源码版本：redis 2.4.4
redis的snapshot通过将内存中的所有数据写入文件，以达到持久化的目的。
需要注意的是：
1）snapshot方式不是追加，而是将内存所有数据写入文件，snapshot间隔短的话，会造成磁盘IO频繁
2）在上一次做snapshot到当前，如果机器crash，期间修改过的数据会丢失
redis支持两种方式做snapshot
1）客户端发送bgsave命令（save命令也可以，其会阻塞主线程执行，不推荐）
2）根据配置的周期，定期执行
snapshot配置（redis.conf）：
save A B
在A秒后，如果至少B个数据发生变化则做snapshot。eg： save 3000100即在3000s后，如果至少100个key发生变化则做snapshot
rdbcompression yes
是否启用压缩（压缩消耗cpu）
dbfilename dump.rdb
snapshot dump的文件名
dir ./
指定工作目录，snapshot文件将保存在该目录，同时，aof文件也将保存在该目录
源码
执行bgsave时执行：
void bgsaveCommand(redisClient *c) { if (server.bgsavechildpid != -1) { //无snapshot子进程运行 addReplyError(c,"Background save already in progress"); } else if (server.bgrewritechildpid != -1) { //无重写aof进程运行 addReplyError(c,"Can't BGSAVE while AOF log rewriting is in progress"); } else if (rdbSaveBackground(server.dbfilename) == REDIS_OK) { //执行snapshot addReplyStatus(c,"Background saving started"); } else { addReply(c,shared.err); } }检查是否满足snapshot条件：
int serverCron(struct aeEventLoop *eventLoop, long long id, void *clientData) { ..... /* If there is not a background saving in progress check if * we have to save now */ for (j = 0; j < server.saveparamslen; j++) { struct saveparam *sp = server.saveparams+j; if (server.dirty >= sp->changes && now-server.lastsave > sp->seconds) { redisLog(REDIS_NOTICE,"%d changes in %d seconds. Saving...", sp->changes, sp->seconds); rdbSaveBackground(server.dbfilename); break; } } ...... }执行snapshot：
int rdbSaveBackground(char *filename) { pid_t childpid; long long start; if (server.bgsavechildpid != -1) return REDIS_ERR; if (server.vm_enabled) waitEmptyIOJobsQueue(); server.dirty_before_bgsave = server.dirty; start = ustime(); if ((childpid = fork()) == 0) { //fork子进程做snapshot /* Child */ if (server.vm_enabled) vmReopenSwapFile(); if (server.ipfd > 0) close(server.ipfd); if (server.sofd > 0) close(server.sofd); if (rdbSave(filename) == REDIS_OK) { _exit(0); } else { _exit(1); } } else { /* Parent */ server.stat_fork_time = ustime()-start; if (childpid == -1) { redisLog(REDIS_WARNING,"Can't save in background: fork: %s", strerror(errno)); return REDIS_ERR; } redisLog(REDIS_NOTICE,"Background saving started by pid %d",childpid); server.bgsavechildpid = childpid; updateDictResizePolicy(); return REDIS_OK; } return REDIS_OK; /* unreached */ }利用copy on write调用rdbsave将当前数据状态写入文件。
int rdbSave(char *filename) { dictIterator *di = NULL; dictEntry *de; FILE *fp; char tmpfile[256]; int j; time_t now = time(NULL); /* 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; }在serverCron 中检查做bgsave的子进程是否结束
/* Check if a background saving or AOF rewrite in progress terminated */ if (server.bgsavechildpid != -1 || server.bgrewritechildpid != -1) { //是否有做snapshot或者aof rewrite的子进程存在 int statloc; pid_t pid; if ((pid = wait3(&statloc,WNOHANG,NULL)) != 0) { if (pid == server.bgsavechildpid) { //结束了对其做后处理 backgroundSaveDoneHandler(statloc); } else { backgroundRewriteDoneHandler(statloc); } updateDictResizePolicy(); } } else { time_t now = time(NULL);
void backgroundSaveDoneHandler(int statloc) { int exitcode = WEXITSTATUS(statloc); int bysignal = WIFSIGNALED(statloc); //判断做snapshot是否成功 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); }