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collStats

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  • Definition
  • Behavior
  • Example
  • Output

Definition

collStats

The collStats command returns a variety of storage statistics for a given collection.

Tip

In the mongo Shell, this command can also be run through the stats() helper method. Specific fields in the collStats output can be accessed using the dataSize(), estimatedDocumentCount(), isCapped(), latencyStats(), storageSize(), totalIndexSize(), and totalSize() helper methods.

Helper methods are convenient for mongo users, but they may not return the same level of information as database commands. In cases where the convenience is not needed or the additional return fields are required, use the database command.

To run collStats, use the db.runCommand( { <command> } ) method.

The collStats command has the following syntax:

{
   collStats: <string>,
   scale: <int>
}

The command takes the following fields:

Field
Type
Description
collStats
string
The name of the target collection.
scale
int

Optional. The scale factor for the various size data (with the exception of those sizes that specify the unit of measurement in the field name). The value defaults to 1 to return size data in bytes. To display kilobytes rather than bytes, specify a scale value of 1024.

If you specify a non-integer scale factor, MongoDB uses the integer part of the specified factor. For example, if you specify a scale factor of 1023.999, MongoDB uses 1023 as the scale factor.

The scale factor rounds the affected size values to whole numbers.

Starting in version 4.2, the output includes the scaleFactor used to scale the size values.

Note

Starting in version 4.2, MongoDB removes the MMAPv1 storage engine. In conjunction with this change, MongoDB removes the MMAPv1 specific collStats option verbose.

Behavior

Scaled Sizes

Unless otherwise specified by the metric name (such as "bytes currently in the cache"), values related to size are displayed in bytes and can be overridden by scale.

The scale factor rounds the affected size values to whole numbers.

Accuracy after Unexpected Shutdown

After an unclean shutdown of a mongod using the Wired Tiger storage engine, size statistics reported by collStats 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.

In-Progress Indexes

Starting in MongoDB 4.2, the collStats includes information on indexes currently being built. For details, see:

Replica Set Member State Restriction

Starting in MongoDB 4.4, to run on a replica set member, collStats operations require the member to be in PRIMARY or SECONDARY state. If the member is in another state, such as STARTUP2, the operation errors.

In previous versions, the operations also run when the member is in STARTUP2. The operations wait until the member transitioned to RECOVERING.

Non-Existent Collections

If you run collStats for a non-existent collection, then depending on your database implementation, collStats might return 0 values in the output fields instead of returning an error.

For example:

db.runCommand( { collStats : "nonExistentCollection" } )

Example output with 0 values in the fields:

{
   ns: 'test.nonExistentCollection',
   size: 0,
   count: 0,
   ...
}

Example

The following operation runs the collStats command on the restaurants collection, specifying a scale of 1024 bytes:

db.runCommand( { collStats : "restaurants", scale: 1024 } )

The following document provides a representation of the collStats output. Depending on the configuration of your collection and the storage engine, the output fields may vary.

{
  "ns" : <string>,
  "size" : <number>,
  "count" : <number>,
  "avgObjSize" : <number>,
  "storageSize" : <number>,
  "freeStorageSize" : <number>,
  "capped" : <boolean>,
  "max" : <number>,
  "maxSize" :  <number>,
  "wiredTiger" : {
     "metadata" : {
        "formatVersion" : <num>
     },
     "creationString" : <string>
     "type" :  <string>,
     "uri" :  <string>,
     "LSM" : {
        "bloom filter false positives" : <number>,
        "bloom filter hits" : <number>,
        "bloom filter misses" : <number>,
        "bloom filter pages evicted from cache" : <number>,
        "bloom filter pages read into cache" : <number>,
        "bloom filters in the LSM tree" : <number>,
        "total size of bloom filters" : <number>,
        "chunks in the LSM tree" : <number>,
        "highest merge generation in the LSM tree" : <number>,
        "queries that could have benefited from a Bloom filter that did not exist" : <number>,
        "sleep for LSM checkpoint throttle" : <number>,
        "sleep for LSM merge throttle" : <number>
        "total size of bloom filters" : <number>
     },
     "block-manager" : {
         "allocations requiring file extension" : <number>,
         "blocks allocated" : <number>,
         "blocks freed" : <number>,
         "checkpoint size" : <number>,
         "file allocation unit size" : <number>,
         "file bytes available for reuse" : <number>,
         "file magic number" : <number>,
         "file major version number" : <number>,
         "file size in bytes" : <number>,
         "minor version number" : <number>
     },
     "btree" : {
         "btree checkpoint generation" : <number>,
         "column-store fixed-size leaf pages" : <number>,
         "column-store internal pages" : <number>,
         "column-store variable-size RLE encoded values" : <number>,
         "column-store variable-size deleted values" : <number>,
         "column-store variable-size leaf pages" : <number>,
         "fixed-record size" : <number>,
         "maximum internal page key size" : <number>,
         "maximum internal page size" : <number>,
         "maximum leaf page key size" : <number>,
         "maximum leaf page size" : <number>,
         "maximum leaf page value size" : <number>,
         "maximum tree depth" : <number>,
         "number of key/value pairs" : <number>,
         "overflow pages" : <number>,
         "pages rewritten by compaction" : <number>,
         "row-store empty values" : <number>,
         "row-store internal pages" : <number>,
         "row-store leaf pages" : <number>
     },
     "cache" : {
        "bytes currently in the cache" : <number>,
        "bytes dirty in the cache cumulative" : <number>,
        "bytes read into cache" : <number>,
        "bytes written from cache" : <number>,
        "checkpoint blocked page eviction" : <number>,
        "data source pages selected for eviction unable to be evicted" : <number>,
        "eviction walk passes of a file" : <number>,
        "eviction walk target pages histogram - 0-9" : <number>,
        "eviction walk target pages histogram - 10-31" : <number>,
        "eviction walk target pages histogram - 128 and higher" : <number>,
        "eviction walk target pages histogram - 32-63" : <number>,
        "eviction walk target pages histogram - 64-128" : <number>,
        "eviction walks abandoned" : <number>,
        "eviction walks gave up because they restarted their walk twice" : <number>,
        "eviction walks gave up because they saw too many pages and found no candidates" : <number>,
        "eviction walks gave up because they saw too many pages and found too few candidates" : <number>,
        "eviction walks reached end of tree" : <number>,
        "eviction walks started from root of tree" : <number>,
        "eviction walks started from saved location in tree" : <number>,
        "hazard pointer blocked page eviction" : <number>,
        "in-memory page passed criteria to be split" : <number>,
        "in-memory page splits" : <number>,
        "internal pages evicted" : <number>,
        "internal pages split during eviction" : <number>,
        "leaf pages split during eviction" : <number>,
        "modified pages evicted" : <number>,
        "overflow pages read into cache" : <number>,
        "page split during eviction deepened the tree" : <number>,
        "page written requiring cache overflow records" : <number>,
        "pages read into cache" : <number>,
        "pages read into cache after truncate" : <number>,
        "pages read into cache after truncate in prepare state" : <number>,
        "pages read into cache requiring cache overflow entries" : <number>,
        "pages requested from the cache" : <number>,
        "pages seen by eviction walk" : <number>,
        "pages written from cache" : <number>,
        "pages written requiring in-memory restoration" : <number>,
        "tracked dirty bytes in the cache" : <number>,
        "unmodified pages evicted" : <number>
     },
     "cache_walk" : {
        "Average difference between current eviction generation when the page was last considered" : <number>,
        "Average on-disk page image size seen" : <number>,
        "Average time in cache for pages that have been visited by the eviction server" : <number>,
        "Average time in cache for pages that have not been visited by the eviction server" : <number>,
        "Clean pages currently in cache" : <number>,
        "Current eviction generation" : <number>,
        "Dirty pages currently in cache" : <number>,
        "Entries in the root page" : <number>,
        "Internal pages currently in cache" : <number>,
        "Leaf pages currently in cache" : <number>,
        "Maximum difference between current eviction generation when the page was last considered" : <number>,
        "Maximum page size seen" : <number>,
        "Minimum on-disk page image size seen" : <number>,
        "Number of pages never visited by eviction server" : <number>,
        "On-disk page image sizes smaller than a single allocation unit" : <number>,
        "Pages created in memory and never written" : <number>,
        "Pages currently queued for eviction" : <number>,
        "Pages that could not be queued for eviction" : <number>,
        "Refs skipped during cache traversal" : <number>,
        "Size of the root page" : <number>,
        "Total number of pages currently in cache" : <number>
     },
     "compression" : {
        "compressed page maximum internal page size prior to compression" : <number>,
        "compressed page maximum leaf page size prior to compression " : <number>,
        "compressed pages read" : <number>,
        "compressed pages written" : <number>,
        "page written failed to compress" : <number>,
        "page written was too small to compress" : 1
     },
     "cursor" : {
        "bulk loaded cursor insert calls" : <number>,
        "cache cursors reuse count" : <number>,
        "close calls that result in cache" : <number>,
        "create calls" : <number>,
        "insert calls" : <number>,
        "insert key and value bytes" : <number>,
        "modify" : <number>,
        "modify key and value bytes affected" : <number>,
        "modify value bytes modified" : <number>,
        "next calls" : <number>,
        "open cursor count" : <number>,
        "operation restarted" : <number>,
        "prev calls" : <number>,
        "remove calls" : <number>,
        "remove key bytes removed" : <number>,
        "reserve calls" : <number>,
        "reset calls" : <number>,
        "search calls" : <number>,
        "search near calls" : <number>,
        "truncate calls" : <number>,
        "update calls" : <number>,
        "update key and value bytes" : <number>,
        "update value size change" : <num>
     },
     "reconciliation" : {
        "dictionary matches" : <number>,
        "fast-path pages deleted" : <number>,
        "internal page key bytes discarded using suffix compression" : <number>,
        "internal page multi-block writes" : <number>,
        "internal-page overflow keys" : <number>,
        "leaf page key bytes discarded using prefix compression" : <number>,
        "leaf page multi-block writes" : <number>,
        "leaf-page overflow keys" : <number>,
        "maximum blocks required for a page" : <number>,
        "overflow values written" : <number>,
        "page checksum matches" : <number>,
        "page reconciliation calls" : <number>,
        "page reconciliation calls for eviction" : <number>,
        "pages deleted" : <number>
     },
     "session" : {
        "object compaction" : <number>,
     },
     "transaction" : {
        "update conflicts" : <number>
     }
  },
  "nindexes" : <number>,
  "indexDetails" : {
     "_id_" : {
        "metadata" : {
           "formatVersion" : <number>
        },
        ...
     },
     ...
  },
  "indexBuilds" : [            // Available starting in MongoDB 4.2
     <string>,
  ],
  "totalIndexSize" : <number>,
  "totalSize" : <number>,      // Available starting in MongoDB 4.4
  "indexSizes" : {
          "_id_" : <number>,
          "<indexName>" : <number>,
          ...
  },
  "scaleFactor" : <number> // Available starting in MongoDB 4.2
  "ok" : <number>
}

Output

collStats.ns

The namespace of the current collection, which follows the format [database].[collection].

collStats.size

The total uncompressed size in memory of all records in a collection. The size does not include the size of any indexes associated with the collection, which the totalIndexSize field reports.

The scale argument affects this value. Data compression does not affect this value.

collStats.count

The number of objects or documents in this collection.

collStats.avgObjSize

The average size of an object in the collection. The scale argument does not affect this value.

collStats.storageSize

The total amount of storage allocated to this collection for document storage. The scale argument affects this value.

If collection data is compressed (which is the default for WiredTiger), the storage size reflects the compressed size and may be smaller than the value for collStats.size.

storageSize does not include index size. See totalIndexSize for index sizing.

collStats.freeStorageSize

Unavailable for the In-Memory Storage Engine

The amount of storage available for reuse. The scale argument affects this value.

The field is only available if storage is available for reuse (i.e. greater than zero).

New in version 4.4.

collStats.nindexes

The number of indexes on the collection. All collections have at least one index on the _id field.

Starting in MongoDB 4.2, nindexes includes in its count those indexes currently being built.

collStats.indexDetails

A document that reports data from the WiredTiger storage engine for each index in the collection. Other storage engines will return an empty document.

The fields in this document are the names of the indexes, while the values themselves are documents that contain statistics for the index provided by the storage engine. These statistics are for internal diagnostic use.

Starting in MongoDB 4.2, indexDetails includes details on indexes currently being built.

collStats.indexBuilds

An array that contains the names of the indexes that are currently being built on the collection. Once an index build completes, the index does not appear in the indexBuilds.

New in version 4.2.

collStats.totalIndexSize

The total size of all indexes. The scale argument affects this value.

If an index uses prefix compression (which is the default for WiredTiger), the returned size reflects the compressed size for any such indexes when calculating the total.

Starting in MongoDB 4.2, totalIndexSize includes in its total the size of those indexes currently being built.

collStats.totalSize

The sum of the storageSize and totalIndexSize. The scale argument affects this value.

New in version 4.4.

collStats.indexSizes

This field specifies the key and size of every existing index on the collection. The scale argument affects this value.

If an index uses prefix compression (which is the default for WiredTiger), the returned size reflects the compressed size.

Starting in MongoDB 4.2, indexSizes includes sizes of indexes currently being built.

collStats.scaleFactor

The scale value used by the command.

If you had specified a non-integer scale factor, MongoDB uses the integer part of the specified factor. For example, if you specify a scale factor of 1023.999, MongoDB uses 1023 as the scale factor.

New in version 4.2.

collStats.capped

This field will be "true" if the collection is capped.

collStats.max

Shows the maximum number of documents that may be present in a capped collection.

collStats.maxSize

Shows the maximum size of a capped collection.

collStats.wiredTiger

wiredTiger only appears when using the WiredTiger storage engine.

This document contains data reported directly by the WiredTiger engine and other data for internal diagnostic use.

collStats.inMemory

inMemory only appears when using the in-memory storage engine.

This document contains data reported directly by the storage engine and other data for internal diagnostic use.

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