{"Entry":{"collection":"guc","key":"multixact_member_buffers","name":"multixact_member_buffers","aliases":[],"metadata":{"baseline":false,"boot_human":"Not specified","boot_val":null,"category":"Resource Usage / Memory","category_zh":"","changed_in":[],"changes":[{"documentation_changed":false,"fields":{},"from":"16","status":"added","to":"17"},{"documentation_changed":true,"fields":{},"from":"17","status":"changed","to":"18"}],"content_hash":"e1e89b20ec6a61a4c7f9ffde958381ff8837bab306d0c34c3108e85807eab70e","context":"","default_changed_in":[],"default_history":[{"from":"17","to":"19","value":"256 KiB (32 × 8kB)"}],"editorial":{"advice":{"olap":"Analytical workload labels alone do not justify changing multixact_member_buffers; tune only when the underlying transaction-state facility, not table scans, is the measured bottleneck.","oltp":"Leave multixact_member_buffers at automatic or upstream sizing unless SLRU-specific I/O and contention prove this cache is undersized. A larger number consumes shared memory for the entire server lifetime.","small":"Keep multixact_member_buffers at its default on a small host. Moving scarce shared memory into an internal cache without direct evidence can reduce room for more valuable caches and processes."},"mechanism":["multixact_member_buffers allocates a dedicated shared buffer pool for pg_multixact/members, which caches MultiXact member entries. This is real startup shared memory, not a planner estimate.","The configured block count is allocated at server start. The unit is BLCKSZ blocks, normally 8kB.","A larger cache can reduce SLRU read/write churn for unusually heavy use of the underlying feature, but it permanently consumes shared memory and does not increase the feature's logical capacity. Its postmaster context fixes the value at server start; changing it requires a restart."],"pitfalls":["Changing multixact_member_buffers without applying its documented unit and configuration context.","Optimizing an isolated benchmark while ignoring concurrent aggregate resource use.","Assuming a configured value guarantees operating-system or storage behavior.","Failing to retest startup, failover, and workload latency after the change."],"references":[{"title":"PostgreSQL 19 Beta 4: multixact_member_buffers","url":"https://www.postgresql.org/docs/19/runtime-config-resource.html#GUC-MULTIXACT-MEMBER-BUFFERS"},{"title":"PostgreSQL 19 release notes","url":"https://www.postgresql.org/docs/19/release-19.html"}],"related":["multixact_offset_buffers","shared_buffers","max_locks_per_transaction"],"summary":"multixact_member_buffers — Sets the size of the dedicated buffer pool used for the MultiXact member cache. Observed in PG17–19 Beta 4; its last measured boot default is 256 KiB (32 × 8kB) in PG19 Beta 4, with postmaster context. This is a beta-snapshot fact and can change before PostgreSQL 19 GA."},"enumvals":[],"first_version":"17","group":"Resource Usage","group_slug":"resource","imported_at":"2026-09-27T17:57:31.789207+08:00","intro_commit":{"authored_at":"2024-02-28T17:05:31+01:00","discussion":["https://postgr.es/m/2BEC2B3F-9B61-4C1D-9FB5-5FAB0F05EF86@yandex-team.ru","https://postgr.es/m/CAFiTN-vzDvNz=ExGXz6gdyjtzGixKSqs0mKHMmaQ8sOSEFZ33A@mail.gmail.com"],"hash":"53c2a97a92665be6bd7d70bd62ae6158fe4db96e","subject":"Improve performance of subsystems on top of SLRU","url":"https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=53c2a97a92665be6bd7d70bd62ae6158fe4db96e"},"key":"multixact_member_buffers","last_version":"20","max_val":"","min_val":"","name":"multixact_member_buffers","position":298,"present_in":["17","18","19","20"],"short_desc":"Specifies the amount of shared memory to use to cache the contents of pg_multixact/members (see Table 66.1).","short_desc_zh":"","source_rev":"english-manuals:367651b17f6ece5c85814f55d14003885187fbfcf7020ca823a4db7ab306eeb1","unit":"","vartype":"integer"}},"Definition":{"Collection":"guc","Key":"multixact_member_buffers","SourceDatabase":"center","Version":"18","SourceTable":"guc","SourceKey":"multixact_member_buffers","SourceRevision":"english-manuals:367651b17f6ece5c85814f55d14003885187fbfcf7020ca823a4db7ab306eeb1","Facts":{"boot_val":"32","category":"Resource Usage / Memory","context":"postmaster","description":"Specifies the amount of shared memory to use to cache the contents of pg_multixact/members (see Table 66.1). If this value is specified without units, it is taken as blocks, that is BLCKSZ bytes, typically 8kB. The default value is 32. This parameter can only be set at server start.","doc":{"anchor":"GUC-MULTIXACT-MEMBER-BUFFERS","file":"runtime-config-resource.html","lang":"en","sha256":"2e207c599f0884dfbe594558c478a2c7c132578981071e980e2015f31dfce28c","slug":"18"},"documented":true,"enumvals":null,"extra_desc":null,"lang":"en","max_val":"131072","metadata_version":"18","min_val":"16","name":"multixact_member_buffers","short_desc":"Sets the size of the dedicated buffer pool used for the MultiXact member cache.","source":"pg-settings-source-snapshot","unit":"8kB","vartype":"integer"},"ManualEvidence":{"doc":{"anchor":"GUC-MULTIXACT-MEMBER-BUFFERS","file":"runtime-config-resource.html","lang":"en","sha256":"2e207c599f0884dfbe594558c478a2c7c132578981071e980e2015f31dfce28c","slug":"18"}},"MeasuredEvidence":{"metadata_version":"18"}},"Text":{"Collection":"guc","Key":"multixact_member_buffers","SourceDatabase":"pgweb","Version":"18","Locale":"zh-Hans","Title":"multixact_member_buffers","Summary":"","BodyHTML":"\u003cp\u003e指定用于缓存\u003ccode\u003epg_multixact/members\u003c/code\u003e内容的共享内存量（参见\u003ca href=\"/docs/18/storage-file-layout.html#PGDATA-CONTENTS-TABLE\" rel=\"nofollow\"\u003e表 66.1\u003c/a\u003e）。如果未指定单位，则视为块，即\u003ccode\u003eBLCKSZ\u003c/code\u003e字节，通常为 8kB。默认值为 \u003ccode\u003e32\u003c/code\u003e。此参数只能在服务器启动时设置。\u003c/p\u003e","SourceRevision":"2026-09-11@29c86d9","ContentHash":"0123cf02a001154cd63ce6404defce2142eb346b09bcfb0d51e82fbfd9f0e961","Payload":{"carried_from":"","carry_reason":"","doc_html":"\u003cp\u003e指定用于缓存\u003ccode class=\"literal\"\u003epg_multixact/members\u003c/code\u003e内容的共享内存量（参见\u003ca href=\"/docs/18/storage-file-layout.html#PGDATA-CONTENTS-TABLE\" title=\"表 66.1. PGDATA 的内容\"\u003e表 66.1\u003c/a\u003e）。如果未指定单位，则视为块，即\u003ccode class=\"symbol\"\u003eBLCKSZ\u003c/code\u003e字节，通常为 8kB。默认值为 \u003ccode class=\"literal\"\u003e32\u003c/code\u003e。此参数只能在服务器启动时设置。\u003c/p\u003e","doc_same_as":""}},"RequestedLocale":"zh-Hans","Fallback":false,"Versions":["17","18","19","20"],"Locales":["en","zh-Hans"],"Signatures":null,"Spellings":null,"SQLState":null,"Evidence":null}