Remove Shards from an Existing Sharded Cluster
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To remove a shard you must ensure the shard's data is migrated to the remaining shards in the cluster. This procedure describes how to safely migrate data and how to remove a shard.
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.
This procedure describes how to remove a shard from a cluster. Do not use this procedure to migrate an entire cluster to new hardware. To migrate, see Migrate a Self-Managed Sharded Cluster to Different Hardware instead.
To remove a shard, first connect to one of the cluster's
mongos
instances using mongosh
. Then use
the sequence of tasks in this document to remove a shard from the
cluster.
Considerations
A shard removal may cause an open change stream cursor to close, and the closed change stream cursor may not be fully resumable.
Ensure the Balancer Process is Enabled
To successfully migrate data from a shard, the balancer process
must be enabled. Check the balancer state using the
sh.getBalancerState()
helper in mongosh
.
For more information, see the section on balancer operations.
Determine the Name of the Shard to Remove
To determine the name of the shard, connect to a mongos
instance with mongosh
and either:
Use the
listShards
command, as in the following:db.adminCommand( { listShards: 1 } ) Run either the
sh.status()
or thedb.printShardingStatus()
method.
The shards._id
field lists the name of each shard.
Remove Chunks from the Shard
From the admin
database, run the removeShard
command.
This begins "draining" chunks
from the shard you are removing to other shards in the cluster. For
example, for a shard named mongodb0
, run:
db.adminCommand( { removeShard: "mongodb0" } )
mongos
converts the
write concern of the
removeShard
command to "majority"
.
This operation returns with the following response:
{ "msg" : "draining started successfully", "state" : "started", "shard" : "mongodb0", "note" : "you need to drop or movePrimary these databases", "dbsToMove" : [ "fiz", "buzz" ], "ok" : 1, "operationTime" : Timestamp(1575398919, 2), "$clusterTime" : { "clusterTime" : Timestamp(1575398919, 2), "signature" : { "hash" : BinData(0,"Oi68poWCFCA7b9kyhIcg+TzaGiA="), "keyId" : NumberLong("6766255701040824328") } } }
The balancer begins migrating chunks from the shard named mongodb0
to other shards in the cluster. These migrations happens slowly to
avoid placing undue load on the overall cluster.
Depending on your network capacity and the amount of data, this
operation can take from a few minutes to several days to complete.
Note
The output includes the field dbsToMove
indicating the
databases, if any, for which the shard is the primary shard. After all chunks have been drained from the shard,
you must either movePrimary
for the database(s) or
alternatively, drop the databases (which deletes the associated data
files).
Check the Status of the Migration
To check the progress of the migration at any stage in the process, run
removeShard
from the admin
database again. For example,
for a shard named mongodb0
, run:
db.adminCommand( { removeShard: "mongodb0" } )
mongos
converts the
write concern of the
removeShard
command to "majority"
.
The command returns output similar to the following:
{ "msg" : "draining ongoing", "state" : "ongoing", "remaining" : { "chunks" : NumberLong(2), "dbs" : NumberLong(2), "jumboChunks" : NumberLong(0) }, "note" : "you need to drop or movePrimary these databases", "dbsToMove" : [ "fizz", "buzz" ], "ok" : 1, "operationTime" : Timestamp(1575399086, 1655), "$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 |
---|---|
| Total number of chunks currently remaining on the shard. |
| Total number of databases whose primary shard is the shard. These databases are specified in
the |
| Of the total number of If the After the |
Continue checking the status of the removeShard
command until the
number of chunks remaining is 0
.
db.adminCommand( { removeShard: "mongodb0" } )
Move Databases to Another Primary Shard
If the shard is the primary shard for one or more databases in
the cluster, then you must make that database use a different shard
as its primary shard. removeShard
lists any databases
that you need to move in the dbsToMove
field in the command output.
If the shard is
not the primary shard for any databases, skip to the next task,
Finalize the Migration.
To move a database to another shard, use the movePrimary
command.
Important
To ensure a smooth migration, refer to the considerations in the movePrimary
command
documentation before running movePrimary
.
To migrate the fizz
database from
mongodb0
to mongodb1
, issue the following command:
db.adminCommand( { movePrimary: "fizz", to: "mongodb1" })
mongos
uses "majority"
write concern for
movePrimary
.
This command does not return until MongoDB completes moving all data. The response from this command will resemble the following:
{ "ok" : 1, "operationTime" : Timestamp(1575400369, 9), "$clusterTime" : { "clusterTime" : Timestamp(1575400369, 9), "signature" : { "hash" : BinData(0,"2Nz8QCcVXB0LJLm1hsXfpTCaM0M="), "keyId" : NumberLong("6766255701040824328") } } }
Finalize the Migration
To clean up all metadata information and finalize the removal, run
removeShard
again. For example, for a shard named
mongodb0
, run:
db.adminCommand( { removeShard: "mongodb0" } )
mongos
converts the
write concern of the
removeShard
command to "majority"
.
A success message appears at completion:
{ "msg" : "removeshard completed successfully", "state" : "completed", "shard" : "mongodb0", "ok" : 1, "operationTime" : Timestamp(1575400370, 2), "$clusterTime" : { "clusterTime" : Timestamp(1575400370, 2), "signature" : { "hash" : BinData(0,"JjSRciHECXDBXo0e5nJv9mdRG8M="), "keyId" : NumberLong("6766255701040824328") } } }
Once the value of the state
field is "completed", you may safely
stop the instances comprising the mongodb0
shard.