initial Number of Chunks¶
If you shard an empty collection using a hashed shard key, MongoDB automatically creates and migrates empty chunks so that each shard has two chunks. To control how many chunks MongoDB creates when sharding the collection, use shardCollection with the numInitialChunks parameter.
Important
MongoDB 2.4 adds support for hashed shard keys. After sharding a collection with a hashed shard key, you must use the MongoDB 2.4 or higher mongos and mongod instances in your sharded cluster.
Warning
MongoDB hashed indexes truncate floating point numbers to 64-bit integers before hashing. For example, a hashed index would store the same value for a field that held a value of 2.3, 2.2, and 2.9. To prevent collisions, do not use a hashed index for floating point numbers that cannot be reliably converted to 64-bit integers (and then back to floating point). MongoDB hashed indexes do not support floating point values larger than 253.
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