Redis源码解析(8) AOF持久化
Redis源码解析(1) 动态字符串与链表
Redis源码解析(2) 字典与迭代器
Redis源码解析(3) 跳跃表
Redis源码解析(4) 整数集合
Redis源码解析(5) 压缩列表
Redis源码解析(6) 键的过期处理策略
Redis源码解析(7) 发布订阅机制
Redis源码解析(8) AOF持久化
Redis源码解析(9) RDB持久化
Redis源码解析(10) 网络框架
Redis源码解析(11) 内存淘汰策略
Redis源码解析(12) 命令执行过程
Redis源码解析(13) 主从复制
Redis源码解析(14) 哨兵机制[1] 结构与初始化
Redis源码解析(15) 哨兵机制[2] 信息同步与TILT模式
Redis源码解析(16) 哨兵机制[3] 判断下线
Redis源码解析(17) 哨兵机制[4] 故障转移
Redis源码解析(18) 集群[1]初始化,握手与心跳检测
Redis源码解析(19) 集群[2] 主从复制,故障检测与故障转移
Redis源码解析(20) 集群[3] 键的存储,重新分片与重定向
Redis源码解析(21) 集群[4] 故障转移failover与slave迁移
Redis源码解析(22) 事务
Redis源码解析(23) SCAN命令实现
引言
AOF持久化是为了弥补RDB持久化中数据丢失的不可控性和在执行RDB时消耗的大量资源(需要把所有数据库的所有键值都拷贝到RDB文件中)而存在的一种持久化.AOF非常类似与MySQL中的redo日志,它们都是记录每条使得服务器状态做出改变的命令,以顺序IO插入到AOF文件(mysql中当然不是这个)中.
AOF策略
在说清楚AOF持久化的策略之前,我觉得我们得清楚数据在数据库中到底经历了哪些事情
- 由write类系统调用写入内核,但内核中为了减少磁盘IO的次数,其实是维护了缓冲区的,即我们的数据进入内核缓冲区
- 由内核缓冲区写入磁盘驱动程序,其中可能会有磁盘的缓存
- 数据真正的写入到物理介质上(磁盘)
那么数据的丢失又有哪些情况呢?
- 服务器这个进程宕机,但是操作系统没有挂,也就是说上面三步只要执行了第一步就可以,操作系统就可以确保数据写回磁盘.
- 断电等不可控因素,就算写到内核缓冲或者磁盘缓冲还是没用,得真正写入到磁盘上才可以.即我们得执行第三步
这里我们不讨论磁盘上的缓存,那有专门的人操心,我们只需要保证数据能够发给磁盘即可.由上面我们举的数据进入磁盘的三步,操作系统为前两部提供了API,使得我们可以更好的让程序按照我们想要的那样运行,即write和(fsync/fdatasync).
AOF的策略保证了我们能够在断电的情况下不丢失超过一秒的数据(其实是两秒,下面会说),分为三种策略,这里不详细讨论,网上说这个的博客多如牛毛.我们只说它们的区别,就是多长时间进行一次fsync,即把数据从内核缓冲刷入磁盘.因为如果数据已经写入磁盘,我们就可以认为数据已经持久化了,所以fsync的间隔是保证redis数据安全的最重要的要素.
下面说下为什么会丢失超过两秒的数据,可以参考flushAppendOnlyFile中的代码解析.因为fsync这个操作可能执行时间比较长,如果在下一次执行flush的时候上一次fsync还未完成且此次flush距上次间隔在两秒之内的话,此次flush的不会执行,会直接退出,这些数据就会只存在在内存中,这样就会导致在有些情况下两秒的数据丢失.
首先列出在我们执行AOF时redisserver中我们需要使用的参数
struct rediserver{
// AOF 文件的当前字节大小
off_t aof_current_size; /* AOF current size. */
int aof_rewrite_scheduled; /* Rewrite once BGSAVE terminates. */
// 负责进行 AOF 重写的子进程 ID
pid_t aof_child_pid; /* PID if rewriting process */
// AOF 重写缓存链表，链接着多个缓存块 用于在AOF执行时缓存到来的命令
list *aof_rewrite_buf_blocks; /* Hold changes during an AOF rewrite. */
// AOF 缓冲区
sds aof_buf; /* AOF buffer, written before entering the event loop */
// AOF 文件的描述符
int aof_fd; /* File descriptor of currently selected AOF file */
// AOF 的当前目标数据库
int aof_selected_db; /* Currently selected DB in AOF */
.......
// 推迟 write 操作的时间
time_t aof_flush_postponed_start; /* UNIX time of postponed AOF flush */
// 最后一直执行 fsync 的时间
time_t aof_last_fsync; /* UNIX time of last fsync() */
time_t aof_rewrite_time_last; /* Time used by last AOF rewrite run. */
// AOF 重写的开始时间
time_t aof_rewrite_time_start; /* Current AOF rewrite start time. */
..........
}
首先是在每次事件循环结束时要执行的flush操作flushAppendOnlyFile,它的作用是把我们写入在aof_buf中的缓冲数据通过write写入系统缓冲区,然后通过系统中的配置appendfsync来决定fsync的频率.上面已经提到了.接下来我们就来看看源码吧
事件循环中的flush操作
void flushAppendOnlyFile(int force) {
ssize_t nwritten;
int sync_in_progress = 0; //是否正在进行fsync 如果正在进行需要判断是否已经进行了两秒
// 缓冲区中没有任何内容，直接返回
if (sdslen(server.aof_buf) == 0) return;
// 策略为每秒 FSYNC
if (server.aof_fsync == AOF_FSYNC_EVERYSEC)
// 是否有 SYNC 正在后台进行?
// 避免write和fsync同时操作同一个文件描述符 write阻塞主线程
sync_in_progress = bioPendingJobsOfType(REDIS_BIO_AOF_FSYNC) != 0;
// 每秒 fsync,并且强制写入为假
if (server.aof_fsync == AOF_FSYNC_EVERYSEC && !force) {
/* With this append fsync policy we do background fsyncing.
*
* 当 fsync 策略为每秒钟一次时， fsync 在后台执行。
*
* If the fsync is still in progress we can try to delay
* the write for a couple of seconds.
*
* 如果后台仍在执行 FSYNC ，那么我们可以延迟写操作一两秒
* （如果强制执行 write 的话，服务器主线程将阻塞在 write 上面）
*/
if (sync_in_progress) {
// 有 fsync 正在后台进行 。。。
if (server.aof_flush_postponed_start == 0) {
/* No previous write postponinig, remember that we are
* postponing the flush and return.
*
* 前面没有推迟过 write 操作，这里将推迟写操作的时间记录下来
* 然后就返回，不执行 write 或者 fsync
*/
server.aof_flush_postponed_start = server.unixtime;
return;
} else if (server.unixtime - server.aof_flush_postponed_start < 2) {
/* We were already waiting for fsync to finish, but for less
* than two seconds this is still ok. Postpone again.
*
* 如果之前已经因为 fsync 而推迟了 write 操作
* 但是推迟的时间不超过 2 秒，那么直接返回
* 不执行 write 或者 fsync
* 防止write阻塞主线程
*/
return;
}
/* Otherwise fall trough, and go write since we can't wait
* over two seconds.
*
* 如果后台还有 fsync 在执行，并且 write 已经推迟 >= 2 秒
* 那么执行写操作（write 将被阻塞）
*/
server.aof_delayed_fsync++;
redisLog(REDIS_NOTICE,"Asynchronous AOF fsync is taking too long (disk is busy?). Writing the AOF buffer without waiting for fsync to complete, this may slow down Redis.");
}
}
/* If you are following this code path, then we are going to write so
* set reset the postponed flush sentinel to zero.
*
* 执行到这里，程序会对 AOF 文件进行写入。
*
* 清零延迟 write 的时间记录
*/
server.aof_flush_postponed_start = 0;
/* We want to perform a single write. This should be guaranteed atomic
* at least if the filesystem we are writing is a real physical one.
*
* 执行单个 write 操作，如果写入设备是物理的话，那么这个操作应该是原子的
*
* While this will save us against the server being killed I don't think
* there is much to do about the whole server stopping for power problems
* or alike
*
* 当然，如果出现像电源中断这样的不可抗现象，那么 AOF 文件也是可能会出现问题的
* 这时就要用 redis-check-aof 程序来进行修复。
* 下面的write向aof文件写存储在aof_buf中的数据
*/
nwritten = write(server.aof_fd,server.aof_buf,sdslen(server.aof_buf));
//可能磁盘缓冲不足 导致写入数据不够 或其他原因导致写入失败
if (nwritten != (signed)sdslen(server.aof_buf)) {
static time_t last_write_error_log = 0;
int can_log = 0;
/* Limit logging rate to 1 line per AOF_WRITE_LOG_ERROR_RATE seconds. */
// 将日志的记录频率限制在每行 AOF_WRITE_LOG_ERROR_RATE 秒
if ((server.unixtime - last_write_error_log) > AOF_WRITE_LOG_ERROR_RATE) {
can_log = 1;
last_write_error_log = server.unixtime;
}
/* Lof the AOF write error and record the error code. */
// 如果写入出错，那么尝试将该情况写入到日志里面
if (nwritten == -1) {
if (can_log) {
redisLog(REDIS_WARNING,"Error writing to the AOF file: %s",
strerror(errno));
server.aof_last_write_errno = errno;
}
} else {
if (can_log) {
redisLog(REDIS_WARNING,"Short write while writing to "
"the AOF file: (nwritten=%lld, "
"expected=%lld)",
(long long)nwritten,
(long long)sdslen(server.aof_buf));
}
// 我们不希望数据不完整 所以尝试移除新追加的不完整内容
if (ftruncate(server.aof_fd, server.aof_current_size) == -1) {
if (can_log) {
redisLog(REDIS_WARNING, "Could not remove short write "
"from the append-only file. Redis may refuse "
"to load the AOF the next time it starts. "
"ftruncate: %s", strerror(errno));
}
} else {
/* If the ftrunacate() succeeded we can set nwritten to
* -1 since there is no longer partial data into the AOF. */
nwritten = -1;
}
//ENOSPC -> /* No space left on device */
server.aof_last_write_errno = ENOSPC; //磁盘缓冲空间不足
}
/* Handle the AOF write error. */
// 处理写入 AOF 文件时出现的错误
// 当模式为ALWAYS是 写入错误时无法处理 直接结束进程
if (server.aof_fsync == AOF_FSYNC_ALWAYS) {
/* We can't recover when the fsync policy is ALWAYS since the
* reply for the client is already in the output buffers, and we
* have the contract with the user that on acknowledged write data
* is synched on disk. */
redisLog(REDIS_WARNING,"Can't recover from AOF write error when the AOF fsync policy is 'always'. Exiting...");
exit(1);
} else {
/* Recover from failed write leaving data into the buffer. However
* set an error to stop accepting writes as long as the error
* condition is not cleared. */
server.aof_last_write_status = REDIS_ERR;
/* Trim the sds buffer if there was a partial write, and there
* was no way to undo it with ftruncate(2). */
if (nwritten > 0) {
server.aof_current_size += nwritten;
sdsrange(server.aof_buf,nwritten,-1); //把缓冲区中读指针向后移动 下次再尝试写入
}
return; /* We'll try again on the next call... */
}
} else {
/* Successful write(2). If AOF was in error state, restore the
* OK state and log the event. */
// 写入成功，更新最后写入状态
if (server.aof_last_write_status == REDIS_ERR) {
redisLog(REDIS_WARNING,
"AOF write error looks solved, Redis can write again.");
server.aof_last_write_status = REDIS_OK;
}
}
// 更新写入后的 AOF 文件大小
server.aof_current_size += nwritten;
/* Re-use AOF buffer when it is small enough. The maximum comes from the
* arena size of 4k minus some overhead (but is otherwise arbitrary).
*
* 如果 AOF 缓存的大小足够小的话，那么重用这个缓存，
* 否则的话，释放 AOF 缓存。
*/
//这个式子加起来是缓冲区的大小
if ((sdslen(server.aof_buf)+sdsavail(server.aof_buf)) < 4000) {
// 清空缓存中的内容，等待重用
sdsclear(server.aof_buf);
} else {
// 释放缓存
sdsfree(server.aof_buf);
server.aof_buf = sdsempty(); //防止缓存过大 在后面的存储中存不满 导致碎片 重新分配
}
/* Don't fsync if no-appendfsync-on-rewrite is set to yes and there are
* children doing I/O in the background.
*
* 如果 no-appendfsync-on-rewrite 选项为开启状态，
* 并且有 BGSAVE 或者 BGREWRITEAOF 正在进行的话，
* 那么不执行 fsync
*/
if (server.aof_no_fsync_on_rewrite &&
(server.aof_child_pid != -1 || server.rdb_child_pid != -1))
return;
/* Perform the fsync if needed. */
// 下面就是将数据从内核缓冲区刷到磁盘驱动最重要的一步
// 总是执行 fsnyc
if (server.aof_fsync == AOF_FSYNC_ALWAYS) {
/* aof_fsync is defined as fdatasync() for Linux in order to avoid
* flushing metadata. */
aof_fsync(server.aof_fd); /* Let's try to get this data on the disk */
// 更新最后一次执行 fsnyc 的时间
server.aof_last_fsync = server.unixtime;
// 策略为每秒 fsnyc ，并且距离上次 fsync 已经超过 1 秒
} else if ((server.aof_fsync == AOF_FSYNC_EVERYSEC &&
server.unixtime > server.aof_last_fsync)) {
// 放到后台执行
if (!sync_in_progress) aof_background_fsync(server.aof_fd);
// 更新最后一次执行 fsync 的时间
server.aof_last_fsync = server.unixtime;
}
}
重写操作
我们再来看看在执行BGREWRITEAOF进行后台重写的时候的实现rewriteAppendOnlyFileBackground,其中当然也包括了重写的真正实现函数rewriteAppendOnlyFile
/*
* 父进程会捕捉子进程的退出信号，
* 如果子进程的退出状态是 OK 的话，
* 那么父进程将新输入命令的缓存追加到临时文件，
* 然后使用 rename(2) 对临时文件改名，用它代替旧的 AOF 文件，
* 至此，后台 AOF 重写完成。
*/
int rewriteAppendOnlyFileBackground(void) {
pid_t childpid; //子进程进程号
long long start; //记录fork开始前的时间,计算fork耗时用
// 已经有进程在进行 AOF 重写了
if (server.aof_child_pid != -1) return REDIS_ERR;
start = ustime();
//我们可以看到在重写时并没有把资源池化 而是使用很笨重的操作fork
if ((childpid = fork()) == 0) {
char tmpfile[256]; //保存重写使用的临时文件名
/* Child */
// 关闭网络连接 fd 这一步为什么?
closeListeningSockets(0);
// 为进程设置名字，方便记认
redisSetProcTitle("redis-aof-rewrite");
// 创建临时文件，并进行 AOF 重写
snprintf(tmpfile,256,"temp-rewriteaof-bg-%d.aof", (int) getpid());
if (rewriteAppendOnlyFile(tmpfile) == REDIS_OK) { //重写返回OK 完成重写操作 并没有exec 当然也没有必要
size_t private_dirty = zmalloc_get_private_dirty();
if (private_dirty) {
redisLog(REDIS_NOTICE,
"AOF rewrite: %zu MB of memory used by copy-on-write",
private_dirty/(1024*1024));
}
// 通知父进程
// 发送重写成功信号
exitFromChild(0);
} else {
// 发送重写失败信号
exitFromChild(1);
}
} else {
/* Parent */
// 记录执行 fork 所消耗的时间
server.stat_fork_time = ustime()-start;
if (childpid == -1) { //fork失败
redisLog(REDIS_WARNING,
"Can't rewrite append only file in background: fork: %s",
strerror(errno));
return REDIS_ERR;
}
redisLog(REDIS_NOTICE,
"Background append only file rewriting started by pid %d",childpid);
// 记录 AOF 重写的信息
server.aof_rewrite_scheduled = 0;
server.aof_rewrite_time_start = time(NULL);
server.aof_child_pid = childpid;
// 关闭字典自动 rehash
updateDictResizePolicy();
/* We set appendseldb to -1 in order to force the next call to the
* feedAppendOnlyFile() to issue a SELECT command, so the differences
* accumulated by the parent into server.aof_rewrite_buf will start
* with a SELECT statement and it will be safe to merge.
*
* 将 aof_selected_db 设为 -1 ，
* 强制让 feedAppendOnlyFile() 下次执行时引发一个 SELECT 命令，
* 从而确保之后新添加的命令会设置到正确的数据库中
*/
server.aof_selected_db = -1;
replicationScriptCacheFlush();
return REDIS_OK;
}
return REDIS_OK; /* unreached */
}
int rewriteAppendOnlyFile(char *filename) { //子进程执行的重写函数,
dictIterator *di = NULL; //遍历字典时要使用的迭代器
dictEntry *de; //得到的每一项
rio aof; //
FILE *fp; //临时文件的文件指针
char tmpfile[256];
int j;
long long now = mstime(); //当前时间
/* Note that we have to use a different temp name here compared to the
* one used by rewriteAppendOnlyFileBackground() function.
*
* 创建临时文件
*
* 注意这里创建的文件名和 rewriteAppendOnlyFileBackground() 创建的文件名稍有不同
*/
snprintf(tmpfile,256,"temp-rewriteaof-%d.aof", (int) getpid());
fp = fopen(tmpfile,"w");
if (!fp) {
redisLog(REDIS_WARNING, "Opening the temp file for AOF rewrite in rewriteAppendOnlyFile(): %s", strerror(errno));
return REDIS_ERR;
}
// 初始化文件 io
rioInitWithFile(&aof,fp);
// 设置每写入 REDIS_AOF_AUTOSYNC_BYTES 字节
// 就执行一次 FSYNC
// 防止缓存中积累太多命令内容，造成 I/O 阻塞时间过长
if (server.aof_rewrite_incremental_fsync)
rioSetAutoSync(&aof,REDIS_AOF_AUTOSYNC_BYTES);
// 遍历所有数据库 dbnum是数据库的总数
for (j = 0; j < server.dbnum; j++) {
char selectcmd[] = "*2\r\n$6\r\nSELECT\r\n";
redisDb *db = server.db+j;
// 指向键空间
dict *d = db->dict;
if (dictSize(d) == 0) continue;
// 创建键空间迭代器
di = dictGetSafeIterator(d); //是一个安全迭代器 安全迭代器存在的时候不进行rehash
if (!di) {
fclose(fp);
return REDIS_ERR;
}
/* SELECT the new DB
*
* 每个数据库记录的开头首先写入SELECT命令,确保之后的数据会被插入到正确的数据库上
*/
if (rioWrite(&aof,selectcmd,sizeof(selectcmd)-1) == 0) goto werr;
if (rioWriteBulkLongLong(&aof,j) == 0) goto werr;
/* Iterate this DB writing every entry
*
* 遍历数据库所有键,并通过命令将它们的当前状态（值）记录到新 AOF 文件中
*/
while((de = dictNext(di)) != NULL) {
sds keystr;
robj key, *o;
long long expiretime;
// 取出键
keystr = dictGetKey(de);
// 取出值
o = dictGetVal(de);
initStaticStringObject(key,keystr);
// 取出过期时间
expiretime = getExpire(db,&key);
/* If this key is already expired skip it
*
* 如果键已经过期,那么跳过它,不保存 这也是使得文件变小的很重要的一点
*/
if (expiretime != -1 && expiretime < now) continue;
/* Save the key and associated value
*
* 根据值的类型，选择适当的命令来保存值
* 值得一提的是每个支持多参数的命令不会使得参数超过64个 防止客户端缓存溢出
* 每一种类型对应的写入方法都是不一样的 这里不解析每种写入方法 有兴趣可以看看
*/
if (o->type == REDIS_STRING) {
/* Emit a SET command */
char cmd[]="*3\r\n$3\r\nSET\r\n";
if (rioWrite(&aof,cmd,sizeof(cmd)-1) == 0) goto werr;
/* Key and value */
if (rioWriteBulkObject(&aof,&key) == 0) goto werr;
if (rioWriteBulkObject(&aof,o) == 0) goto werr;
} else if (o->type == REDIS_LIST) {
if (rewriteListObject(&aof,&key,o) == 0) goto werr;
} else if (o->type == REDIS_SET) {
if (rewriteSetObject(&aof,&key,o) == 0) goto werr;
} else if (o->type == REDIS_ZSET) {
if (rewriteSortedSetObject(&aof,&key,o) == 0) goto werr;
} else if (o->type == REDIS_HASH) {
if (rewriteHashObject(&aof,&key,o) == 0) goto werr;
} else {
redisPanic("Unknown object type");
}
/* Save the expire time
*
* 保存键的过期时间
*/
if (expiretime != -1) {
char cmd[]="*3\r\n$9\r\nPEXPIREAT\r\n";
// 写入 PEXPIREAT expiretime 命令
if (rioWrite(&aof,cmd,sizeof(cmd)-1) == 0) goto werr;
if (rioWriteBulkObject(&aof,&key) == 0) goto werr;
if (rioWriteBulkLongLong(&aof,expiretime) == 0) goto werr;
}
}
// 释放迭代器
dictReleaseIterator(di);
}
/* Make sure data will not remain on the OS's output buffers */
// 冲洗并关闭新 AOF 文件
if (fflush(fp) == EOF) goto werr;
if (aof_fsync(fileno(fp)) == -1) goto werr;
if (fclose(fp) == EOF) goto werr;
/* Use RENAME to make sure the DB file is changed atomically only
* if the generate DB file is ok.
*
* 原子地改名，用重写后的新 AOF 文件覆盖旧 AOF 文件
*/
if (rename(tmpfile,filename) == -1) {
redisLog(REDIS_WARNING,"Error moving temp append only file on the final destination: %s", strerror(errno));
unlink(tmpfile); //减少这个文件的引用计数 也是remove删除文件的时候调用的函数
return REDIS_ERR;
}
redisLog(REDIS_NOTICE,"SYNC append only file rewrite performed");
return REDIS_OK;
werr: //当有任何错误的时候都会goto到这里来
fclose(fp);
unlink(tmpfile);
redisLog(REDIS_WARNING,"Write error writing append only file on disk: %s", strerror(errno));
if (di) dictReleaseIterator(di);
return REDIS_ERR;
}
重写结束后主线程操作
我们在rewriteAppendOnlyFileBackground中看到在子进程在执行完毕后会发出信号,那么父进程在接收到信号后会干些什么呢?
其实简单来说就是把重写缓存中的数据写入临时AOF文件以后再改个名字,然后再在redisserver中把相关的状态改变,比如AOF文件大小,AOF文件fd,文件指针等.当然我们再进行完重写以后也要释放重写缓冲中的资源,和移除临时文件.这样的话所有的状态就恢复到重写之前了.
//当子线程完成 AOF 重写时,即接父进程收到信号的时候调用这个函数
void backgroundRewriteDoneHandler(int exitcode, int bysignal) {
if (!bysignal && exitcode == 0) {
int newfd, oldfd;
char tmpfile[256];
long long now = ustime();
redisLog(REDIS_NOTICE, //首先记录日志 重写完成
"Background AOF rewrite terminated with success");
/* Flush the differences accumulated by the parent to the
* rewritten AOF. */
// 打开保存新 AOF 文件内容的临时文件
snprintf(tmpfile,256,"temp-rewriteaof-bg-%d.aof",
(int)server.aof_child_pid);
newfd = open(tmpfile,O_WRONLY|O_APPEND);//o_app是因为我们还需要添加缓存信息
if (newfd == -1) {
redisLog(REDIS_WARNING,
"Unable to open the temporary AOF produced by the child: %s", strerror(errno));
goto cleanup;
}
// 将累积的重写缓存写入到临时文件中
// 这个函数调用的 write 操作会阻塞主进程
// 重写缓存的组织方式为缓存链表
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.
*
* 剩下的工作就是将临时文件改名为 AOF 程序指定的文件名，
* 并将新文件的 fd 设为 AOF 程序的写目标。
*
* 不过这里有一个问题 ——
* 我们不想 close(2) 或者 rename(2) 在删除旧文件时阻塞。
*
* There are two possible scenarios:
*
* 以下是两个可能的场景：
*
* 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.
*
* AOF 被关闭，这个是一次单次的写操作。
* 临时文件会被改名为 AOF 文件。
* 本来已经存在的 AOF 文件会被 unlink ，这可能会阻塞服务器。
*
* 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.
*
* AOF 被开启，并且重写后的 AOF 文件会立即被用于接收新的写入命令。
* 当临时文件被改名为 AOF 文件时，原来的 AOF 文件描述符会被关闭。
* 因为 Redis 会是最后一个引用这个文件的进程，
* 所以关闭这个文件会引起 unlink ，这可能会阻塞服务器。
*
* 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.
*
* 为了避免出现阻塞现象，程序会将 close(2) 放到后台线程执行，
* 这样服务器就可以持续处理请求，不会被中断。
*/
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".
*
* 对临时文件进行改名，替换现有的 AOF 文件。
*
* 旧的 AOF 文件不会在这里被 unlink ，因为 oldfd 引用了它。
*/
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;
}
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.
*
* AOF 被关闭，直接关闭 AOF 文件，
* 因为关闭 AOF 本来就会引起阻塞，所以这里就算 close 被阻塞也无所谓
*/
close(newfd);
} else {
/* AOF enabled, replace the old fd with the new one.
*
* 用新 AOF 文件的 fd 替换原来 AOF 文件的 fd
*/
oldfd = server.aof_fd;
server.aof_fd = newfd;
// 因为前面进行了 AOF 重写缓存追加，所以这里立即 fsync 一次
if (server.aof_fsync == AOF_FSYNC_ALWAYS)
aof_fsync(newfd);
else if (server.aof_fsync == AOF_FSYNC_EVERYSEC)
aof_background_fsync(newfd);
// 强制引发 SELECT
server.aof_selected_db = -1; /* Make sure SELECT is re-issued */
// 更新 AOF 文件的大小
aofUpdateCurrentSize();
// 记录前一次重写时的大小
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.
*
* 清空 AOF 缓存，因为它的内容已经被写入过了，没用了
*/
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
*
* 如果是第一次创建 AOF 文件，那么更新 AOF 状态
*/
if (server.aof_state == REDIS_AOF_WAIT_REWRITE)
server.aof_state = REDIS_AOF_ON;
/* Asynchronously close the overwritten AOF.
*
* 异步关闭旧 AOF 文件
*/
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);
// BGREWRITEAOF 重写出错
} else if (!bysignal && exitcode != 0) {
server.aof_lastbgrewrite_status = REDIS_ERR;
redisLog(REDIS_WARNING,
"Background AOF rewrite terminated with error");
// 未知错误
} else {
server.aof_lastbgrewrite_status = REDIS_ERR;
redisLog(REDIS_WARNING,
"Background AOF rewrite terminated by signal %d", bysignal);
}
cleanup:
// 清空 AOF 缓冲区
aofRewriteBufferReset();
// 移除临时文件
aofRemoveTempFile(server.aof_child_pid);
// 重置默认属性
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;
}
