$shardedDataDistribution (aggregation)

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Definition

Syntax
Output Fields
Behavior
Examples
Return All Sharded Data Distibution Metrics
Return Metrics for a Specific Shard
Return Metrics for a Namespace
Confirm No Orphaned Documents Remain

Definition

$shardedDataDistribution

New in version 6.0.3.

Returns information on the distribution of data in sharded collections.

Note

This aggregation stage is only available on mongos.

This aggregation stage must be run on the admin database. The user must have the shardedDataDistribution privilege action.

Syntax

The shardedDataDistribution stage has the following syntax:

db.aggregate( [
   { $shardedDataDistribution: { } }
] )

Output Fields

The $shardedDataDistribution stage outputs an array of documents for each sharded collection in the database. These documents contain the following fields:

Field Name	Data Type	Description
`ns`	string	Namespace of the sharded collection.
`shards`	array	Shards in the collection with the data distribution information for each shard.
`shards.numOrphanedDocs`	integer	Number of orphaned documents in the shard.
`shards.numOwnedDocuments`	integer	Number of documents owned by the shard.
`shards.ownedSizeBytes`	integer	Size in bytes of documents owned by the shard when uncompressed.
`shards.orphanedSizeBytes`	integer	Size in bytes of orphaned documents in the shard when uncompressed.

Behavior

After an unclean shutdown of a mongod using the Wired Tiger storage engine, size and count statistics reported by $shardedDataDistribution may be inaccurate.

The amount of drift depends on the number of insert, update, or delete operations performed between the last checkpoint and the unclean shutdown. Checkpoints usually occur every 60 seconds. However, mongod instances running with non-default --syncdelay settings may have more or less frequent checkpoints.

Run validate on each collection on the mongod to restore statistics after an unclean shutdown.

After an unclean shutdown:

validate updates the count statistic in the collStats output with the latest value.
Other statistics like the number of documents inserted or removed in the collStats output are estimates.

Examples

Return All Sharded Data Distibution Metrics

To return all sharded data distribution metrics, run the following:

db.aggregate([
   { $shardedDataDistribution: { } }
])

Example output:

[
  {
    "ns": "test.names",
    "shards": [
      {
        "shardName": "shard-1",
        "numOrphanedDocs": 0,
        "numOwnedDocuments": 6,
        "ownedSizeBytes": 366,
        "orphanedSizeBytes": 0
      },
      {
        "shardName": "shard-2",
        "numOrphanedDocs": 0,
        "numOwnedDocuments": 6,
        "ownedSizeBytes": 366,
        "orphanedSizeBytes": 0
      }
    ]
  }
]

Return Metrics for a Specific Shard

To return sharded data distribution metrics for a specific shard, run the following:

db.aggregate([
   { $shardedDataDistribution: { } },
   { $match: {  "shards.shardName": "<name of the shard>" } }
])

Return Metrics for a Namespace

To return sharded data distribution data for a namespace, run the following:

db.aggregate([
   { $shardedDataDistribution: { } },
   { $match: {  "ns": "<database>.<collection>" } }
])

Confirm No Orphaned Documents Remain

Starting in MongoDB 6.0.3, you can run an aggregation using the $shardedDataDistribution stage to confirm no orphaned documents remain:

db.aggregate([
   { $shardedDataDistribution: { } },
   { $match: { "ns": "<database>.<collection>" } }
])

$shardedDataDistribution has output similar to the following:

[
  {
    "ns": "test.names",
    "shards": [
      {
        "shardName": "shard-1",
        "numOrphanedDocs": 0,
        "numOwnedDocuments": 6,
        "ownedSizeBytes": 366,
        "orphanedSizeBytes": 0
      },
      {
        "shardName": "shard-2",
        "numOrphanedDocs": 0,
        "numOwnedDocuments": 6,
        "ownedSizeBytes": 366,
        "orphanedSizeBytes": 0
      }
    ]
  }
]

Ensure that "numOrphanedDocs" is 0 for each shard in the cluster.

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