Redis資料持久化機制AOF原理分析二
阿新 • • 發佈:2019-01-03
至此,AOF資料持久化已經全部結束了,剩下的就是一些細節的處理,以及一些Linux庫函式的理解,對於rename、unlink、wait3等庫函式的深入認識就去問Google吧。/* A background append only file rewriting (BGREWRITEAOF) terminated its work. * Handle this. */ void backgroundRewriteDoneHandler(int exitcode, int bysignal) { if (!bysignal && exitcode == 0) {//子程序退出狀態正確 int newfd, oldfd; char tmpfile[256]; long long now = ustime(); redisLog(REDIS_NOTICE, "Background AOF rewrite terminated with success"); /* Flush the differences accumulated by the parent to the * rewritten AOF. */ snprintf(tmpfile,256,"temp-rewriteaof-bg-%d.aof", (int)server.aof_child_pid); newfd = open(tmpfile,O_WRONLY|O_APPEND); if (newfd == -1) { redisLog(REDIS_WARNING, "Unable to open the temporary AOF produced by the child: %s", strerror(errno)); goto cleanup; } //處理server.aof_rewrite_buf_blocks中DIFF資料 if (aofRewriteBufferWrite(newfd) == -1) { redisLog(REDIS_WARNING, "Error trying to flush the parent diff to the rewritten AOF: %s", strerror(errno)); close(newfd); goto cleanup; } redisLog(REDIS_NOTICE, "Parent diff successfully flushed to the rewritten AOF (%lu bytes)", aofRewriteBufferSize()); /* The only remaining thing to do is to rename the temporary file to * the configured file and switch the file descriptor used to do AOF * writes. We don't want close(2) or rename(2) calls to block the * server on old file deletion. * * There are two possible scenarios: * * 1) AOF is DISABLED and this was a one time rewrite. The temporary * file will be renamed to the configured file. When this file already * exists, it will be unlinked, which may block the server. * * 2) AOF is ENABLED and the rewritten AOF will immediately start * receiving writes. After the temporary file is renamed to the * configured file, the original AOF file descriptor will be closed. * Since this will be the last reference to that file, closing it * causes the underlying file to be unlinked, which may block the * server. * * To mitigate the blocking effect of the unlink operation (either * caused by rename(2) in scenario 1, or by close(2) in scenario 2), we * use a background thread to take care of this. First, we * make scenario 1 identical to scenario 2 by opening the target file * when it exists. The unlink operation after the rename(2) will then * be executed upon calling close(2) for its descriptor. Everything to * guarantee atomicity for this switch has already happened by then, so * we don't care what the outcome or duration of that close operation * is, as long as the file descriptor is released again. */ if (server.aof_fd == -1) { /* AOF disabled */ /* Don't care if this fails: oldfd will be -1 and we handle that. * One notable case of -1 return is if the old file does * not exist. */ oldfd = open(server.aof_filename,O_RDONLY|O_NONBLOCK); } else { /* AOF enabled */ oldfd = -1; /* We'll set this to the current AOF filedes later. */ } /* Rename the temporary file. This will not unlink the target file if * it exists, because we reference it with "oldfd". */ //把臨時檔案改名為正常的AOF檔名。由於當前oldfd已經指向這個之前的正常檔名的檔案, //所以當前不會造成unlink操作,得等那個oldfd被close的時候,核心判斷該檔案沒有指向了,就刪除之。 if (rename(tmpfile,server.aof_filename) == -1) { redisLog(REDIS_WARNING, "Error trying to rename the temporary AOF file: %s", strerror(errno)); close(newfd); if (oldfd != -1) close(oldfd); goto cleanup; } //如果AOF關閉了,那隻要處理新檔案,直接關閉這個新的檔案即可 //但是這裡會不會導致伺服器卡呢?這個newfd應該是臨時檔案的最後一個fd了,不會的, //因為這個檔案在本函式不會寫入資料,因為stopAppendOnly函式會清空aof_rewrite_buf_blocks列表。 if (server.aof_fd == -1) { /* AOF disabled, we don't need to set the AOF file descriptor * to this new file, so we can close it. */ close(newfd); } else { /* AOF enabled, replace the old fd with the new one. */ oldfd = server.aof_fd; //指向新的fd,此時這個fd由於上面的rename語句存在,已經為正常aof檔名 server.aof_fd = newfd; //fsync到硬碟 if (server.aof_fsync == AOF_FSYNC_ALWAYS) aof_fsync(newfd); else if (server.aof_fsync == AOF_FSYNC_EVERYSEC) aof_background_fsync(newfd); server.aof_selected_db = -1; /* Make sure SELECT is re-issued */ aofUpdateCurrentSize(); server.aof_rewrite_base_size = server.aof_current_size; /* Clear regular AOF buffer since its contents was just written to * the new AOF from the background rewrite buffer. */ //rewrite得到的肯定是最新的資料,所以aof_buf中的資料沒有意義,直接清空 sdsfree(server.aof_buf); server.aof_buf = sdsempty(); } server.aof_lastbgrewrite_status = REDIS_OK; redisLog(REDIS_NOTICE, "Background AOF rewrite finished successfully"); /* Change state from WAIT_REWRITE to ON if needed */ //下面判斷是否需要開啟AOF,比如bgrewriteaofCommand就不需要開啟AOF。 if (server.aof_state == REDIS_AOF_WAIT_REWRITE) server.aof_state = REDIS_AOF_ON; /* Asynchronously close the overwritten AOF. */ //讓後臺執行緒去關閉這個舊的AOF檔案FD,只要CLOSE就行,會自動unlink的,因為上面已經有rename if (oldfd != -1) bioCreateBackgroundJob(REDIS_BIO_CLOSE_FILE,(void*)(long)oldfd,NULL,NULL); redisLog(REDIS_VERBOSE, "Background AOF rewrite signal handler took %lldus", ustime()-now); } else if (!bysignal && exitcode != 0) { server.aof_lastbgrewrite_status = REDIS_ERR; redisLog(REDIS_WARNING, "Background AOF rewrite terminated with error"); } else { server.aof_lastbgrewrite_status = REDIS_ERR; redisLog(REDIS_WARNING, "Background AOF rewrite terminated by signal %d", bysignal); } cleanup: aofRewriteBufferReset(); aofRemoveTempFile(server.aof_child_pid); server.aof_child_pid = -1; server.aof_rewrite_time_last = time(NULL)-server.aof_rewrite_time_start; server.aof_rewrite_time_start = -1; /* Schedule a new rewrite if we are waiting for it to switch the AOF ON. */ if (server.aof_state == REDIS_AOF_WAIT_REWRITE) server.aof_rewrite_scheduled = 1; }