compact

发布于 2015-09-14 15:03:32 | 271 次阅读 | 评论: 0 | 来源: 网络整理

compact¶

2.0 新版功能.

The compact command rewrites and defragments a single collection. Additionally, the command drops all indexes at the beginning of compaction and rebuilds the indexes at the end. compact is conceptually similar to repairDatabase, but works on a single collection rather than an entire database.

The command has the following syntax:

{ compact: <collection name> }

You may also specify the following options:

参数:

参数:	force – 在 2.2 版更改: `compact` blocks activities only for the database it is compacting. The `force` specifies whether the `compact` command can run on the primary node in a replica set. Set to `true` to run the `compact` command on the primary node in a replica set. Otherwise, the `compact` command returns an error when invoked on a replica set primary because the command blocks all other activity. paddingFactor – 2.2 新版功能. Default: `1.0` Minimum: `1.0` (no padding.) Maximum: `4.0` The `paddingFactor` describes the record size allocated for each document as a factor of the document size. 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 } ) paddingBytes – 2.2 新版功能. The `paddingBytes` sets the padding as an absolute number of bytes. 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 1KB, 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). With the following command, you can use the `paddingBytes` option of the `compact` command to set the padding size to `100` bytes on the collection named by `<collection>`: db.runCommand ( { compact: '<collection>', paddingBytes: 100 } )

force –
在 2.2 版更改: compact blocks activities only for the database it is compacting.

The force specifies whether the compact command can run on the primary node in a replica set. Set to true to run the compact command on the primary node in a replica set. Otherwise, the compact command returns an error when invoked on a replica set primary because the command blocks all other activity.
paddingFactor –
2.2 新版功能.

Default: 1.0

Minimum: 1.0 (no padding.)

Maximum: 4.0

The paddingFactor describes the record size allocated for each document as a factor of the document size. 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 } )
```
paddingBytes –
2.2 新版功能.

The paddingBytes sets the padding as an absolute number of bytes. 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 1KB, 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).

With the following command, you can use the paddingBytes option of the compact command to set the padding size to 100 bytes on the collection named by <collection>:
```
db.runCommand ( { compact: '<collection>', paddingBytes: 100 } )
```

警告

Always have an up-to-date backup before performing server maintenance such as the compact operation.

Note the following behaviors:

compact blocks all other activity. In MongoDB 2.2, compact blocks activities only for its database. You may view the intermediate progress either by viewing the mongod log file, or by running the db.currentOp() in another shell instance.
compact removes any padding factor in the collection when issued without either the paddingFactor option or the paddingBytes option. This may impact performance if the documents grow regularly. However, compact retains existing paddingFactor statistics for the collection that MongoDB will use to calculate the padding factor for future inserts.
compact generally uses less disk space than repairDatabase and is faster. However,the compact command is still slow and does block other database use. Only use compact during scheduled maintenance periods.
If you terminate the operation with the db.killOp() method or restart the server before it has finished:
- If you have journaling enabled, the data remains consistent 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’s impossible to guarantee that the data is in a consistent 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.
compact may increase the total size and number of our 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.
compact requires a small amount of additional disk space while running but unlike repairDatabase it does not free space on the file system.
You may also wish to run the collStats command before and after compaction to see how the storage space changes for the collection.
compact commands do not replicate to secondaries in a replica set:
- Compact each member separately.
- Ideally, compaction runs on a secondary. See option force:true above for information regarding compacting the primary.
警告

If you run compact on a secondary, the secondary will enter a RECOVERING state to prevent clients from sending read operations during compaction. Once the compaction finishes the secondary will automatically return to SECONDARY state. See state for more information on states.

Refer to the “partial script for automating step down and compaction” for an example of this procedure.
compact is a command issued to a mongod. In a sharded environment, run compact on each shard separately as a maintenance operation.
It is not possible to compact capped collections because they don’t have padding, and documents cannot grow in these collections. However, the documents of a capped collections are not subject to fragmentation.

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repairDatabase

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