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collMod makes it possible to add flags to a collection to modify the behavior of MongoDB. Flags include usePowerOf2Sizes and index. The command takes the following prototype form:

db.runCommand( {"collMod" : <collection> , "<flag>" : <value> } )

In this command substitute <collection> with the name of a collection in the current database, and <flag> and <value> with the flag and value you want to set.

Use the userFlags field in the db.collection.stats() output to check enabled collection flags.


TTL Collection Expiration Time


The index flag changes the expiration time of a TTL Collection.

Specify the key and new expiration time with a document of the form:

{keyPattern: <index_spec>, expireAfterSeconds: <seconds> }

In this example, <index_spec> is an existing index in the collection and seconds is the number of seconds to subtract from the current time.

On success collMod returns a document with fields expireAfterSeconds_old and expireAfterSeconds_new set to their respective values.

On failure, collMod returns a document with no expireAfterSeconds field to update if there is no existing expireAfterSeconds field or cannot find index { **key**: 1.0 } for ns **namespace** if the specified keyPattern does not exist.

Record Allocation

Disable All Record Padding


New in version 3.0.0.

noPadding flag is available for the MMAPv1 storage engine only.

noPadding disables record padding for the collection. Without padding, the record allocation size corresponds to the document size, and any updates that results in document growth will require a new allocation.


Only set noPadding to true for collections whose workloads have no update operations that cause documents to grow, such as for collections with workloads that are insert-only. For more information, see No Padding Allocation Strategy.

Powers of Two Record Allocation


Deprecated since version 3.0.0.

usePowerOf2Sizes flag is available for the MMAPv1 storage engine only.

usePowerOf2Sizes has no effect on the allocation strategy. MongoDB 3.0 uses the power of 2 allocation as the default record allocation strategy for MMAPv1.

To disable the power of 2 allocation for a collection, use the collMod command with the noPadding flag or the db.createCollection() method with the noPadding option. For more information, see MMAPv1 Record Allocation Behavior Changes.

Example: Change Expiration Value for Indexes

To update the expiration value for a collection named sessions indexed on a lastAccess field from 30 minutes to 60 minutes, use the following operation:

db.runCommand({collMod: "sessions",
               index: {keyPattern: {lastAccess:1},
                       expireAfterSeconds: 3600}})

Which will return the document:

{ "expireAfterSeconds_old" : 1800, "expireAfterSeconds_new" : 3600, "ok" : 1 }
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