PostgreSQL启动恢复过程中日志源的切换
日志源会在XLOG_FROM_ARCHIVE->XLOG_FROM_STREAM->XLOG_FROM_ARCHIVE直接切换,只有读取过程中出错,就会切换到另外一个日志源。但实际执行过程中,XLOG_FROM_ARCHIVE出错后会到XLOG_FROM_PG_WAL读取,但是日志源的变量并不会改变。这个需要注意。
static int XLogFileReadAnyTLI(XLogSegNo segno, int emode, int source) { if (expectedTLEs) tles = expectedTLEs; else tles = readTimeLineHistory(recoveryTargetTLI); /* 1、WaitForWALToBecomeAvailable调用时,source是XLOG_FROM_ARCHIVE时,会使用XLOG_FROM_ANY 2、使用XLOG_FROM_ANY,会首先从归档中读取xlog,如果open失败,则会使用XLOG_FROM_PG_WAL 3、外部日志源变量并没有切换 */ foreach(cell, tles){ TimeLineID tli = ((TimeLineHistoryEntry *) lfirst(cell))->tli; if (tli < curFileTLI) break; /* don't bother looking at too-old TLIs */ if (source == XLOG_FROM_ANY || source == XLOG_FROM_ARCHIVE){ fd = XLogFileRead(segno, emode, tli,XLOG_FROM_ARCHIVE, true); if (fd != -1){ if (!expectedTLEs) expectedTLEs = tles; return fd; } } if (source == XLOG_FROM_ANY || source == XLOG_FROM_PG_WAL){ fd = XLogFileRead(segno, emode, tli, XLOG_FROM_PG_WAL, true); if (fd != -1){ if (!expectedTLEs) expectedTLEs = tles; return fd; } } } return -1; }