/* * CheckArchiveTimeout -- check for archive_timeout and switch xlog files * * This will switch to a new WAL file and force an archive file write if * meaningful activity is recorded in the current WAL file. This includes most * writes, including just a single checkpoint record, but excludes WAL records * that were inserted with the XLOG_MARK_UNIMPORTANT flag being set (like * snapshots of running transactions). Such records, depending on * configuration, occur on regular intervals and don't contain important * information. This avoids generating archives with a few unimportant * records. */ static void CheckArchiveTimeout(void) { pg_time_t now; pg_time_t last_time; XLogRecPtr last_switch_lsn; if (XLogArchiveTimeout <= 0 || RecoveryInProgress()) return; now = (pg_time_t) time(NULL); /* First we do a quick check using possibly-stale local state. */ if ((int) (now - last_xlog_switch_time) < XLogArchiveTimeout) return; /* * Update local state ... note that last_xlog_switch_time is the last time * a switch was performed *or requested*. */ last_time = GetLastSegSwitchData(&last_switch_lsn); last_xlog_switch_time = Max(last_xlog_switch_time, last_time); /* Now we can do the real checks */ if ((int) (now - last_xlog_switch_time) >= XLogArchiveTimeout) { /* * Switch segment only when "important" WAL has been logged since the * last segment switch (last_switch_lsn points to end of segment * switch occurred in). */ if (GetLastImportantRecPtr() > last_switch_lsn) { XLogRecPtr switchpoint; /* mark switch as unimportant, avoids triggering checkpoints */ switchpoint = RequestXLogSwitch(true); /* * If the returned pointer points exactly to a segment boundary, * assume nothing happened. */ if ((switchpoint % XLogSegSize) != 0) elog(DEBUG1, "write-ahead log switch forced (archive_timeout=%d)", XLogArchiveTimeout); } /* * Update state in any case, so we don't retry constantly when the * system is idle. */ last_xlog_switch_time = now; } }
/* * Main entry point for bgwriter process * * This is invoked from AuxiliaryProcessMain, which has already created the * basic execution environment, but not enabled signals yet. */ void BackgroundWriterMain(void) { sigjmp_buf local_sigjmp_buf; MemoryContext bgwriter_context; bool prev_hibernate; WritebackContext wb_context; /* * Properly accept or ignore signals the postmaster might send us. * * bgwriter doesn't participate in ProcSignal signalling, but a SIGUSR1 * handler is still needed for latch wakeups. */ pqsignal(SIGHUP, BgSigHupHandler); /* set flag to read config file */ pqsignal(SIGINT, SIG_IGN); pqsignal(SIGTERM, ReqShutdownHandler); /* shutdown */ pqsignal(SIGQUIT, bg_quickdie); /* hard crash time */ pqsignal(SIGALRM, SIG_IGN); pqsignal(SIGPIPE, SIG_IGN); pqsignal(SIGUSR1, bgwriter_sigusr1_handler); pqsignal(SIGUSR2, SIG_IGN); /* * Reset some signals that are accepted by postmaster but not here */ pqsignal(SIGCHLD, SIG_DFL); pqsignal(SIGTTIN, SIG_DFL); pqsignal(SIGTTOU, SIG_DFL); pqsignal(SIGCONT, SIG_DFL); pqsignal(SIGWINCH, SIG_DFL); /* We allow SIGQUIT (quickdie) at all times */ sigdelset(&BlockSig, SIGQUIT); /* * Create a resource owner to keep track of our resources (currently only * buffer pins). */ CurrentResourceOwner = ResourceOwnerCreate(NULL, "Background Writer"); /* * We just started, assume there has been either a shutdown or * end-of-recovery snapshot. */ last_snapshot_ts = GetCurrentTimestamp(); /* * Create a memory context that we will do all our work in. We do this so * that we can reset the context during error recovery and thereby avoid * possible memory leaks. Formerly this code just ran in * TopMemoryContext, but resetting that would be a really bad idea. */ bgwriter_context = AllocSetContextCreate(TopMemoryContext, "Background Writer", ALLOCSET_DEFAULT_SIZES); MemoryContextSwitchTo(bgwriter_context); WritebackContextInit(&wb_context, &bgwriter_flush_after); /* * If an exception is encountered, processing resumes here. * * See notes in postgres.c about the design of this coding. */ if (sigsetjmp(local_sigjmp_buf, 1) != 0) { /* Since not using PG_TRY, must reset error stack by hand */ error_context_stack = NULL; /* Prevent interrupts while cleaning up */ HOLD_INTERRUPTS(); /* Report the error to the server log */ EmitErrorReport(); /* * These operations are really just a minimal subset of * AbortTransaction(). We don't have very many resources to worry * about in bgwriter, but we do have LWLocks, buffers, and temp files. */ LWLockReleaseAll(); ConditionVariableCancelSleep(); AbortBufferIO(); UnlockBuffers(); /* buffer pins are released here: */ ResourceOwnerRelease(CurrentResourceOwner, RESOURCE_RELEASE_BEFORE_LOCKS, false, true); /* we needn't bother with the other ResourceOwnerRelease phases */ AtEOXact_Buffers(false); AtEOXact_SMgr(); AtEOXact_Files(); AtEOXact_HashTables(false); /* * Now return to normal top-level context and clear ErrorContext for * next time. */ MemoryContextSwitchTo(bgwriter_context); FlushErrorState(); /* Flush any leaked data in the top-level context */ MemoryContextResetAndDeleteChildren(bgwriter_context); /* re-initialize to avoid repeated errors causing problems */ WritebackContextInit(&wb_context, &bgwriter_flush_after); /* Now we can allow interrupts again */ RESUME_INTERRUPTS(); /* * Sleep at least 1 second after any error. A write error is likely * to be repeated, and we don't want to be filling the error logs as * fast as we can. */ pg_usleep(1000000L); /* * Close all open files after any error. This is helpful on Windows, * where holding deleted files open causes various strange errors. * It's not clear we need it elsewhere, but shouldn't hurt. */ smgrcloseall(); /* Report wait end here, when there is no further possibility of wait */ pgstat_report_wait_end(); } /* We can now handle ereport(ERROR) */ PG_exception_stack = &local_sigjmp_buf; /* * Unblock signals (they were blocked when the postmaster forked us) */ PG_SETMASK(&UnBlockSig); /* * Reset hibernation state after any error. */ prev_hibernate = false; /* * Loop forever */ for (;;) { bool can_hibernate; int rc; /* Clear any already-pending wakeups */ ResetLatch(MyLatch); if (got_SIGHUP) { got_SIGHUP = false; ProcessConfigFile(PGC_SIGHUP); } if (shutdown_requested) { /* * From here on, elog(ERROR) should end with exit(1), not send * control back to the sigsetjmp block above */ ExitOnAnyError = true; /* Normal exit from the bgwriter is here */ proc_exit(0); /* done */ } /* * Do one cycle of dirty-buffer writing. */ can_hibernate = BgBufferSync(&wb_context); /* * Send off activity statistics to the stats collector */ pgstat_send_bgwriter(); if (FirstCallSinceLastCheckpoint()) { /* * After any checkpoint, close all smgr files. This is so we * won't hang onto smgr references to deleted files indefinitely. */ smgrcloseall(); } /* * Log a new xl_running_xacts every now and then so replication can * get into a consistent state faster (think of suboverflowed * snapshots) and clean up resources (locks, KnownXids*) more * frequently. The costs of this are relatively low, so doing it 4 * times (LOG_SNAPSHOT_INTERVAL_MS) a minute seems fine. * * We assume the interval for writing xl_running_xacts is * significantly bigger than BgWriterDelay, so we don't complicate the * overall timeout handling but just assume we're going to get called * often enough even if hibernation mode is active. It's not that * important that log_snap_interval_ms is met strictly. To make sure * we're not waking the disk up unnecessarily on an idle system we * check whether there has been any WAL inserted since the last time * we've logged a running xacts. * * We do this logging in the bgwriter as it is the only process that * is run regularly and returns to its mainloop all the time. E.g. * Checkpointer, when active, is barely ever in its mainloop and thus * makes it hard to log regularly. */ if (XLogStandbyInfoActive() && !RecoveryInProgress()) { TimestampTz timeout = 0; TimestampTz now = GetCurrentTimestamp(); timeout = TimestampTzPlusMilliseconds(last_snapshot_ts, LOG_SNAPSHOT_INTERVAL_MS); /* * Only log if enough time has passed and interesting records have * been inserted since the last snapshot. Have to compare with <= * instead of < because GetLastImportantRecPtr() points at the * start of a record, whereas last_snapshot_lsn points just past * the end of the record. */ if (now >= timeout && last_snapshot_lsn <= GetLastImportantRecPtr()) { last_snapshot_lsn = LogStandbySnapshot(); last_snapshot_ts = now; } } /* * Sleep until we are signaled or BgWriterDelay has elapsed. * * Note: the feedback control loop in BgBufferSync() expects that we * will call it every BgWriterDelay msec. While it's not critical for * correctness that that be exact, the feedback loop might misbehave * if we stray too far from that. Hence, avoid loading this process * down with latch events that are likely to happen frequently during * normal operation. */ rc = WaitLatch(MyLatch, WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH, BgWriterDelay /* ms */ , WAIT_EVENT_BGWRITER_MAIN); /* * If no latch event and BgBufferSync says nothing's happening, extend * the sleep in "hibernation" mode, where we sleep for much longer * than bgwriter_delay says. Fewer wakeups save electricity. When a * backend starts using buffers again, it will wake us up by setting * our latch. Because the extra sleep will persist only as long as no * buffer allocations happen, this should not distort the behavior of * BgBufferSync's control loop too badly; essentially, it will think * that the system-wide idle interval didn't exist. * * There is a race condition here, in that a backend might allocate a * buffer between the time BgBufferSync saw the alloc count as zero * and the time we call StrategyNotifyBgWriter. While it's not * critical that we not hibernate anyway, we try to reduce the odds of * that by only hibernating when BgBufferSync says nothing's happening * for two consecutive cycles. Also, we mitigate any possible * consequences of a missed wakeup by not hibernating forever. */ if (rc == WL_TIMEOUT && can_hibernate && prev_hibernate) { /* Ask for notification at next buffer allocation */ StrategyNotifyBgWriter(MyProc->pgprocno); /* Sleep ... */ rc = WaitLatch(MyLatch, WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH, BgWriterDelay * HIBERNATE_FACTOR, WAIT_EVENT_BGWRITER_HIBERNATE); /* Reset the notification request in case we timed out */ StrategyNotifyBgWriter(-1); } /* * Emergency bailout if postmaster has died. This is to avoid the * necessity for manual cleanup of all postmaster children. */ if (rc & WL_POSTMASTER_DEATH) exit(1); prev_hibernate = can_hibernate; } }