Connection Pool Parameters
When a connection pool is used to access the database, database connections are established and then stored in the memory as objects during system running. When you need to access the database, no new connection is established. Instead, an existing idle connection is selected from the connection pool. After you finish accessing the database, the database does not disable the connection but puts it back into the connection pool. The connection can be used for the next access request.
min_pool_size
Parameter description: Specifies the minimum number of connections between a CN's connection pool and another CN/DN.
Type: POSTMASTER
Value range: an integer ranging from 1 to 65535
Default value: 1
max_pool_size
Parameter description: Specifies the maximum number of connections between a CN's connection pool and another CN/DN.
Type: POSTMASTER
Value range: an integer ranging from 1 to 65535
Default value: 800 for CNs and 5000 for DNs
persistent_datanode_connections
Parameter description: Specifies whether to release the connection for the current session.
Type: USERSET
Value range: Boolean
- off indicates that the connection for the current session will be released.
- on indicates that the connection for the current session will not be released.
After this function is enabled, a session may hold a connection but does not run a query. As a result, other query requests fail to be connected. To fix this problem, the number of sessions must be less than or equal to max_active_statements.
Default value: off
cache_connection
Parameter description: Specifies whether to reclaim the connections of a connection pool.
Type: USERSET
Value range: Boolean
- on indicates that the connections of a connection pool will be reclaimed.
- off indicates that the connections of a connection pool will not be reclaimed.
Default value: on
enable_force_reuse_connections
Parameter description: Specifies whether a session forcibly reuses a new connection.
Type: USERSET
Value range: Boolean
- on indicates that the new connection is forcibly used.
- off indicates that the current connection is used.
Default value: off
enable_pooler_parallel
Parameter description: Specifies whether a CN's connection pool can be connected in parallel mode.
Type: SIGHUP
Value range: Boolean
- on indicates that a CN's connection pool can be connected in parallel mode.
- off indicates that a CN's connection pool cannot be connected in parallel mode.
Default value: on
syscache_clean_policy
Parameter description: Specifies the policy for clearing the memory and number of idle DN connections. This parameter is supported only by clusters of version 9.1.0.100 or later.
Type: SIGHUP
Value range: a string
This parameter policy consists of three values:
- The first value ranges from 0 to 1 and represents the percentage of total available memory used by DNs. When the percentage of used memory reaches this value, 1/4 of the stream threads will be cleared, and the second value will be evaluated.
- The second value ranges from 0 to 1 and represents the percentage of total available memory used by syscache on DNs. When the percentage of syscache memory usage reaches this value, the third value will be evaluated.
- The third value ranges from 0 to INT_MAX and is measured in MB. It represents the size of syscache memory used by idle threads. When the syscache memory usage of an idle thread reaches this value, the syscache used by that thread will be cleared.
Default value: 0.8,0.3,64
- Before setting this parameter, evaluate the memory usage using the pv_session_memory_detail and pv_total_memory_detail views.
- When setting this parameter, follow the specified format, ensuring that the three values are separated by commas without spaces.
- If the parameter is not set according to the specified format and the setting fails, a WARNING log will be generated in the log, and the parameter value displayed when using the SHOW command to query the parameter will be the last successfully set value. If the setting fails and the system is restarted, the parameter will be set to the default value.
- During the Readcommand phase, if a thread on CN times out after 30 seconds, it will clear DNs if syscache is greater than 256 MB. There are two operations:
- If the overall memory usage reaches 80%, an auxiliary thread will monitor the memory usage and clear 1/4 of the stream threads. It will also check if syscache usage exceeds 30% of the total memory usage. If it does, it will clear the syscache of Readcommand phase pg threads greater than 64 MB.
- If a stream thread is idle for more than 30 seconds and syscache usage is greater than 64 MB, it will clear the syscache.
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