removeShard
removeShardRemoves a shard from a sharded cluster. When you run
removeShard, MongoDB drains the shard by using the balancer to move the shard's chunks to other shards in the cluster. Once the shard is drained, MongoDB removes the shard from the cluster.
Syntax
To run, from a mongos instance, issue the command
against the admin database:
db.adminCommand( { removeShard : <shardToRemove> } )
Behavior
No Cluster Back Ups During Shard Removal
You cannot back up the cluster data during shard removal.
Concurrent removeShard Operations
Starting in MongoDB 4.4 (and 4.2.1), you can have more than one
removeShard operation in progress.
In MongoDB 4.2.0 and earlier, removeShard returns an error
if another removeShard operation is in progress.
Access Requirements
If you have authorization enabled, you must have the
clusterManager role or any role that
includes the removeShard action.
Database Migration Requirements
Each database in a sharded cluster has a primary shard. If the shard you
want to remove is also the primary of one of the cluster's databases, then
you must manually move the databases to a new shard after migrating
all data from the shard. See the movePrimary command and
the Remove Shards from an Existing Sharded Cluster for more information.
Chunk Balancing
When you remove a shard in a cluster with an uneven chunk distribution, the balancer first removes the chunks from the draining shard and then balances the remaining uneven chunk distribution.
Write Concern
mongos converts the
write concern of the
removeShard command to "majority".
Change Streams
A shard removal may cause an open change stream cursor to close, and the closed change stream cursor may not be fully resumable.
Example
From the mongo shell, the removeShard
operation resembles the following:
db.adminCommand( { removeShard : "bristol01" } )
Replace bristol01 with the name of the shard to remove. When you
run removeShard, the command returns with a message that
resembles the following:
{ "msg" : "draining started successfully", "state" : "started", "shard" : "bristol01", "note" : "you need to drop or movePrimary these databases", "dbsToMove" : [ "fizz", "buzz" ], "ok" : 1, "operationTime" : Timestamp(1575398919, 2), "$clusterTime" : { "clusterTime" : Timestamp(1575398919, 2), "signature" : { "hash" : BinData(0,"Oi68poWCFCA7b9kyhIcg+TzaGiA="), "keyId" : NumberLong("6766255701040824328") } } }
The balancer begins migrating ("draining") chunks from the shard named
bristol01 to other shards in the cluster. These migrations happen
slowly in order to avoid placing undue load on the cluster.
The output includes the field dbsToMove indicating the databases
for which bristol01 is the primary shard.
After all chunks have been drained from the shard, you must either
movePrimary for the database(s) or alternatively, drop
these databases.
Note
If the shard you are removing is not the primary shard for any
database, the dbsToMove array will be empty and
removeShard can complete the migration without
intervention.
If you run the command again, removeShard returns the
current status of the process. For example, if the operaton is in an
ongoing state, the command returns an output that resembles the
following:
{ "msg" : "draining ongoing", "state" : "ongoing", "remaining" : { "chunks" : NumberLong(2), "dbs" : NumberLong(2), "jumboChunks" : NumberLong(0) // Available starting in 4.2.2 (and 4.0.14) }, "note" : "you need to drop or movePrimary these databases", "dbsToMove" : [ "fizz", "buzz" ], "ok" : 1, "operationTime" : Timestamp(1575399086, 1655), "$clusterTime" : { "clusterTime" : Timestamp(1575399086, 1655), "signature" : { "hash" : BinData(0,"XBrTmjMMe82fUtVLRm13GBVtRE8="), "keyId" : NumberLong("6766255701040824328") } } }
In the output, the remaining field includes the following fields:
Field | Description |
|---|---|
chunks | Total number of chunks currently remaining on the shard. |
dbs | Total number of databases whose primary shard is the shard. These databases are specified in
the dbsToMove output field. |
jumboChunks | Of the total number of If the After the Available starting in 4.2.2 (and 4.0.14) |
Continue checking the status of the removeShard command
(i.e. rerun the command) until the number of chunks remaining is 0.
{ "msg" : "draining ongoing", "state" : "ongoing", "remaining" : { "chunks" : NumberLong(0), // All chunks have moved "dbs" : NumberLong(2), "jumboChunks" : NumberLong(0) }, "note" : "you need to drop or movePrimary these databases", "dbsToMove" : [ "fizz", "buzz" ], "ok" : 1, "operationTime" : Timestamp(1575400343, 1), "$clusterTime" : { "clusterTime" : Timestamp(1575400343, 1), "signature" : { "hash" : BinData(0,"9plu5B/hw4uWAgEmjjBP3syw1Zk="), "keyId" : NumberLong("6766255701040824328") } } }
After all chunks have been drained from the shard, if you have
dbsToMove, you can either movePrimary for those
databases or alternatively, drop the databases (which deletes the
associated data files).
After the balancer completes moving all chunks off the shard and you
have handled the dbsToMove, removeShard can finish.
Running removeShard again returns output that resembles
the following:
{ "msg" : "removeshard completed successfully", "state" : "completed", "shard" : "bristol01", "ok" : 1, "operationTime" : Timestamp(1575400370, 2), "$clusterTime" : { "clusterTime" : Timestamp(1575400370, 2), "signature" : { "hash" : BinData(0,"JjSRciHECXDBXo0e5nJv9mdRG8M="), "keyId" : NumberLong("6766255701040824328") } } }