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- Configure Behavior of Balancer Process in Sharded Clusters
Configure Behavior of Balancer Process in Sharded Clusters¶
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The balancer is a process that runs on one of the mongos
instances in a cluster and ensures that chunks are
evenly distributed throughout a sharded cluster. In most deployments,
the default balancer configuration is sufficient for normal
operation. However, administrators might need to modify balancer
behavior depending on application or operational requirements. If you
encounter a situation where you need to modify the behavior of the
balancer, use the procedures described in this document.
For conceptual information about the balancer, see Sharded Collection Balancing and Cluster Balancer.
Schedule a Window of Time for Balancing to Occur¶
You can schedule a window of time during which the balancer can migrate chunks, as described in the following procedures:
The mongos
instances use their own local timezones when
respecting balancer window.
Configure Default Chunk Size¶
The default chunk size for a sharded cluster is 64 megabytes. In most situations, the default size is appropriate for splitting and migrating chunks. For information on how chunk size affects deployments, see details, see Chunk Size.
Changing the default chunk size affects chunks that are processes during migrations and auto-splits but does not retroactively affect all chunks.
To configure default chunk size, see Modify Chunk Size in a Sharded Cluster.
Change the Maximum Storage Size for a Given Shard¶
The maxSize
field in the shards
collection in the
config database sets the maximum size for a
shard, allowing you to control whether the balancer will migrate chunks
to a shard. If mapped
size [1] is above a shard’s
maxSize
, the balancer will not move chunks to the shard. Also, the
balancer will not move chunks off an overloaded shard. This must happen
manually. The maxSize
value only affects the balancer’s selection of
destination shards.
By default, maxSize
is not specified, allowing shards to consume the
total amount of available space on their machines if necessary.
You can set maxSize
both when adding a shard and once a shard is
running.
To set maxSize
when adding a shard, set the addShard
command’s maxSize
parameter to the maximum size in megabytes. For
example, the following command run in the mongo
shell adds a
shard with a maximum size of 125 megabytes:
To set maxSize
on an existing shard, insert or update the
maxSize
field in the shards
collection in the
config database. Set the maxSize
in
megabytes.
Example
Assume you have the following shard without a maxSize
field:
Run the following sequence of commands in the mongo
shell
to insert a maxSize
of 125 megabytes:
To later increase the maxSize
setting to 250 megabytes, run the
following:
[1] | This value includes the mapped size of all data
files including the``local`` and admin databases. Account for
this when setting maxSize . |
Change Replication Behavior for Chunk Migration (Secondary Throttle)¶
The _secondaryThrottle
parameter of the balancer and the
moveChunk
command affects the replication behavior during
chunk migration. By default,
_secondaryThrottle
is true
, which means each document move
during chunk migration propagates to at least one secondary before the
balancer proceeds with its next operation. For more information on the
replication behavior during various steps of chunk migration, see
Chunk Migration and Replication.
To change the balancer’s _secondaryThrottle
value, connect to a
mongos
instance and directly update the
_secondaryThrottle
value in the settings
collection
of the config database. For example, from a
mongo
shell connected to a mongos
, issue the
following command:
The effects of changing the _secondaryThrottle
value may not be
immediate. To ensure an immediate effect, stop and restart the balancer
to enable the selected value of _secondaryThrottle
. See
Manage Sharded Cluster Balancer for details.