对 health_check_period与healht_check_timeout，官方解释如下：
health_check_periodThis parameter specifies the interval between the health checks in seconds. Default is 0, which means health check is disabled. You need to reload pgpool.conf if you change health_check_period.
health_check_timeoutpgpool-II periodically tries to connect to the backends to detect any error on the servers or networks. This error check procedure is called "health check". If an error is detected, pgpool-II tries to perform failover or degeneration. This parameter serves to prevent the health check from waiting for a long time in a case such as un unplugged network cable. The timeout value is in seconds. Default value is 20. 0 disables timeout (waits until TCP/IP timeout). This health check requires one extra connection to each backend, so max_connections in the postgresql.conf needs to be incremented as needed. You need to reload pgpool.conf if you change this value.
两者到底是怎样的关系？
看看代码就可以知晓：
int main(int argc, char **argv) { …… /* * This is the main loop */ for (;;) { CHECK_REQUEST; /* do we need health checking for PostgreSQL? */ if (pool_config->health_check_period > 0) { …… if (pool_config->health_check_timeout > 0) { /* * set health checker timeout. we want to detect * communication path failure much earlier before * TCP/IP stack detects it. */ pool_signal(SIGALRM, health_check_timer_handler); alarm(pool_config->health_check_timeout); } /* * do actual health check. trying to connect to the backend */ errno = 0; health_check_timer_expired = 0; POOL_SETMASK(&UnBlockSig); sts = health_check(); POOL_SETMASK(&BlockSig); if (pool_config->parallel_mode || pool_config->enable_query_cache) sys_sts = system_db_health_check(); /** 着里面有根据结果进行failover处理的逻辑，省略*/ if ((sts > 0 || sys_sts < 0) && (errno != EINTR || (errno == EINTR && health_check_timer_expired))) { …… } if (pool_config->health_check_timeout > 0) { /* seems ok. cancel health check timer */ pool_signal(SIGALRM, SIG_IGN); } /** 请注意这里的sleep处理 */ sleep_time = pool_config->health_check_period; pool_sleep(sleep_time); } else { for (;;) { int r; struct timeval t = { 3, 0}; POOL_SETMASK(&UnBlockSig); r = pool_pause(&t); POOL_SETMASK(&BlockSig); if (r > 0) break; } } } pool_shmem_exit(0); } 也就是说首先，health_check是否发生，要看 health_check_period是否大于0。
在这个前提下，如果health_check_timeout也大于零，就埋下一个定时器，
到达health_check_timeout的秒数时，从定时器激活 healht_check函数。与此同时，
在主循环中进行了 health_check处理后，如果结果OK，那么进行一番整理后，要开始睡眠一段时间，睡眠的时间间隔就是: health_check_period。睡醒了，再回到循环起始处，继续循环。