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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 user their own local timezones to 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 . |
Require Replication During Chunk Migration (Secondary Throttle)¶
New in version 2.2.1: _secondaryThrottle
became an option to the balancer and to
moveChunk
in 2.2.1. _secondaryThrottle
makes it
possible to require the balancer to wait for replication to
secondaries for all documents during migrations.
Changed in version 2.4: _secondaryThrottle
became the default mode for all balancer and
moveChunk
operations.
Before 2.2.1, the write operations required to migrate chunks between shards do not need to replicate to secondaries in order to succeed. However, you can configure the balancer to require write operations during the migration to replicate to secondaries. This throttles or slows the migration process and in doing so reduces the potential impact of migrations on a sharded cluster.
You can throttle migrations by enabling the balancer’s
_secondaryThrottle
parameter. When enabled, secondary throttle
requires a { w : 2 }
write concern on delete and insertion
operations, so that every operation propagates to at least one
secondary before the balancer issues the next operation.
Starting with version 2.4 the default secondaryThrottle value is
true
. To revert to previous behavior, set _secondaryThrottle
to false
.
You enable or disable _secondaryThrottle
directly in the
settings
collection in the config database by running the following commands from a
mongo
shell, connected to a mongos
instance:
You also can enable secondary throttle when issuing the
moveChunk
command by setting _secondaryThrottle
to
true
. For more information, see moveChunk
.