RDS for PostgreSQL memory - Amazon Relational Database Service (original) (raw)
RDS for PostgreSQL memory is divided into shared and local.
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Shared memory in RDS for PostgreSQL
RDS for PostgreSQL allocates shared memory when the instance starts. Shared memory is divided into multiple subareas. The following sections provide descriptions of the most important ones.
Shared buffers
The shared buffer pool is an RDS for PostgreSQL memory area that holds all pages that are or were being used by application connections. A page is the memory version of a disk block. The shared buffer pool caches the data blocks read from disk. The pool reduces the need to reread data from disk, making the database operate more efficiently.
Every table and index is stored as an array of pages of a fixed size. Each block contains multiple tuples, which correspond to rows. A tuple can be stored in any page.
The shared buffer pool has finite memory. If a new request requires a page that isn't in memory, and no more memory exists, RDS for PostgreSQL evicts a less frequently used page to accommodate the request. The eviction policy is implemented by a clock sweep algorithm.
The shared_buffers parameter determines how much memory the server dedicates to caching data. The default value is set to{DBInstanceClassMemory/32768} bytes, based on the available memory for the DB instance.
Write ahead log (WAL) buffers
A write-ahead log (WAL) buffer holds transaction data that RDS for PostgreSQL later writes to persistent storage. Using the WAL mechanism, RDS for PostgreSQL can do the following:
- Recover data after a failure
- Reduce disk I/O by avoiding frequent writes to disk
When a client changes data, RDS for PostgreSQL writes the changes to the WAL buffer. When the client issues a COMMIT, the WAL writer process writes transaction data to the WAL file.
The wal_level parameter determines how much information is written to the WAL, with possible values such as minimal,replica, and logical.
Local memory in RDS for PostgreSQL
Every backend process allocates local memory for query processing.
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Work memory area
The work memory area holds temporary data for queries that performs sorts and hashes. For example, a query with an ORDER BY clause performs a sort. Queries use hash tables in hash joins and aggregations.
The work_mem parameter the amount of memory to be used by internal sort operations and hash tables before writing to temporary disk files, measured in megabytes. The default value is 4 MB. Multiple sessions can run simultaneously, and each session can run maintenance operations in parallel. For this reason, the total work memory used can be multiples of the work_mem setting.
Maintenance work memory area
The maintenance work memory area caches data for maintenance operations. These operations include vacuuming, creating an index, and adding foreign keys.
The maintenance_work_mem parameter specifies the maximum amount of memory to be used by maintenance operations, measured in megabytes. The default value is 64 MB. A database session can only run one maintenance operation at a time.
Temporary buffer area
The temporary buffer area caches temporary tables for each database session.
Each session allocates temporary buffers as needed up to the limit you specify. When the session ends, the server clears the buffers.
The temp_buffers parameter sets the maximum number of temporary buffers used by each session, measured in megabytes. The default value is 8 MB. Before the first use of temporary tables within a session, you can change the temp_buffers value.