FAQ: MongoDB Storage for Self-Managed Deployments用于自我管理部署的MongoDB存储

What is a storage engine?什么是存储引擎？

A storage engine is the part of a database that is responsible for managing how data is stored, both in memory and on disk. Many databases support multiple storage engines, where different engines perform better for specific workloads. 存储引擎是数据库的一部分，负责管理数据在内存和磁盘上的存储方式。许多数据库支持多个存储引擎，其中不同的引擎对特定的工作负载表现更好。For example, one storage engine might offer better performance for read-heavy workloads, and another might support a higher throughput for write operations.例如，一个存储引擎可能为读取繁重的工作负载提供更好的性能，另一个可能为写入操作支持更高的吞吐量。

How many collections and indexes can be in a cluster?一个集群中可以有多少个集合和索引？

Cluster performance might degrade once the combined number of collections and indexes reaches beyond 100,000. In addition, many large collections have a greater impact on performance than smaller collections.一旦集合和索引的总数超过100000，集群性能可能会下降。此外，许多大型集合对性能的影响比小型集合大。

Can I upgrade an existing deployment to WiredTiger?我可以将现有部署升级到WiredTiger吗？

Yes. See:对。请参阅：

• Change a Self-Managed Standalone to WiredTiger将自管理独立设备更改为WiredTiger
• Change a Self-Managed Replica Set to WiredTiger将自我管理副本集更改为WiredTiger
• Change a Self-Managed Sharded Cluster to WiredTiger将自管理分片集群更改为WiredTiger

How much compression does WiredTiger provide?WiredTiger提供了多少压缩？

The ratio of compressed data to uncompressed data depends on your data and the compression library used. 压缩数据与未压缩数据的比率取决于数据和使用的压缩库。By default, collection data in WiredTiger use Snappy block compression; zlib and zstd compression is also available. 默认情况下，WiredTiger中的采集数据使用Snappy块压缩；zlib和zstd压缩也是可用的。Index data use prefix compression by default.默认情况下，索引数据使用前缀压缩。

To what size should I set the WiredTiger internal cache?我应该将WiredTiger内部缓存设置为多大？

With WiredTiger, MongoDB utilizes both the WiredTiger internal cache and the filesystem cache.使用WiredTiger，MongoDB同时利用WiredTigeer内部缓存和文件系统缓存。

The default WiredTiger internal cache size is the larger of either:默认的WiredTiger内部缓存大小是以下两者中的较大值：

• 50% of (RAM - 1 GB), or
• 256 MB.

For example, on a system with a total of 4GB of RAM the WiredTiger cache uses 1.5GB of RAM (0.5 * (4 GB - 1 GB) = 1.5 GB). 例如，在总共有4GB RAM的系统上，WiredTiger缓存使用1.5GB RAM（0.5 * (4 GB - 1 GB) = 1.5 GB）。Conversely, on a system with a total of 1.25 GB of RAM WiredTiger allocates 256 MB to the WiredTiger cache because that is more than half of the total RAM minus one gigabyte (0.5 * (1.25 GB - 1 GB) = 128 MB < 256 MB).相反，在总共有1.25 GB RAM的系统上，WiredTiger将256 MB分配给WiredTigeer缓存，因为这超过了总RAM减去1 GB的一半（0.5 * (1.25 GB - 1 GB) = 128 MB < 256 MB）。

By default, WiredTiger uses Snappy block compression for all collections and prefix compression for all indexes. Compression defaults are configurable at a global level and can also be set on a per-collection and per-index basis during collection and index creation.默认情况下，WiredTiger对所有集合使用Snappy块压缩，对所有索引使用前缀压缩。压缩默认值可以在全局级别进行配置，也可以在集合和索引创建期间根据每个集合和每个索引进行设置。

Different representations are used for data in the WiredTiger internal cache versus the on-disk format:与磁盘格式相比，WiredTiger内部缓存中的数据使用了不同的表示方式：

• Data in the filesystem cache is the same as the on-disk format, including benefits of any compression for data files. The filesystem cache is used by the operating system to reduce disk I/O.文件系统缓存中的数据与磁盘上的格式相同，包括对数据文件进行任何压缩的好处。操作系统使用文件系统缓存来减少磁盘I/O。
• Indexes loaded in the WiredTiger internal cache have a different data representation to the on-disk format, but can still take advantage of index prefix compression to reduce RAM usage. Index prefix compression deduplicates common prefixes from indexed fields.加载在WiredTiger内部缓存中的索引具有与磁盘上格式不同的数据表示形式，但仍然可以利用索引前缀压缩来减少RAM使用。索引前缀压缩可消除索引字段中的常见前缀的重复。
• Collection data in the WiredTiger internal cache is uncompressed and uses a different representation from the on-disk format. Block compression can provide significant on-disk storage savings, but data must be uncompressed to be manipulated by the server.WiredTiger内部缓存中的集合数据是未压缩的，并使用与磁盘格式不同的表示形式。块压缩可以显著节省磁盘存储空间，但数据必须解压缩才能由服务器操作。

With the filesystem cache, MongoDB automatically uses all free memory that is not used by the WiredTiger cache or by other processes.使用文件系统缓存，MongoDB会自动使用WiredTiger缓存或其他进程未使用的所有空闲内存。

To adjust the size of the WiredTiger internal cache, see --wiredTigerCacheSizeGB and storage.wiredTiger.engineConfig.cacheSizeGB. 要调整WiredTiger内部缓存的大小，请参阅--wiredTigerCacheSizeGB和storage.wiredTiger.engineConfig.cacheSizeGB。Avoid increasing the WiredTiger internal cache size above its default value. 避免将WiredTiger内部缓存大小增加到默认值以上。If your use case requires increased internal cache size, see --wiredTigerCacheSizePct and storage.wiredTiger.engineConfig.cacheSizePct.如果用例需要增加内部缓存大小，请参阅--wiredTigerCacheSizePct和storage.wiredTiger.engineConfig.cacheSizePct。

The default WiredTiger internal cache size value assumes that there is a single mongod instance per machine. 默认的WiredTiger内部缓存大小值假定每台机器只有一个mongod实例。If a single machine contains multiple MongoDB instances, decrease the setting to accommodate the other mongod instances.如果一台机器包含多个MongoDB实例，请减小设置以容纳其他MongoDB实例。

If you run mongod in a container (for example, lxc, cgroups, Docker, etc.) that does not have access to all of the RAM available in a system, you must set storage.wiredTiger.engineConfig.cacheSizeGB to a value less than the amount of RAM available in the container. 如果你在一个无法访问系统中所有可用RAM的容器（例如lxc、cgroups、Docker等）中运行mongod，你必须将storage.wiredTiger.engineConfig.cacheSizeGB设置为小于容器中可用RAM量的值。The exact amount depends on the other processes running in the container. See memLimitMB.确切的数量取决于容器中运行的其他进程。请参阅memLimitMB。

You can only provide one of either storage.wiredTiger.engineConfig.cacheSizeGB or storage.wiredTiger.engineConfig.cacheSizePct.您只能提供storage.wiredTiger.engineConfig.cacheSizeGB或storage.wiredTiger.engineConfig.cacheSizePct中的一个。

To view statistics on the cache and eviction rate, see the wiredTiger.cache field returned from the serverStatus command.要查看缓存和驱逐率的统计信息，请参阅serverStatus命令返回的wiredTigercache字段。

How much memory does MongoDB allocate per connection?MongoDB为每个连接分配多少内存？

Each connection uses up to 1 megabyte of RAM.每个连接最多使用1兆字节的RAM。

To optimize memory use for connections, ensure that you:要优化连接的内存使用，请确保：

• Monitor the number of open connections to your deployment. 监视与部署的打开连接数。Too many open connections result in excessive use of RAM and reduce available memory for the working set.过多的打开连接会导致RAM的过度使用，并减少工作集的可用内存。
• Close connection pools when they are no longer needed. 当不再需要连接池时，关闭它们。A connection pool is a cache of open, ready-to-use database connections maintained by the driver. Closing unneeded pools makes additional memory resources available.连接池是由驱动程序维护的开放、即用型数据库连接的缓存。关闭不需要的池会使额外的内存资源可用。

• Manage the size of your connection pool. 管理连接池的大小。The maxPoolSize connection string option specifies the maximum number of open connections in the pool. maxPoolSize连接字符串选项指定池中打开的连接的最大数量。By default, you can have up to 100 open connections in the pool. Lowering the maxPoolSize reduces the maximum amount of RAM used for connections.默认情况下，池中最多可以有100个打开的连接。降低maxPoolSize会减少用于连接的最大RAM量。

  Tip

  To configure your connection pool, see Connection Pool Configuration Settings.要配置连接池，请参阅连接池配置设置。

How frequently does WiredTiger write to disk?WiredTiger多久写入一次磁盘？

Checkpoints检查点

MongoDB configures WiredTiger to create checkpoints, specifically, writing the snapshot data to disk at intervals of 60 seconds.MongoDB配置WiredTiger以创建检查点，具体来说，每隔60秒将快照数据写入磁盘。

Journal Data日记账数据

WiredTiger syncs the buffered journal records to disk upon any of the following conditions: WiredTiger在以下任何一种情况下将缓冲的日志记录同步到磁盘：

• For replica set members (primary and secondary members):对于副本集成员（主要和次要成员）：

  • If a write operation includes or implies a write concern of j: true.如果写操作包含或暗示了j: true的写关注。
  • Additionally for secondary members, after every batch application of the oplog entries.此外，对于次要成员，每次批量应用oplog条目后。

  Note

  Write concern "majority" implies j: true if the writeConcernMajorityJournalDefault is true.如果writeConcernMajorityJournalDefault为真，则写关注"majority"意味着j:true。

• At every 100 milliseconds (See storage.journal.commitIntervalMs).每100毫秒一次（请参阅storage.journal.commitIntervalMs）。
• When WiredTiger creates a new journal file. Because MongoDB uses a journal file size limit of 100 MB, WiredTiger creates a new journal file approximately every 100 MB of data.当WiredTiger创建新的日志文件时。因为MongoDB使用100MB的日志文件大小限制，所以WiredTiger大约每100MB的数据创建一个新的日志文件。

How do I reclaim disk space in WiredTiger?如何在WiredTiger中回收磁盘空间？

The WiredTiger storage engine maintains lists of empty records in data files as it deletes documents. This space can be reused by WiredTiger, but will not be returned to the operating system unless under very specific circumstances.WiredTiger存储引擎在删除文档时维护数据文件中的空记录列表。WiredTiger可以重用此空间，但除非在非常特殊的情况下，否则不会将其返回给操作系统。

The amount of empty space available for reuse by WiredTiger is reflected in the output of db.collection.stats() under the heading wiredTiger.block-manager.file bytes available for reuse.可供WiredTiger重用的空白空间量反映在db.collection.stats()的输出中，标题为可供重用的wiredTiger.block-manager.file字节。

To allow the WiredTiger storage engine to release this empty space to the operating system, you can de-fragment your data file. 为了允许WiredTiger存储引擎将此空白空间释放到操作系统，您可以对数据文件进行分片化处理。This can be achieved by resyncing a replica set member or using the compact command.这可以通过重新同步副本集成员或使用compact命令来实现。

How can I check the size of a collection?我如何查看集合的大小？

To view the statistics for a collection, including the data size, use the db.collection.stats() method from within mongosh. 要查看集合的统计信息，包括数据大小，请使用mongosh中的db.collection.stats()方法。The following example issues db.collection.stats() for the orders collection:以下示例为orders集合发出db.collection.stats()：

db.orders.stats();

MongoDB also provides the following methods to return specific sizes for the collection:MongoDB还提供了以下方法来返回集合的特定大小：

• db.collection.dataSize() to return the uncompressed data size in bytes for the collection.返回集合的未压缩数据大小（以字节为单位）。
• db.collection.storageSize() to return the size in bytes of the collection on disk storage. 返回磁盘存储上集合的大小（以字节为单位）。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 returned by db.collection.dataSize().如果集合数据被压缩（这是WiredTiger的默认值），则存储大小反映了压缩后的大小，并且可能小于db.collection.dataSize()返回的值。
• db.collection.totalIndexSize() to return the index sizes in bytes for the collection. 返回集合的索引大小（以字节为单位）。If an index uses prefix compression (which is the default for WiredTiger), the returned size reflects the compressed size.如果索引使用前缀压缩（这是WiredTiger的默认值），则返回的大小反映了压缩后的大小。

The following script prints the statistics for each database:以下脚本打印每个数据库的统计信息：

db.adminCommand("listDatabases").databases.forEach(function (d) {
   mdb = db.getSiblingDB(d.name);
   printjson(mdb.stats());
})

The following script prints the statistics for each collection in each database:以下脚本打印每个数据库中每个集合的统计信息：

db.adminCommand("listDatabases").databases.forEach(function (d) {
   mdb = db.getSiblingDB(d.name);
   mdb.getCollectionNames().forEach(function(c) {
      s = mdb[c].stats();
      printjson(s);
   })
})

How can I check the size of the individual indexes for a collection?如何检查集合的单个索引的大小？

To view the size of the data allocated for each index, use the db.collection.stats() method and check the indexSizes field in the returned document.要查看为每个索引分配的数据大小，请使用db.collection.stats()方法并检查返回文档中的indexSizes字段。

If an index uses prefix compression (which is the default for WiredTiger), the returned size for that index reflects the compressed size.如果索引使用前缀压缩（这是WiredTiger的默认值），则该索引的返回大小反映了压缩后的大小。

How can I get information on the storage use of a database?我如何获取数据库的存储使用信息？

The db.stats() method in mongosh returns the current state of the "active" database. mongosh中的db.stats()方法返回“活动”数据库的当前状态。For the description of the returned fields, see dbStats Output.有关返回字段的描述，请参阅dbStats输出。