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Data Partitioning with Chunks

Data Partitioning with Chunks¶

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MongoDB uses the shard key associated to the collection to partition the data into chunks. A chunk consists of a subset of sharded data. Each chunk has a inclusive lower and exclusive upper range based on the shard key.

Diagram of the shard key value space segmented into smaller ranges or chunks.

The mongos routes writes to the appropriate chunk based on the shard key value. MongoDB splits chunks when they grow beyond the configured chunk size. Both inserts and updates can trigger a chunk split.

The smallest range a chunk can represent is a single unique shard key value. A chunk that only contains documents with a single shard key value cannot be split.

Chunk Size¶

The default chunk size in MongoDB is 64 megabytes. You can increase or reduce the chunk size. Consider the implications of changing the default chunk size:

Small chunks lead to a more even distribution of data at the expense of more frequent migrations. This creates expense at the query routing (mongos) layer.
Large chunks lead to fewer migrations. This is more efficient both from the networking perspective and in terms of internal overhead at the query routing layer. But, these efficiencies come at the expense of a potentially uneven distribution of data.
Chunk size affects the Maximum Number of Documents Per Chunk to Migrate.
Chunk size affects the maximum collection size when sharding an existing collection. Post-sharding, chunk size does not constrain collection size.

For many deployments, it makes sense to avoid frequent and potentially spurious migrations at the expense of a slightly less evenly distributed data set.

Limitations¶

Changing the chunk size affects when chunks split but there are some limitations to its effects.

Automatic splitting only occurs during inserts or updates. If you lower the chunk size, it may take time for all chunks to split to the new size.
Splits cannot be “undone”. If you increase the chunk size, existing chunks must grow through inserts or updates until they reach the new size.

Chunk Splits¶

Splitting is a process that keeps chunks from growing too large. When a chunk grows beyond a specified chunk size, or if the number of documents in the chunk exceeds Maximum Number of Documents Per Chunk to Migrate, MongoDB splits the chunk based on the shard key values the chunk represent. A chunk may be split into multiple chunks where necessary. Inserts and updates may trigger splits. Splits are an efficient meta-data change. To create splits, MongoDB does not migrate any data or affect the shards.

Diagram of a shard with a chunk that exceeds the default chunk size of 64 MB and triggers a split of the chunk into two chunks.

Splits may lead to an uneven distribution of the chunks for a collection across the shards. In such cases, the balancer redistributes chunks across shards. See Cluster Balancer for more details on balancing chunks across shards.

Chunk Migration¶

MongoDB migrates chunks in a sharded cluster to distribute the chunks of a sharded collection evenly among shards. Migrations may be either:

Manual. Only use manual migration in limited cases, such as to distribute data during bulk inserts. See Migrating Chunks Manually for more details.
Automatic. The balancer process automatically migrates chunks when there is an uneven distribution of a sharded collection’s chunks across the shards. See Migration Thresholds for more details.

For more information on the sharded cluster balancer, see Sharded Cluster Balancer.

Balancing¶

The balancer is a background process that manages chunk migrations. If the difference in number of chunks between the largest and smallest shard exceed the migration thresholds, the balancer begins migrating chunks across the cluster to ensure an even distribution of data.

Diagram of a collection distributed across three shards. For this collection, the difference in the number of chunks between the shards reaches the *migration thresholds* (in this case, 2) and triggers migration.

You can manage certain aspects of the balancer. The balancer also respects any zones created as a part of configuring zones in a sharded cluster.

See Sharded Cluster Balancer for more information on the balancer.

Indivisible Chunks¶

In some cases, chunks can grow beyond the specified chunk size but cannot undergo a split. The most common scenario is when a chunk represents a single shard key value. Since the chunk cannot split, it continues to grow beyond the chunk size, becoming a jumbo chunk. These jumbo chunks can become a performance bottleneck as they continue to grow, especially if the shard key value occurs with high frequency.

The addition of new data or new shards can result in data distribution imbalances within the cluster. A particular shard may acquire more chunks than another shard, or the size of a chunk may grow beyond the configured maximum chunk size.

MongoDB ensures a balanced cluster using two processes: chunk splitting and the balancer.

`moveChunk` directory¶

In MongoDB 2.6 and MongoDB 3.0, sharding.archiveMovedChunks is enabled by default. All other MongoDB versions have this disabled by default. With sharding.archiveMovedChunks enabled, the source shard archives the documents in the migrated chunks in a directory named after the collection namespace under the moveChunk directory in the storage.dbPath.

If some error occurs during a migration, these files may be helpful in recovering documents affected during the migration.

Once the migration has completed successfully and there is no need to recover documents from these files, you may safely delete these files. Or, if you have an existing backup of the database that you can use for recovery, you may also delete these files after migration.

To determine if all migrations are complete, run sh.isBalancerRunning() while connected to a mongos instance.

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