MongoRocks¶
MongoRocks is a storage engine for MongoDB based on the RocksDB key-value store optimized for fast storage. It is developed by Facebook and designed to handle write-intensive workloads.
The MongoRocks storage engine is available in Percona Server for MongoDB along with the standard MongoDB engines (the original MMAPv1 and the default WiredTiger), as well as Percona Memory Engine.
Using MongoRocks¶
As of version 3.2, Percona Server for MongoDB runs with WiredTiger by default.
You can select a storage engine
using the --storageEngine
command-line option when you start mongod
.
Alternatively, you can set the storage.engine
variable
in the configuration file (by default, /etc/mongod.conf
):
Data created by one storage engine is not compatible with other storage engines, because each one has its own data model. When changing the storage engine, you have to do one of the following:
If you simply want to temporarily test MongoRocks, change to a different data directory with the
--dbpath
command-line option:$ service mongod stop $ mongod --storageEngine rocksdb --dbpath <newDataDir>
Note
Make sure that the user running
mongod
has read and write permissons for the new data directory.If you want to permanently switch to MongoRocks and do not have any valuable data in your database, clean out the default data directory and edit the configuration file:
$ service mongod stop $ rm -rf /var/lib/mongodb/* $ sed -i '/engine: .*rocksdb/s/#//g' /etc/mongod.conf $ service mongod start
If there is data that you want to migrate and make compatible with MongoRocks, use the
mongodump
andmongorestore
utilities:$ mongodump --out <dumpDir> $ service mongod stop $ rm -rf /var/lib/mongodb/* $ sed -i '/engine: .*rocksdb/s/#//g' /etc/mongod.conf $ service mongod start $ mongorestore <dumpDir>
Configuring MongoRocks¶
You can configure MongoRocks using either command-line options
or corresponding parameters in the /etc/mongod.conf
file.
The configuration file is formatted in YAML.
For example, the following sample configuration is suggested
as the default for running Percona Server for MongoDB with MongoRocks:
storage:
engine: rocksdb
rocksdb:
cacheSizeGB: 1
compression: snappy
maxWriteMBPerSec: 1024
crashSafeCounters: false
counters: true
singleDeleteIndex: false
Setting parameters in the previous example configuration file
is the same as starting the mongod
daemon with the following options:
mongod --storageEngine=rocksdb \
--rocksdbCacheSizeGB=1 \
--rocksdbCompression=snappy \
--rocksdbMaxWriteMBPerSec=1024 \
--rocksdbCrashSafeCounters=false \
--rocksdbCounters=true \
--rocksdbSingleDeleteIndex=false
The following options are available (with corresponding YAML configuration file parameters):
-
option
--rocksdbCacheSizeGB
¶ Variable: storage.rocksdb.cacheSizeGB
Type: Integer Default: 30% of physical memory Specifies the amount of memory (in gigabytes) to allocate for block cache. Block cache is used to store uncompressed pages. Compressed pages are stored in kernel’s page cache.
To configure block cache size dynamically, set the
rocksdbRuntimeConfigCacheSizeGB
parameter at runtime:db.adminCommand({setParameter:1, rocksdbRuntimeConfigCacheSizeGB: 10})
-
option
--rocksdbCompression
¶ Variable: storage.rocksdb.compression
Type: String Default: snappy
Specifies the block compression algorithm for data collection. Possible values:
none
,snappy
,zlib
,lz4
,lz4hc
.
-
option
--rocksdbMaxWriteMBPerSec
¶ Variable: storage.rocksdb.maxWriteMBPerSec
Type: Integer Default: 1024
(1 GB/sec)Specifies the maximum speed at which MongoRocks writes to storage (in megabytes per second). Decrease this value to reduce read latency spikes during compactions. However, reducing it too much might slow down writes.
To configure write speed dynamically, set the
rocksdbRuntimeConfigMaxWriteMBPerSec
parameter at runtime:db.adminCommand({setParameter:1, rocksdbRuntimeConfigMaxWriteMBPerSec:30})
-
option
--rocksdbCrashSafeCounters
¶ Variable: storage.rocksdb.crashSafeCounters
Type: Boolean Default: false
Specifies whether to correct counters after a crash. Enabling this can affect write performance.
-
option
--rocksdbCounters
¶ Variable: storage.rocksdb.counters
Type: Boolean Default: true
Specifies whether to use advanced counters for MongoRocks. You can disable them to improve write performance.
-
option
--rocksdbSingleDeleteIndex
¶ Variable: storage.rocksdb.singleDeleteIndex
Type: Boolean Default: false
This is an experimental feature. Enable it only if you know what you are doing.
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