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- Remove Shards from an Existing Sharded Cluster
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 safely remove a single shard. Do not use this procedure to migrate an entire cluster to new hardware. To migrate an entire shard to new hardware, migrate individual shards as if they were independent replica sets.
To remove a shard, first connect to one of the cluster’s
mongos
instances using mongo
shell. 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 the mongo
shell.
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 the mongo
shell and either:
Use the
listShards
command, as in the following: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:
This operation returns immediately, with the following response:
The balancer begins migrating chunks from the shard named bristol01
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
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,
removeShard
lists the database in the dbsToMove
field.
To finish removing the shard, you must either move the database
to a new shard after migrating all data from the shard or drop the
database, deleting 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:
The command returns output similar to the following:
In the output, the remaining
document displays the remaining number
of chunks that MongoDB must migrate to other shards and the number of
MongoDB databases that have “primary” status on this shard.
Continue checking the status of the removeShard command until the
number of chunks remaining is 0
. Always run the command on the
admin
database. If you are on a database other than admin
, you can
use sh._adminCommand
to run the command on admin
.
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:
This command does not return until MongoDB completes moving all data. The response from this command will resemble the following:
Warning
If you use the movePrimary
command to move un-sharded
collections, you must either restart all mongos
instances,
or use the flushRouterConfig
command on all
mongos
instances before reading or writing any data to any
unsharded collections that were moved. This action ensures that the
mongos
is aware of the new shard for these collections.
If you do not update the mongos
instances’ metadata cache
after using movePrimary
, the mongos
may miss data
on reads, and may not write data to the correct shard. To recover, you must
manually intervene.
Finalize the Migration¶
To clean up all metadata information and finalize the removal, run
removeShard
again. For example, for a shard named
mongodb0
, run:
A success message appears at completion:
Once the value of the state
field is “completed”, you may safely
stop the instances comprising the mongodb0
shard.