PostgreSQL中mdread函数有什么作用

本篇内容主要讲解"PostgreSQL中mdread函数有什么作用"，感兴趣的朋友不妨来看看。本文介绍的方法操作简单快捷，实用性强。下面就让小编来带大家学习"PostgreSQL中mdread函数有什么作用"吧!

PostgreSQL存储管理的mdread函数是magnetic disk存储管理中负责读取的函数.

一、数据结构

smgrsw
f_smgr函数指针结构体定义了独立的存储管理模块和smgr.c之间的API函数.
md是magnetic disk的缩写.
除了md,先前PG还支持Sony WORM optical disk jukebox and persistent main memory这两种存储方式,
但在后面只剩下magnetic disk,其余的已被废弃不再支持.
"magnetic disk"本身的名称也存在误导,实际上md可以支持操作系统提供标准文件系统的任何类型的设备.

 /* * This struct of function pointers defines the API between smgr.c and * any individual storage manager module.  Note that smgr subfunctions are * generally expected to report problems via elog(ERROR).  An exception is * that smgr_unlink should use elog(WARNING), rather than erroring out, * because we normally unlink relations during post-commit/abort cleanup, * and so it's too late to raise an error.  Also, various conditions that * would normally be errors should be allowed during bootstrap and/or WAL * recovery --- see comments in md.c for details. * 函数指针结构体定义了独立的存储管理模块和smgr.c之间的API函数. * 注意smgr子函数通常会通过elog(ERROR)报告错误. * 其中一个例外是smgr_unlink应该使用elog(WARNING),而不是把错误抛出, *   因为通过来说在事务提交/回滚清理期间才会解链接(unlinke)关系, *   因此这时候抛出错误就显得太晚了. * 同时,在bootstrap和/或WAL恢复期间,各种可能会出现错误的情况也应被允许 --- 详细可查看md.c中的注释. */typedef struct f_smgr{    void        (*smgr_init) (void);    /* may be NULL */    void        (*smgr_shutdown) (void);    /* may be NULL */    void        (*smgr_close) (SMgrRelation reln, ForkNumber forknum);    void        (*smgr_create) (SMgrRelation reln, ForkNumber forknum,                                bool isRedo);    bool        (*smgr_exists) (SMgrRelation reln, ForkNumber forknum);    void        (*smgr_unlink) (RelFileNodeBackend rnode, ForkNumber forknum,                                bool isRedo);    void        (*smgr_extend) (SMgrRelation reln, ForkNumber forknum,                                BlockNumber blocknum, char *buffer, bool skipFsync);    void        (*smgr_prefetch) (SMgrRelation reln, ForkNumber forknum,                                  BlockNumber blocknum);    void        (*smgr_read) (SMgrRelation reln, ForkNumber forknum,                              BlockNumber blocknum, char *buffer);    void        (*smgr_write) (SMgrRelation reln, ForkNumber forknum,                               BlockNumber blocknum, char *buffer, bool skipFsync);    void        (*smgr_writeback) (SMgrRelation reln, ForkNumber forknum,                                   BlockNumber blocknum, BlockNumber nblocks);    BlockNumber (*smgr_nblocks) (SMgrRelation reln, ForkNumber forknum);    void        (*smgr_truncate) (SMgrRelation reln, ForkNumber forknum,                                  BlockNumber nblocks);    void        (*smgr_immedsync) (SMgrRelation reln, ForkNumber forknum);    void        (*smgr_pre_ckpt) (void);    /* may be NULL */    void        (*smgr_sync) (void);    /* may be NULL */    void        (*smgr_post_ckpt) (void);   /* may be NULL */} f_smgr;/*md是magnetic disk的缩写.除了md,先前PG还支持Sony WORM optical disk jukebox and persistent main memory这两种存储方式,但在后面只剩下magnetic disk,其余的已被废弃不再支持."magnetic disk"本身的名称也存在误导,实际上md可以支持操作系统提供标准文件系统的任何类型的设备.*/static const f_smgr smgrsw[] = {    /* magnetic disk */    {        .smgr_init = mdinit,        .smgr_shutdown = NULL,        .smgr_close = mdclose,        .smgr_create = mdcreate,        .smgr_exists = mdexists,        .smgr_unlink = mdunlink,        .smgr_extend = mdextend,        .smgr_prefetch = mdprefetch,        .smgr_read = mdread,        .smgr_write = mdwrite,        .smgr_writeback = mdwriteback,        .smgr_nblocks = mdnblocks,        .smgr_truncate = mdtruncate,        .smgr_immedsync = mdimmedsync,        .smgr_pre_ckpt = mdpreckpt,        .smgr_sync = mdsync,        .smgr_post_ckpt = mdpostckpt    }};

MdfdVec
magnetic disk存储管理在自己的描述符池中跟踪打开的文件描述符.
之所以这样做是因为便于支持超过os文件大小上限(通常是2GB)的关系.
为了达到这个目的,我们拆分关系为多个比OS文件大小上限要小的"segment"文件.
段大小通过pg_config.h中定义的RELSEG_SIZE配置参数设置.

/* *  The magnetic disk storage manager keeps track of open file *  descriptors in its own descriptor pool.  This is done to make it *  easier to support relations that are larger than the operating *  system's file size limit (often 2GBytes).  In order to do that, *  we break relations up into "segment" files that are each shorter than *  the OS file size limit.  The segment size is set by the RELSEG_SIZE *  configuration constant in pg_config.h. *  magnetic disk存储管理在自己的描述符池中跟踪打开的文件描述符. *  之所以这样做是因为便于支持超过os文件大小上限(通常是2GB)的关系. *  为了达到这个目的,我们拆分关系为多个比OS文件大小上限要小的"segment"文件. *  段大小通过pg_config.h中定义的RELSEG_SIZE配置参数设置. * *  On disk, a relation must consist of consecutively numbered segment *  files in the pattern *      -- Zero or more full segments of exactly RELSEG_SIZE blocks each *      -- Exactly one partial segment of size 0 <= size < RELSEG_SIZE blocks *      -- Optionally, any number of inactive segments of size 0 blocks. *  The full and partial segments are collectively the "active" segments. *  Inactive segments are those that once contained data but are currently *  not needed because of an mdtruncate() operation.  The reason for leaving *  them present at size zero, rather than unlinking them, is that other *  backends and/or the checkpointer might be holding open file references to *  such segments.  If the relation expands again after mdtruncate(), such *  that a deactivated segment becomes active again, it is important that *  such file references still be valid --- else data might get written *  out to an unlinked old copy of a segment file that will eventually *  disappear. *  在磁盘上,关系必须由按照某种模式连续编号的segment files组成. *    -- 每个RELSEG_SIZE块的另段或多个完整段 *    -- 大小满足0 <= size < RELSEG_SIZE blocks的一个部分段 *    -- 可选的,大小为0 blocks的N个非活动段 *  完整和部分段统称为活动段.非活动段指的是哪些因为mdtruncate()操作而出现的包含数据但目前不需要的. *  保留这些大小为0的非活动段而不是unlinking的原因是其他进程和/或checkpointer进程可能 *    持有这些段的文件依赖. *  如果关系在mdtruncate()之后再次扩展了,这样一个无效的会重新变为活动段, *    因此文件依赖仍然保持有效是很重要的  *    --- 否则数据可能写出到未经链接的旧segment file拷贝上,会时不时的出现数据丢失. * *  File descriptors are stored in the per-fork md_seg_fds arrays inside *  SMgrRelation. The length of these arrays is stored in md_num_open_segs. *  Note that a fork's md_num_open_segs having a specific value does not *  necessarily mean the relation doesn't have additional segments; we may *  just not have opened the next segment yet.  (We could not have "all *  segments are in the array" as an invariant anyway, since another backend *  could extend the relation while we aren't looking.)  We do not have *  entries for inactive segments, however; as soon as we find a partial *  segment, we assume that any subsequent segments are inactive. *  文件描述符在SMgrRelation中的per-fork md_seg_fds数组存储. *  这些数组的长度存储在md_num_open_segs中. *  注意一个fork的md_num_open_segs有一个特定值并不必要意味着关系不能有额外的段, *    我们只是还没有打开下一个段而已. *  (但不管怎样,我们不可能把"所有段都放在数组中"作为一个不变式看待, *   因为其他后台进程在尚未检索时已经扩展了关系) *  但是,我们不需要持有非活动段的条目,只要我们一旦发现部分段,那么就可以假定接下来的段是非活动的. * *  The entire MdfdVec array is palloc'd in the MdCxt memory context. *  整个MdfdVec数组通过palloc在MdCxt内存上下文中分配. */typedef struct _MdfdVec{    //文件描述符池中该文件的编号    File        mdfd_vfd;       /* fd number in fd.c's pool */    //段号,从0起算    BlockNumber mdfd_segno;     /* segment number, from 0 */} MdfdVec;

二、源码解读

mdread() - 从relation中读取相应的block.
源码较为简单,主要是调用FileRead函数执行实际的读取操作.

/* *  mdread() -- Read the specified block from a relation. *  mdread() -- 从relation中读取相应的block */voidmdread(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,       char *buffer){    off_t       seekpos;//seek的位置    int         nbytes;//bytes    MdfdVec    *v;//md文件描述符向量数组    TRACE_POSTGRESQL_SMGR_MD_READ_START(forknum, blocknum,                                        reln->smgr_rnode.node.spcNode,                                        reln->smgr_rnode.node.dbNode,                                        reln->smgr_rnode.node.relNode,                                        reln->smgr_rnode.backend);    //获取向量数组    v = _mdfd_getseg(reln, forknum, blocknum, false,                     EXTENSION_FAIL | EXTENSION_CREATE_RECOVERY);    //获取block偏移    seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));    //验证    Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);    //读取文件,读入buffer中,返回读取的字节数    nbytes = FileRead(v->mdfd_vfd, buffer, BLCKSZ, seekpos, WAIT_EVENT_DATA_FILE_READ);    //跟踪    TRACE_POSTGRESQL_SMGR_MD_READ_DONE(forknum, blocknum,                                       reln->smgr_rnode.node.spcNode,                                       reln->smgr_rnode.node.dbNode,                                       reln->smgr_rnode.node.relNode,                                       reln->smgr_rnode.backend,                                       nbytes,                                       BLCKSZ);    if (nbytes != BLCKSZ)    {        //读取的字节数不等于块大小,报错        if (nbytes < 0)            ereport(ERROR,                    (errcode_for_file_access(),                     errmsg("could not read block %u in file \"%s\": %m",                            blocknum, FilePathName(v->mdfd_vfd))));        /*         * Short read: we are at or past EOF, or we read a partial block at         * EOF.  Normally this is an error; upper levels should never try to         * read a nonexistent block.  However, if zero_damaged_pages is ON or         * we are InRecovery, we should instead return zeroes without         * complaining.  This allows, for example, the case of trying to         * update a block that was later truncated away.         * Short read:处于EOF或者在EOF之后,或者在EOF处读取了一个部分块.         * 通常来说,这是一个错误,高层代码不应尝试读取一个不存在的block.         *  但是,如果zero_damaged_pages参数设置为ON或者处于InRecovery状态,那么应该返回0而不报错.         * 比如,这可以允许尝试更新一个块但随后就给截断的情况.         */        if (zero_damaged_pages || InRecovery)            MemSet(buffer, 0, BLCKSZ);        else            ereport(ERROR,                    (errcode(ERRCODE_DATA_CORRUPTED),                     errmsg("could not read block %u in file \"%s\": read only %d of %d bytes",                            blocknum, FilePathName(v->mdfd_vfd),                            nbytes, BLCKSZ)));    }}

三、跟踪分析

测试脚本

11:15:11 (xdb@[local]:5432)testdb=# insert into t1(id) select generate_series(100,500);

启动gdb,跟踪
查看调用栈

(gdb) b mdreadBreakpoint 3 at 0x8b669b: file md.c, line 738.(gdb) cContinuing.Breakpoint 3, mdread (reln=0x2d09be0, forknum=MAIN_FORKNUM, blocknum=50, buffer=0x7f3823369c00 "") at md.c:738738     TRACE_POSTGRESQL_SMGR_MD_READ_START(forknum, blocknum,(gdb) bt#0  mdread (reln=0x2d09be0, forknum=MAIN_FORKNUM, blocknum=50, buffer=0x7f3823369c00 "") at md.c:738#1  0x00000000008b92d5 in smgrread (reln=0x2d09be0, forknum=MAIN_FORKNUM, blocknum=50, buffer=0x7f3823369c00 "")    at smgr.c:628#2  0x00000000008793f9 in ReadBuffer_common (smgr=0x2d09be0, relpersistence=112 'p', forkNum=MAIN_FORKNUM, blockNum=50,     mode=RBM_NORMAL, strategy=0x0, hit=0x7ffd5fb2948b) at bufmgr.c:890#3  0x0000000000878cd4 in ReadBufferExtended (reln=0x7f3836e1e788, forkNum=MAIN_FORKNUM, blockNum=50, mode=RBM_NORMAL,     strategy=0x0) at bufmgr.c:664#4  0x0000000000878bb1 in ReadBuffer (reln=0x7f3836e1e788, blockNum=50) at bufmgr.c:596#5  0x00000000004eeb96 in ReadBufferBI (relation=0x7f3836e1e788, targetBlock=50, bistate=0x0) at hio.c:87#6  0x00000000004ef387 in RelationGetBufferForTuple (relation=0x7f3836e1e788, len=32, otherBuffer=0, options=0,     bistate=0x0, vmbuffer=0x7ffd5fb295ec, vmbuffer_other=0x0) at hio.c:415#7  0x00000000004df1f8 in heap_insert (relation=0x7f3836e1e788, tup=0x2ca6770, cid=0, options=0, bistate=0x0)    at heapam.c:2468#8  0x0000000000709dda in ExecInsert (mtstate=0x2ca4c40, slot=0x2ca3418, planSlot=0x2ca3418, estate=0x2ca48d8,     canSetTag=true) at nodeModifyTable.c:529#9  0x000000000070c475 in ExecModifyTable (pstate=0x2ca4c40) at nodeModifyTable.c:2159#10 0x00000000006e05cb in ExecProcNodeFirst (node=0x2ca4c40) at execProcnode.c:445#11 0x00000000006d552e in ExecProcNode (node=0x2ca4c40) at ../../../src/include/executor/executor.h:247#12 0x00000000006d7d66 in ExecutePlan (estate=0x2ca48d8, planstate=0x2ca4c40, use_parallel_mode=false,     operation=CMD_INSERT, sendTuples=false, numberTuples=0, direction=ForwardScanDirection, dest=0x2d41a30,     execute_once=true) at execMain.c:1723#13 0x00000000006d5af8 in standard_ExecutorRun (queryDesc=0x2ca24b8, direction=ForwardScanDirection, count=0,     execute_once=true) at execMain.c:364#14 0x00000000006d5920 in ExecutorRun (queryDesc=0x2ca24b8, direction=ForwardScanDirection, count=0, execute_once=true)    at execMain.c:307#15 0x00000000008c1092 in ProcessQuery (plan=0x2d418b8,     sourceText=0x2c7eec8 "insert into t1(id) select generate_series(100,500);", params=0x0, queryEnv=0x0, dest=0x2d41a30, ---Type  to continue, or q  to quit---    completionTag=0x7ffd5fb29b80 "") at pquery.c:161#16 0x00000000008c29a1 in PortalRunMulti (portal=0x2ce4488, isTopLevel=true, setHoldSnapshot=false, dest=0x2d41a30,     altdest=0x2d41a30, completionTag=0x7ffd5fb29b80 "") at pquery.c:1286#17 0x00000000008c1f7a in PortalRun (portal=0x2ce4488, count=9223372036854775807, isTopLevel=true, run_once=true,     dest=0x2d41a30, altdest=0x2d41a30, completionTag=0x7ffd5fb29b80 "") at pquery.c:799#18 0x00000000008bbf16 in exec_simple_query (query_string=0x2c7eec8 "insert into t1(id) select generate_series(100,500);")    at postgres.c:1145#19 0x00000000008c01a1 in PostgresMain (argc=1, argv=0x2ca8af8, dbname=0x2ca8960 "testdb", username=0x2c7bba8 "xdb")    at postgres.c:4182#20 0x000000000081e07c in BackendRun (port=0x2ca0940) at postmaster.c:4361#21 0x000000000081d7ef in BackendStartup (port=0x2ca0940) at postmaster.c:4033#22 0x0000000000819be9 in ServerLoop () at postmaster.c:1706#23 0x000000000081949f in PostmasterMain (argc=1, argv=0x2c79b60) at postmaster.c:1379#24 0x0000000000742941 in main (argc=1, argv=0x2c79b60) at main.c:228(gdb)

获取读取的偏移

(gdb) n744     v = _mdfd_getseg(reln, forknum, blocknum, false,(gdb) 747     seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));(gdb) p *v$1 = {mdfd_vfd = 26, mdfd_segno = 0}(gdb) p BLCKSZ$2 = 8192(gdb) p blocknum$3 = 50(gdb) p RELSEG_SIZE$4 = 131072(gdb) n749     Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);(gdb) p seekpos$5 = 409600(gdb)

执行读取操作

(gdb) n751     if (FileSeek(v->mdfd_vfd, seekpos, SEEK_SET) != seekpos)(gdb) 757     nbytes = FileRead(v->mdfd_vfd, buffer, BLCKSZ, WAIT_EVENT_DATA_FILE_READ);(gdb) 759     TRACE_POSTGRESQL_SMGR_MD_READ_DONE(forknum, blocknum,(gdb) p nbytes$6 = 8192(gdb) p *buffer$7 = 1 '\001'(gdb) n767     if (nbytes != BLCKSZ)(gdb) 792 }(gdb) smgrread (reln=0x2d09be0, forknum=MAIN_FORKNUM, blocknum=50, buffer=0x7f3823369c00 "\001") at smgr.c:629629 }(gdb)

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