本节简单介绍了PostgreSQL缓存管理(Buffer Manager)中的其中一个实现函数ReadBufferExtended,该函数返回对应请求关系数据块的buffer.。

Relation
关系的内存结构.

/* * Here are the contents of a relation cache entry. */typedef struct RelationData{ RelFileNode rd_node; /* relation physical identifier */ /* use "struct" here to avoid needing to include smgr.h: */ struct SMgrRelationData *rd_smgr; /* cached file handle, or NULL */ int rd_refcnt; /* reference count */ BackendId rd_backend; /* owning backend id, if temporary relation */ bool rd_islocaltemp; /* rel is a temp rel of this session */ bool rd_isnailed; /* rel is nailed in cache */ bool rd_isvalid; /* relcache entry is valid */ char rd_indexvalid; /* state of rd_indexlist: 0 = not valid, 1 = * valid, 2 = temporarily forced */ bool rd_statvalid; /* is rd_statlist valid? */ /* * rd_createSubid is the ID of the highest subtransaction the rel has * survived into; or zero if the rel was not created in the current top * transaction. This can be now be relied on, whereas previously it could * be "forgotten" in earlier releases. Likewise, rd_newRelfilenodeSubid is * the ID of the highest subtransaction the relfilenode change has * survived into, or zero if not changed in the current transaction (or we * have forgotten changing it). rd_newRelfilenodeSubid can be forgotten * when a relation has multiple new relfilenodes within a single * transaction, with one of them occurring in a subsequently aborted * subtransaction, e.g. BEGIN; TRUNCATE t; SAVEPOINT save; TRUNCATE t; * ROLLBACK TO save; -- rd_newRelfilenode is now forgotten */ SubTransactionId rd_createSubid; /* rel was created in current xact */ SubTransactionId rd_newRelfilenodeSubid; /* new relfilenode assigned in * current xact */ Form_pg_class rd_rel; /* RELATION tuple */ TupleDesc rd_att; /* tuple descriptor */ Oid rd_id; /* relation's object id */ LockInfoData rd_lockInfo; /* lock mgr's info for locking relation */ RuleLock *rd_rules; /* rewrite rules */ MemoryContext rd_rulescxt; /* private memory cxt for rd_rules, if any */ TriggerDesc *trigdesc; /* Trigger info, or NULL if rel has none */ /* use "struct" here to avoid needing to include rowsecurity.h: */ struct RowSecurityDesc *rd_rsdesc; /* row security policies, or NULL */ /* data managed by RelationGetFKeyList: */ List *rd_fkeylist; /* list of ForeignKeyCacheInfo (see below) */ bool rd_fkeyvalid; /* true if list has been computed */ MemoryContext rd_partkeycxt; /* private memory cxt for the below */ struct PartitionKeyData *rd_partkey; /* partition key, or NULL */ MemoryContext rd_pdcxt; /* private context for partdesc */ struct PartitionDescData *rd_partdesc; /* partitions, or NULL */ List *rd_partcheck; /* partition CHECK quals */ /* data managed by RelationGetIndexList: */ List *rd_indexlist; /* list of OIDs of indexes on relation */ Oid rd_oidindex; /* OID of unique index on OID, if any */ Oid rd_pkindex; /* OID of primary key, if any */ Oid rd_replidindex; /* OID of replica identity index, if any */ /* data managed by RelationGetStatExtList: */ List *rd_statlist; /* list of OIDs of extended stats */ /* data managed by RelationGetIndexAttrBitmap: */ Bitmapset *rd_indexattr; /* columns used in non-projection indexes */ Bitmapset *rd_projindexattr; /* columns used in projection indexes */ Bitmapset *rd_keyattr; /* cols that can be ref'd by foreign keys */ Bitmapset *rd_pkattr; /* cols included in primary key */ Bitmapset *rd_idattr; /* included in replica identity index */ Bitmapset *rd_projidx; /* Oids of projection indexes */ PublicationActions *rd_pubactions; /* publication actions */ /* * rd_options is set whenever rd_rel is loaded into the relcache entry. * Note that you can NOT look into rd_rel for this data. NULL means "use * defaults". */ bytea *rd_options; /* parsed pg_class.reloptions */ /* These are non-NULL only for an index relation: */ Form_pg_index rd_index; /* pg_index tuple describing this index */ /* use "struct" here to avoid needing to include htup.h: */ struct HeapTupleData *rd_indextuple; /* all of pg_index tuple */ /* * index access support info (used only for an index relation) * * Note: only default support procs for each opclass are cached, namely * those with lefttype and righttype equal to the opclass's opcintype. The * arrays are indexed by support function number, which is a sufficient * identifier given that restriction. * * Note: rd_amcache is available for index AMs to cache private data about * an index. This must be just a cache since it may get reset at any time * (in particular, it will get reset by a relcache inval message for the * index). If used, it must point to a single memory chunk palloc'd in * rd_indexcxt. A relcache reset will include freeing that chunk and * setting rd_amcache = NULL. */ Oid rd_amhandler; /* OID of index AM's handler function */ MemoryContext rd_indexcxt; /* private memory cxt for this stuff */ /* use "struct" here to avoid needing to include amapi.h: */ struct IndexAmRoutine *rd_amroutine; /* index AM's API struct */ Oid *rd_opfamily; /* OIDs of op families for each index col */ Oid *rd_opcintype; /* OIDs of opclass declared input data types */ RegProcedure *rd_support; /* OIDs of support procedures */ FmgrInfo *rd_supportinfo; /* lookup info for support procedures */ int16 *rd_indoption; /* per-column AM-specific flags */ List *rd_indexprs; /* index expression trees, if any */ List *rd_indpred; /* index predicate tree, if any */ Oid *rd_exclops; /* OIDs of exclusion operators, if any */ Oid *rd_exclprocs; /* OIDs of exclusion ops' procs, if any */ uint16 *rd_exclstrats; /* exclusion ops' strategy numbers, if any */ void *rd_amcache; /* available for use by index AM */ Oid *rd_indcollation; /* OIDs of index collations */ /* * foreign-table support * * rd_fdwroutine must point to a single memory chunk palloc'd in * CacheMemoryContext. It will be freed and reset to NULL on a relcache * reset. */ /* use "struct" here to avoid needing to include fdwapi.h: */ struct FdwRoutine *rd_fdwroutine; /* cached function pointers, or NULL */ /* * Hack for CLUSTER, rewriting ALTER TABLE, etc: when writing a new * version of a table, we need to make any toast pointers inserted into it * have the existing toast table's OID, not the OID of the transient toast * table. If rd_toastoid isn't InvalidOid, it is the OID to place in * toast pointers inserted into this rel. (Note it's set on the new * version of the main heap, not the toast table itself.) This also * causes toast_save_datum() to try to preserve toast value OIDs. */ Oid rd_toastoid; /* Real TOAST table's OID, or InvalidOid */ /* use "struct" here to avoid needing to include pgstat.h: */ struct PgStat_TableStatus *pgstat_info; /* statistics collection area */} RelationData;typedef struct RelationData *Relation;

BufferAccessStrategy
buffer访问策略

/* * Buffer identifiers. * Buffer标识符 * * Zero is invalid, positive is the index of a shared buffer (1..NBuffers), * negative is the index of a local buffer (-1 .. -NLocBuffer). * 0表示无效,正整数表示共享buffer的索引(1..N), * 负数是本地buffer的索引(-1..-N) */typedef int Buffer;#define InvalidBuffer 0/* * Buffer access strategy objects. * Buffer访问策略对象 * * BufferAccessStrategyData is private to freelist.c * BufferAccessStrategyData对freelist.c来说是私有的 */typedef struct BufferAccessStrategyData *BufferAccessStrategy; /* * Private (non-shared) state for managing a ring of shared buffers to re-use. * This is currently the only kind of BufferAccessStrategy object, but someday * we might have more kinds. * 私有状态,用于管理可重用的环形缓冲区. * 目前只有这么一种缓冲区访问策略对象,但未来某一天可以拥有更多. */typedef struct BufferAccessStrategyData{ /* Overall strategy type */ //全局的策略类型 BufferAccessStrategyType btype; /* Number of elements in buffers[] array */ //buffers[]中的元素个数 int ring_size; /* * Index of the "current" slot in the ring, ie, the one most recently * returned by GetBufferFromRing. * 环形缓冲区中当前slot的索引,最近访问的通过函数GetBufferFromRing返回. */ int current; /* * True if the buffer just returned by StrategyGetBuffer had been in the * ring already. * 如正好通过StrategyGetBuffer返回的buffer已在环形缓冲区中,则返回T */ bool current_was_in_ring; /* * Array of buffer numbers. InvalidBuffer (that is, zero) indicates we * have not yet selected a buffer for this ring slot. For allocation * simplicity this is palloc'd together with the fixed fields of the * struct. * buffer编号数组. * InvalidBuffer(即:0)表示我们还没有为该slot选择buffer. * 为了分配的简单性，这是palloc'd与结构的固定字段。 */ Buffer buffers[FLEXIBLE_ARRAY_MEMBER];} BufferAccessStrategyData;//Block结构体指针typedef void *Block;/* Possible arguments for GetAccessStrategy() *///GetAccessStrategy()函数可取值的参数typedef enum BufferAccessStrategyType{ //常规的随机访问 BAS_NORMAL, /* Normal random access */ //大规模的只读扫描 BAS_BULKREAD, /* Large read-only scan (hint bit updates are * ok) */ //大量的多块写(如 COPY IN) BAS_BULKWRITE, /* Large multi-block write (e.g. COPY IN) */ //VACUUM BAS_VACUUM /* VACUUM */} BufferAccessStrategyType;

ReadBufferMode
ReadBufferExtended函数所可能使用的读取模式.

/* * In RBM_NORMAL mode, the page is read from disk, and the page header is * validated. An error is thrown if the page header is not valid. (But * note that an all-zero page is considered "valid"; see PageIsVerified().) * 在RBM_NORMAL模式,page从磁盘中读取,page头部已被验证有效.假如page头部是无效的,那会抛出错误. * (但是需要注意,初始化的page被认为是有效的;详细参见PageIsVerified函数) * * RBM_ZERO_ON_ERROR is like the normal mode, but if the page header is not * valid, the page is zeroed instead of throwing an error. This is intended * for non-critical data, where the caller is prepared to repair errors. * RBM_ZERO_ON_ERROR类似于正常模式,但如果page header是无效的,则初始化page(置0),而不是报错. * 在调用者准备修复错误时,针对非关键数据使用. * * In RBM_ZERO_AND_LOCK mode, if the page isn't in buffer cache already, it's * filled with zeros instead of reading it from disk. Useful when the caller * is going to fill the page from scratch, since this saves I/O and avoids * unnecessary failure if the page-on-disk has corrupt page headers. * The page is returned locked to ensure that the caller has a chance to * initialize the page before it's made visible to others. * Caution: do not use this mode to read a page that is beyond the relation's * current physical EOF; that is likely to cause problems in md.c when * the page is modified and written out. P_NEW is OK, though. * 在RBM_ZERO_AND_LOCK模式,如果page还没有处于buffer cache,填充0而不是从磁盘中读取. * 在调用者从scratch填充page时使用,因为这样可以节省I/O并避免不必要的page-on-disk的header错误. * page会被锁定并返回,确保在page可见前由调用者初始化此page. * 特别注意:不要在在关系文件标记位(EOF)后使用这种模式读取page,这会在md.c中,修改page并写出该page后出现问题. * 但是,P_NEW是可以的. * * RBM_ZERO_AND_CLEANUP_LOCK is the same as RBM_ZERO_AND_LOCK, but acquires * a cleanup-strength lock on the page. * RBM_ZERO_AND_CLEANUP_LOCK模式与RBM_ZERO_AND_LOCK模式类似,但在page上请求cleanup-strength lock. * * RBM_NORMAL_NO_LOG mode is treated the same as RBM_NORMAL here. * RBM_NORMAL_NO_LOG模式与RBM_NORMAL一致. *//* Possible modes for ReadBufferExtended() */typedef enum{ RBM_NORMAL, /* Normal read */ RBM_ZERO_AND_LOCK, /* Don't read from disk, caller will * initialize. Also locks the page. */ RBM_ZERO_AND_CLEANUP_LOCK, /* Like RBM_ZERO_AND_LOCK, but locks the page * in "cleanup" mode */ RBM_ZERO_ON_ERROR, /* Read, but return an all-zeros page on error */ RBM_NORMAL_NO_LOG /* Don't log page as invalid during WAL * replay; otherwise same as RBM_NORMAL */} ReadBufferMode;二、源码解读

ReadBufferExtended返回对应请求关系数据块的buffer,实现逻辑比较简单,详见代码.
主要的实现逻辑在ReadBuffer_common中,该函数后续再行介绍.

/* * ReadBufferExtended -- returns a buffer containing the requested * block of the requested relation. If the blknum * requested is P_NEW, extend the relation file and * allocate a new block. (Caller is responsible for * ensuring that only one backend tries to extend a * relation at the same time!) * ReadBufferExtended -- 返回对应请求关系数据块的buffer. * 如果blknum是P_NEW,则扩展关系文件并分配新块. * (调用者有责任确保只有一个后台进程在同一时刻尝试扩展关系) * * Returns: the buffer number for the buffer containing * the block read. The returned buffer has been pinned. * Does not return on error --- elog's instead. * 返回:对应block的buffer编号.返回的buffer已被pinned.不需要返回错误,因为elog已进行处理. * * Assume when this function is called, that reln has been opened already. * 假定调用该函数时,关系reln已被打开. * * In RBM_NORMAL mode, the page is read from disk, and the page header is * validated. An error is thrown if the page header is not valid. (But * note that an all-zero page is considered "valid"; see PageIsVerified().) * 在RBM_NORMAL模式,page从磁盘中读取,page头部已被验证有效.假如page头部是无效的,那会抛出错误. * (但是需要注意,初始化的page被认为是有效的;详细参见PageIsVerified函数) * * RBM_ZERO_ON_ERROR is like the normal mode, but if the page header is not * valid, the page is zeroed instead of throwing an error. This is intended * for non-critical data, where the caller is prepared to repair errors. * RBM_ZERO_ON_ERROR类似于正常模式,但如果page header是无效的,则初始化page(置0),而不是报错. * 在调用者准备修复错误时,针对非关键数据使用. * * In RBM_ZERO_AND_LOCK mode, if the page isn't in buffer cache already, it's * filled with zeros instead of reading it from disk. Useful when the caller * is going to fill the page from scratch, since this saves I/O and avoids * unnecessary failure if the page-on-disk has corrupt page headers. * The page is returned locked to ensure that the caller has a chance to * initialize the page before it's made visible to others. * Caution: do not use this mode to read a page that is beyond the relation's * current physical EOF; that is likely to cause problems in md.c when * the page is modified and written out. P_NEW is OK, though. * 在RBM_ZERO_AND_LOCK模式,如果page还没有处于buffer cache,填充0而不是从磁盘中读取. * 在调用者从scratch填充page时使用,因为这样可以节省I/O并避免不必要的page-on-disk的header错误. * page会被锁定并返回,确保在page可见前由调用者初始化此page. * 特别注意:不要在在关系文件标记位(EOF)后使用这种模式读取page,这会在md.c中,修改page并写出该page后出现问题. * 但是,P_NEW是可以的. * * RBM_ZERO_AND_CLEANUP_LOCK is the same as RBM_ZERO_AND_LOCK, but acquires * a cleanup-strength lock on the page. * RBM_ZERO_AND_CLEANUP_LOCK模式与RBM_ZERO_AND_LOCK模式类似,但在page上请求cleanup-strength lock. * * RBM_NORMAL_NO_LOG mode is treated the same as RBM_NORMAL here. * RBM_NORMAL_NO_LOG模式与RBM_NORMAL一致. * * If strategy is not NULL, a nondefault buffer access strategy is used. * See buffer/README for details. * 如strategy非空,则使用非默认的buffer访问策略.详细参见buffer/README. */BufferReadBufferExtended(Relation reln, ForkNumber forkNum, BlockNumber blockNum, ReadBufferMode mode, BufferAccessStrategy strategy){ bool hit; Buffer buf; /* Open it at the smgr level if not already done */ //打开relation,级别为smgr RelationOpenSmgr(reln); /* * Reject attempts to read non-local temporary relations; we would be * likely to get wrong data since we have no visibility into the owning * session's local buffers. * 拒绝尝试访问非本地临时relations. * 由于没有自己会话的本地缓存可见信息,因此读取临时表会得到错误的数据. */ if (RELATION_IS_OTHER_TEMP(reln)) ereport(ERROR, (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("cannot access temporary tables of other sessions"))); /* * Read the buffer, and update pgstat counters to reflect a cache hit or * miss. * 调用ReadBuffer_common读取buffer,更新pgstat计数器以反映命中还是缺失. */ pgstat_count_buffer_read(reln); buf = ReadBuffer_common(reln->rd_smgr, reln->rd_rel->relpersistence, forkNum, blockNum, mode, strategy, &hit); if (hit) pgstat_count_buffer_hit(reln); return buf;}三、跟踪分析

使用bt查看调用栈

(gdb) bt#0 ReadBufferExtended (reln=0x7f497fe72788, forkNum=MAIN_FORKNUM, blockNum=0, mode=RBM_NORMAL, strategy=0x0) at bufmgr.c:647#1 0x00000000004d974f in heapgetpage (scan=0x1d969d8, page=0) at heapam.c:379#2 0x00000000004daeb2 in heapgettup_pagemode (scan=0x1d969d8, dir=ForwardScanDirection, nkeys=0, key=0x0) at heapam.c:837#3 0x00000000004dcf2b in heap_getnext (scan=0x1d969d8, direction=ForwardScanDirection) at heapam.c:1842#4 0x000000000070ec39 in SeqNext (node=0x1d95890) at nodeSeqscan.c:80#5 0x00000000006e0ab0 in ExecScanFetch (node=0x1d95890, accessMtd=0x70eba9 <SeqNext>, recheckMtd=0x70ec74 <SeqRecheck>) at execScan.c:95#6 0x00000000006e0b22 in ExecScan (node=0x1d95890, accessMtd=0x70eba9 <SeqNext>, recheckMtd=0x70ec74 <SeqRecheck>) at execScan.c:145#7 0x000000000070ecbe in ExecSeqScan (pstate=0x1d95890) at nodeSeqscan.c:129#8 0x00000000006dee2a in ExecProcNodeFirst (node=0x1d95890) at execProcnode.c:445#9 0x00000000007021b8 in ExecProcNode (node=0x1d95890) at ../../../src/include/executor/executor.h:237#10 0x00000000007022dd in ExecLimit (pstate=0x1d95680) at nodeLimit.c:95#11 0x00000000006dee2a in ExecProcNodeFirst (node=0x1d95680) at execProcnode.c:445#12 0x00000000006d3d8d in ExecProcNode (node=0x1d95680) at ../../../src/include/executor/executor.h:237#13 0x00000000006d65c5 in ExecutePlan (estate=0x1d95468, planstate=0x1d95680, use_parallel_mode=false, operation=CMD_SELECT, sendTuples=true, numberTuples=0, direction=ForwardScanDirection, dest=0x1d00ea8, execute_once=true) at execMain.c:1723#14 0x00000000006d4357 in standard_ExecutorRun (queryDesc=0x1cfdc28, direction=ForwardScanDirection, count=0, execute_once=true) at execMain.c:364#15 0x00000000006d417f in ExecutorRun (queryDesc=0x1cfdc28, direction=ForwardScanDirection, count=0, execute_once=true) at execMain.c:307#16 0x00000000008bffd4 in PortalRunSelect (portal=0x1d3ebf8, forward=true, count=0, dest=0x1d00ea8) at pquery.c:932#17 0x00000000008bfc72 in PortalRun (portal=0x1d3ebf8, count=9223372036854775807, isTopLevel=true, run_once=true, dest=0x1d00ea8, altdest=0x1d00ea8, completionTag=0x7ffc1fc513d0 "") at pquery.c:773#18 0x00000000008b9cd4 in exec_simple_query (query_string=0x1cd8ec8 "select * from t1 limit 10;") at postgres.c:1145---Type <return> to continue, or q <return> to quit---#19 0x00000000008bdf5f in PostgresMain (argc=1, argv=0x1d05278, dbname=0x1d050e0 "testdb", username=0x1cd5ba8 "xdb") at postgres.c:4182#20 0x000000000081c16d in BackendRun (port=0x1cfae00) at postmaster.c:4361#21 0x000000000081b8e0 in BackendStartup (port=0x1cfae00) at postmaster.c:4033#22 0x0000000000817cda in ServerLoop () at postmaster.c:1706#23 0x0000000000817590 in PostmasterMain (argc=1, argv=0x1cd3b60) at postmaster.c:1379#24 0x0000000000741003 in main (argc=1, argv=0x1cd3b60) at main.c:228(gdb)

逻辑较为简单,这里不再详细跟踪.

PG Source Code