数据库内核月报 - 2015 / 12

PgSQL · 特性分析 · 备库激活过程分析

前言

PostgreSQL standby 可以通过两种方法来激活成为主库:

  1. trigger file,配置在recovery.conf中。
  2. pg_ctl promote发送SIGUSR1信号给postmaster进程。

同时,PostgreSQL支持快速激活(fast promote)和非快速激活(fallback promote):

  1. fast promote 开启数据库读写前,不需要做检查点。而是推到开启读写之后执行一个CHECKPOINT_FORCE检查点。
  2. fallback_promote 在开启数据库读写前,需要先做一个检查点,现在这个模式已经不对用户开放,需要修改代码,只是用作调试。

实现分析

激活数据库的代码分析如下:

激活过程,根据fast_promote变量判断是否需要先做检查点,再激活。

src/backend/access/transam/xlog.c
        if (InRecovery)
        {
                if (bgwriterLaunched)
                {
                        if (fast_promote)  // 如果是快速promote,在打开数据库读写前,不需要创建检查点。只需要创建一个recovery结束标记
                        {
                                checkPointLoc = ControlFile->prevCheckPoint;

                                record = ReadCheckpointRecord(xlogreader, checkPointLoc, 1, false);
                                if (record != NULL)
                                {
                                        fast_promoted = true;
                                        CreateEndOfRecoveryRecord();
                                }
                        }

                        if (!fast_promoted)  //  如果是fallback_promote模式,须先创建一个检查点,再开启读写模式。
                                RequestCheckpoint(CHECKPOINT_END_OF_RECOVERY |
                                                                  CHECKPOINT_IMMEDIATE |
                                                                  CHECKPOINT_WAIT);
                }
...

        InRecovery = false;    //  开启读写模式,允许接收用户写请求.

        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
        ControlFile->state = DB_IN_PRODUCTION;   //  改写控制文件的数据库状态
        ControlFile->time = (pg_time_t) time(NULL);
        UpdateControlFile();   //  更新控制文件
        LWLockRelease(ControlFileLock);

...
        if (fast_promoted)   //  如果是快速promote,在允许用户写请求后,在这里执行一个检查点。所以提高了数据库的可用时间。
                RequestCheckpoint(CHECKPOINT_FORCE);
......

通过pg_ctl命令行工具,向postmaster发SIGUSR1信号,通知它激活数据库。
首先会写一个promote文件,告诉postmaster,是fast_promote。

src/bin/pg_ctl/pg_ctl.c
/*
 * promote
 */

static void
do_promote(void)
{
        FILE       *prmfile;
        pgpid_t         pid;
        struct stat statbuf;

        pid = get_pgpid(false);

......
        sig = SIGUSR1;
        if (kill((pid_t) pid, sig) != 0)  // 发送SIGUSR1信号,通知postmaster激活数据库。
        {
                write_stderr(_("%s: could not send promote signal (PID: %ld): %s\n"),
                                         progname, pid, strerror(errno));
                if (unlink(promote_file) != 0)
                        write_stderr(_("%s: could not remove promote signal file \"%s\": %s\n"),
                                                 progname, promote_file, strerror(errno));
                exit(1);
        }

        print_msg(_("server promoting\n"));
}

数据恢复时,检查standby是否收到promote请求或是否存在trigger文件。
如果是promote请求,则检查有没有promote文件,或者fallback_promote文件,如果有promote文件,则是fast_promote请求。如果有fallback_promote文件,则不是fast_promote请求(实际上根本不可能检测到fallback_promote文件,因为没有写这个文件的操作)。所以通过pg_ctl promote来激活,一定是fast promote的,即不需要先做检查点再激活。
如果检查到trigger文件,同样也是fast promote激活模式。

src/backend/access/transam/xlog.c
#define PROMOTE_SIGNAL_FILE             "promote"
#define FALLBACK_PROMOTE_SIGNAL_FILE "fallback_promote"

/*
 * Check to see whether the user-specified trigger file exists and whether a
 * promote request has arrived.  If either condition holds, return true.
 */
static bool
CheckForStandbyTrigger(void)
{
        struct stat stat_buf;
        static bool triggered = false;

        if (triggered)
                return true;

        if (IsPromoteTriggered())   // 检查是否收到pg_ctl promote信号
        {
......
                if (stat(PROMOTE_SIGNAL_FILE, &stat_buf) == 0)    // 先检查promote文件是否存在
                {
                        unlink(PROMOTE_SIGNAL_FILE);
                        unlink(FALLBACK_PROMOTE_SIGNAL_FILE);
                        fast_promote = true;    //  快速promote
                }
                else if (stat(FALLBACK_PROMOTE_SIGNAL_FILE, &stat_buf) == 0)  // 否则再检查fallback_promote文件是否存在
                {
                        unlink(FALLBACK_PROMOTE_SIGNAL_FILE);
                        fast_promote = false;    //  先执行checkpoint再promote
                }

                ereport(LOG, (errmsg("received promote request")));

                ResetPromoteTriggered();
                triggered = true;
                return true;
        }

        if (TriggerFile == NULL)   //  检查recovery.conf是否配置了trigger_file
                return false;

        if (stat(TriggerFile, &stat_buf) == 0)
        {
                ereport(LOG,
                                (errmsg("trigger file found: %s", TriggerFile)));
                unlink(TriggerFile);
                triggered = true;
                fast_promote = true;    //   快速promote
                return true;
        }
        else if (errno != ENOENT)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not stat trigger file \"%s\": %m",
                                                TriggerFile)));

        return false;
}

src/backend/postmaster/startup.c
        pqsignal(SIGUSR2, StartupProcTriggerHandler);  // 注册SIGUSR2信号处理函数

/* SIGUSR2: set flag to finish recovery */
static void
StartupProcTriggerHandler(SIGNAL_ARGS)
{
        int                     save_errno = errno;

        promote_triggered = true;
        WakeupRecovery();

        errno = save_errno;
}

bool
IsPromoteTriggered(void)
{
        return promote_triggered;
}

postmaster收到SIGUSER1信号后,检查是否收到promote信号,判断当前的状态是否处于恢复中的任意状态,然后向startup进程发一个SIGUSR2的信号,触发promote。

src/backend/postmaster/postmaster.c

        pqsignal(SIGUSR1, sigusr1_handler); /* message from child process */  // 注册SIGUSR1信号处理函数

/*
 * sigusr1_handler - handle signal conditions from child processes
 */
static void
sigusr1_handler(SIGNAL_ARGS)
{
......
        if (CheckPromoteSignal() && StartupPID != 0 &&
                (pmState == PM_STARTUP || pmState == PM_RECOVERY ||
                 pmState == PM_HOT_STANDBY || pmState == PM_WAIT_READONLY))
        {
                /* Tell startup process to finish recovery */
                signal_child(StartupPID, SIGUSR2);  //  向startup进程发SIGUSR2信号,通知它处理promote
        }
......

src/backend/access/transam/xlog.c
/*
 * Check to see if a promote request has arrived. Should be
 * called by postmaster after receiving SIGUSR1.
 */
bool
CheckPromoteSignal(void)
{
        struct stat stat_buf;

        if (stat(PROMOTE_SIGNAL_FILE, &stat_buf) == 0 ||
                stat(FALLBACK_PROMOTE_SIGNAL_FILE, &stat_buf) == 0)
                return true;

        return false;
}

最后提一点, 9.3以前,曾经出现过pg_ctl promote -m 来指定是否需要fast promote或者fallback promote。