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compact¶
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Definition¶
- compact¶
Rewrites and defragments all data and indexes in a collection. On WiredTiger databases, this command will release unneeded disk space to the operating system.
compact has the following form:
{ compact: <collection name> }
compact takes the following fields:
Field Type Description compact string The name of the collection. force boolean Optional. If true, compact can run on the primary in a replica set. If false, compact returns an error when run on a primary, because the command blocks all other operations.
compact blocks operations only for the database it is compacting.
paddingFactor number Optional. Applicable for the MMAPv1 storage engine only. Specifies the padding to use (as a factor of the document size) during the compact operation.
The paddingFactor does not affect the padding of subsequent record allocations after compact completes. For more information, see paddingFactor.
paddingBytes integer Optional. Applicable for the MMAPv1 storage engine only. Specifies the padding to use (in absolute number of bytes) during the compact operation.
paddingBytes does not affect the padding of subsequent record allocations after compact completes. For more information, see paddingBytes.
preservePadding boolean Optional. Applicable for the MMAPv1 storage engine only. Specifies that the compact process should leave document padding intact.
This option cannot be used with paddingFactor or paddingBytes.
2.6 新版功能.
警告
Always have an up-to-date backup before performing server maintenance such as the compact operation.
paddingFactor¶
注解
Applicable for the MMAPv1 storage engine only; specifying paddingFactor has no effect when used with the WiredTiger storage engine.
The paddingFactor field takes the following range of values:
- Default: 1.0
- Minimum: 1.0 (no padding)
- Maximum: 4.0
If your updates increase the size of the documents, padding will increase the amount of space allocated to each document and avoid expensive document relocation operations within the data files.
You can calculate the padding size by subtracting the document size from the record size or, in terms of the paddingFactor, by subtracting 1 from the paddingFactor:
padding size = (paddingFactor - 1) * <document size>.
For example, a paddingFactor of 1.0 specifies a padding size of 0 whereas a paddingFactor of 1.2 specifies a padding size of 0.2 or 20 percent (20%) of the document size.
With the following command, you can use the paddingFactor option of the compact command to set the record size to 1.1 of the document size, or a padding factor of 10 percent (10%):
db.runCommand ( { compact: '<collection>', paddingFactor: 1.1 } )
compact modifies existing documents, but does not set the padding factor for future documents.
paddingBytes¶
注解
Applicable for the MMAPv1 storage engine only; specifying paddingBytes has no effect when used with the WiredTiger storage engine.
Specifying paddingBytes can be useful if your documents start small but then increase in size significantly.
For example, if your documents are initially 40 bytes long and you grow them by 1 kB, using paddingBytes: 1024 might be reasonable since using paddingFactor: 4.0 would specify a record size of 160 bytes (4.0 times the initial document size), which would only provide a padding of 120 bytes (i.e. record size of 160 bytes minus the document size).
The following command uses the paddingBytes option to set the padding size to 100 bytes on the collection named by <collection>:
db.runCommand ( { compact: '<collection>', paddingBytes: 100 } )
Behavior¶
Blocking¶
compact only blocks operations for the database it is currently operating on. Only use compact during scheduled maintenance periods.
You may view the intermediate progress either by viewing the mongod log file or by running the db.currentOp() in another shell instance.
Operation Termination¶
If you terminate the operation with the db.killOp() method or restart the server before the compact operation has finished, be aware of the following:
- If you have journaling enabled, the data remains valid and usable, regardless of the state of the compact operation. You may have to manually rebuild the indexes.
- If you do not have journaling enabled and the mongod or compact terminates during the operation, it is impossible to guarantee that the data is in a valid state.
- In either case, much of the existing free space in the collection may become un-reusable. In this scenario, you should rerun the compaction to completion to restore the use of this free space.
Disk Space¶
compact has different impacts on available disk space depending on which storage engine is in use.
To see how the storage space changes for the collection, run the collStats command before and after compaction.
WiredTiger¶
On WiredTiger, compact will rewrite the collection and indexes to minimize disk space by releasing unused disk space to the system. This is useful if you have removed a large amount of data from the collection, and do not plan to replace it.
MMAPv1¶
On MMAPv1, compact defragments the collection’s data files and recreates its indexes. Unused disk space is not released to the system, but instead retained for future data. If you wish to reclaim disk space from a MMAPv1 database, you should perform an initial sync.
compact requires up to 2 gigabytes of additional disk space to run on MMAPv1 databases.
Size and Number of Data Files¶
compact may increase the total size and number of your data files, especially when run for the first time. However, this will not increase the total collection storage space since storage size is the amount of data allocated within the database files, and not the size/number of the files on the file system.
Replica Sets¶
compact commands do not replicate to secondaries in a replica set.
- Compact each member separately.
- Ideally run compact on a secondary. See option force:true above for information regarding compacting the primary.
- On secondaries, the command forces the secondary to enter RECOVERING state. Read operations issued to an instance in the RECOVERING state will fail. This prevents clients from reading during the operation. When the operation completes, the secondary returns to SECONDARY state.
- See Replica Set Member States for more information about replica set member states.
See 对复制集节点进行维护 for an example replica set maintenance procedure to maximize availability during maintenance operations.
Sharded Clusters¶
compact only applies to mongod instances. In a sharded environment, run compact on each shard separately as a maintenance operation.
You cannot issue compact against a mongos instance.
Capped Collections¶
It is not possible or necessary to compact capped collections because they lack padding and their documents cannot grow. As a result, they cannot become fragmented.