深入剖析Redis RDB持久化机制 (4)

rdb.c：394

int rdbSave(char *filename) { ... /* Wait for I/O therads to terminate, just in case this is a * foreground-saving, to avoid seeking the swap file descriptor at the * same time. */ if (server.vm_enabled) waitEmptyIOJobsQueue(); snprintf(tmpfile,256,"temp-%d.rdb", (int) getpid()); fp = fopen(tmpfile,"w"); if (!fp) { redisLog(REDIS_WARNING, "Failed saving the DB: %s", strerror(errno)); return REDIS_ERR; } if (fwrite("REDIS0002",9,1,fp) == 0) goto werr; for (j = 0; j < server.dbnum; j++) { redisDb *db = server.db+j; dict *d = db->dict; if (dictSize(d) == 0) continue; di = dictGetSafeIterator(d); if (!di) { fclose(fp); return REDIS_ERR; } /* Write the SELECT DB opcode */ if (rdbSaveType(fp,REDIS_SELECTDB) == -1) goto werr; if (rdbSaveLen(fp,j) == -1) goto werr; /* Iterate this DB writing every entry */ while((de = dictNext(di)) != NULL) { sds keystr = dictGetEntryKey(de); robj key, *o = dictGetEntryVal(de); time_t expiretime; initStaticStringObject(key,keystr); expiretime = getExpire(db,&key); /* Save the expire time */ if (expiretime != -1) { /* If this key is already expired skip it */ if (expiretime < now) continue; if (rdbSaveType(fp,REDIS_EXPIRETIME) == -1) goto werr; if (rdbSaveTime(fp,expiretime) == -1) goto werr; } /* Save the key and associated value. This requires special * handling if the value is swapped out. */ if (!server.vm_enabled || o->storage == REDIS_VM_MEMORY || o->storage == REDIS_VM_SWAPPING) { int otype = getObjectSaveType(o); /* Save type, key, value */ if (rdbSaveType(fp,otype) == -1) goto werr; if (rdbSaveStringObject(fp,&key) == -1) goto werr; if (rdbSaveObject(fp,o) == -1) goto werr; } else { /* REDIS_VM_SWAPPED or REDIS_VM_LOADING */ robj *po; /* Get a preview of the object in memory */ po = vmPreviewObject(o); /* Save type, key, value */ if (rdbSaveType(fp,getObjectSaveType(po)) == -1) goto werr; if (rdbSaveStringObject(fp,&key) == -1) goto werr; if (rdbSaveObject(fp,po) == -1) goto werr; /* Remove the loaded object from memory */ decrRefCount(po); } } dictReleaseIterator(di); } /* EOF opcode */ if (rdbSaveType(fp,REDIS_EOF) == -1) goto werr; /* Make sure data will not remain on the OS\'s output buffers */ fflush(fp); fsync(fileno(fp)); fclose(fp); /* Use RENAME to make sure the DB file is changed atomically only * if the generate DB file is ok. */ if (rename(tmpfile,filename) == -1) { redisLog(REDIS_WARNING,"Error moving temp DB file on the final destination: %s", strerror(errno)); unlink(tmpfile); return REDIS_ERR; } redisLog(REDIS_NOTICE,"DB saved on disk"); server.dirty = 0; server.lastsave = time(NULL); return REDIS_OK; werr: fclose(fp); unlink(tmpfile); redisLog(REDIS_WARNING,"Write error saving DB on disk: %s", strerror(errno)); if (di) dictReleaseIterator(di); return REDIS_ERR; }

对是否有vm线程进行再次判断，因为如果是通过save命令过来的是没有判断过vm线程的。

创建并打开临时文件

写入文件签名“REDIS”和版本号“0002”

遍历所有db中的所有key

对每个key，先判断是否设置了expireTime, 如果设置了，则保存expireTime到rdb文件中。然后判断该key对应的value是否则内存中，如果是在内存中，则取出来写入到rdb文件中保 存，如果被换出到虚拟内存了，则从虚拟内存读取然后写入到rdb文件中。

不同类型有有不同的存储格式，详细见rdb文件格式

最后写入rdb文件的结束符

关闭文件并重命名临时文件名到正式文件名

更新脏数据计数server.dirty为0和最近写rdb文件的时间server.lastsave为当前时间，这个只是在通过save命令触发的情况下有用。因为如果是通过fork一个子进程来写rdb文件的，更新无效，因为更新的是子进程的数据。

如果是通过fork一个子进程来写rdb文件（即不是通过save命令触发的），在写rdb文件的过程中，可能又有一些数据被更改了，那此时的脏数据计数server.dirty怎么更新呢？ redis是怎样处理的呢?

我们来看看写rdb的子进程推出时得处理

Redis.c：605

if (server.bgsavechildpid != -1 || server.bgrewritechildpid != -1) { int statloc; pid_t pid; if ((pid = wait3(&statloc,WNOHANG,NULL)) != 0) { if (pid == server.bgsavechildpid) { backgroundSaveDoneHandler(statloc); } else { backgroundRewriteDoneHandler(statloc); } updateDictResizePolicy(); } }

如果捕捉到写rdb文件的子进程退出，则调用backgroundSaveDoneHandler进行处理

接着看看backgroundSaveDoneHandler函数

Rdb.c：1062

void backgroundSaveDoneHandler(int statloc) { int exitcode = WEXITSTATUS(statloc); int bysignal = WIFSIGNALED(statloc); if (!bysignal && exitcode == 0) { redisLog(REDIS_NOTICE, "Background saving terminated with success"); server.dirty = server.dirty - server.dirty_before_bgsave; server.lastsave = time(NULL); } else if (!bysignal && exitcode != 0) { redisLog(REDIS_WARNING, "Background saving error"); } else { redisLog(REDIS_WARNING, "Background saving terminated by signal %d", WTERMSIG(statloc)); rdbRemoveTempFile(server.bgsavechildpid); } server.bgsavechildpid = -1; /* Possibly there are slaves waiting for a BGSAVE in order to be served * (the first stage of SYNC is a bulk transfer of dump.rdb) */ updateSlavesWaitingBgsave(exitcode == 0 ? REDIS_OK : REDIS_ERR); }

更新脏数据计数server.dirty为0和最近写rdb文件的时间server.lastsave为当前时间

唤醒因为正在保存快照而等待的slave，关于slave的具体内容，见replication

快照导入

当redis因为停电或者某些原因挂掉了，此时重启redis时，我们就需要从rdb文件中读取快照文件，把保存到rdb文件中的数据重新导入到内存中。

先来看看启动时对快照导入的处理