接前面,由于看到对 BasicOpenFile 函数的调用。
自然想到,如果两个进程同时访问一个表(即同一文件),会否有冲突或效率的问题。
开两个psql客户端,连接数据库后,后台生成两个进程,
分别运行 select * from tst01 进行观察...发现各进程之间互相不干扰。
我实验的方法,加入调试代码:
/* * BasicOpenFile --- same as open(2) except can free other FDs if needed * * This is exported for use by places that really want a plain kernel FD, * but need to be proof against running out of FDs. Once an FD has been * successfully returned, it is the caller's responsibility to ensure that * it will not be leaked on ereport()! Most users should *not* call this * routine directly, but instead use the VFD abstraction level, which * provides protection against descriptor leaks as well as management of * files that need to be open for more than a short period of time. * * Ideally this should be the *only* direct call of open() in the backend. * In practice, the postmaster calls open() directly, and there are some * direct open() calls done early in backend startup. Those are OK since * this module wouldn't have any open files to close at that point anyway. */ int BasicOpenFile(FileName fileName, int fileFlags, int fileMode) { int fd; //char *tst01=(char *)fileName; char *tst02="base/12788/16384"; //tst01 table's file is base/12788/16384 tryAgain: fd = open(fileName, fileFlags, fileMode); char *tst01=(char *)fileName; fprintf(stderr,"length of fileName is:%d\n",strlen(fileName)); if ( strcmp( fileName, tst02) == 0) { fprintf(stderr,"Sleeping.....by Process:%d\n",getpid()); sleep(120); } //fd = open(fileName, fileFlags, fileMode); if (fd >= 0) return fd; /* success! */ if (errno == EMFILE || errno == ENFILE) { int save_errno = errno; ereport(LOG, (errcode(ERRCODE_INSUFFICIENT_RESOURCES), errmsg("out of file descriptors: %m; release and retry"))); errno = 0; if (ReleaseLruFile()) goto tryAgain; errno = save_errno; } return -1; /* failure */ }
那么如果同时写入的时候又会如何,这个问题还需进一步地观察代码。
至少,不同的进程同时读一个表,在文件IO级别,没有发生阻塞。