随着redis的运行，aof会不断膨胀（对于一个key会有多条aof日志），导致通过aof恢复数据时，耗费大量不必要的时间。redis提供的解决方案是aof rewrite。根据db的内容，对于每个key，生成一条日志。aof触发的时机：
1）用户调用BGREWRITEAOF命令
2）aof日志大小超过预设的限额
1. AOF Rewrite触发时机
首先看一下，BGREWRITEAOF的处理函数：
void bgrewriteaofCommand(redisClient *c) {
if (server.aof_child_pid != -1) {
addReplyError(c,"Background append only file rewriting already in progress");
} else if (server.rdb_child_pid != -1) {
server.aof_rewrite_scheduled = 1;
addReplyStatus(c,"Background append only file rewriting scheduled");
} else if (rewriteAppendOnlyFileBackground() == REDIS_OK) {
addReplyStatus(c,"Background append only file rewriting started");
} else {
addReply(c,shared.err);
}
}
aof_child_pid指示进行aof rewrite进程的pid，rdb_child_pid指示进行rdb dump的进程pid。
1）如果当前正在进行aof rewrite，则返回客户端错误。
2）如果当前正在进行rdb dump，为了避免对磁盘造成压力，将aof_rewrite_scheduled置为1，随后在没有进行aof rewrite和rdb dump时，再开启rewrite。
3）如果当前没有aof rewrite和rdb dump在进行，则调用rewriteAppendOnlyFileBackground进行aof rewrite。
4）异常情况，直接返回错误。
下面，看一下serverCron中是如何触发aof rewrite的。第一个触发点是，避免与rdb dump冲突，延迟触发rewrite。
/* Start a scheduled AOF rewrite if this was requested by the user while
* a BGSAVE was in progress. */
if (server.rdb_child_pid == -1 && server.aof_child_pid == -1 &&
server.aof_rewrite_scheduled)
{
rewriteAppendOnlyFileBackground();
}
需要确认当前没有aof rewrite和rdb dump在进行，并且设置了aof_rewrite_scheduled，调用rewirteAppendOnlyFileBackground进行aof rewrite。
第二个触发位置是aof文件的大小超过预定的百分比。
/* Trigger an AOF rewrite if needed */
if (server.rdb_child_pid == -1 &&
server.aof_child_pid == -1 &&
server.aof_rewrite_perc &&
server.aof_current_size > server.aof_rewrite_min_size)
{
long long base = server.aof_rewrite_base_size ?
server.aof_rewrite_base_size : 1;
long long growth = (server.aof_current_size*100/base) - 100;
if (growth >= server.aof_rewrite_perc) {
redisLog(REDIS_NOTICE,"Starting automatic rewriting of AOF on %lld%% growth",growth);
rewriteAppendOnlyFileBackground();
}
}
当aof文件超过了预定的最小值，并且超过了上一次aof文件的一定百分比，则会触发aof rewrite。
2. AOF Rewrite
rewrite的大致流程是：创建子进程，获取当前快照，同时将之后的命令记录到aof_rewrite_buf中，子进程遍历db生成aof 临时文件，然后退出；父进程wait子进程，待结束后，将aof_rewrite_buf中的数据追加到该aof文件中，最后重命名该临时文件为正式的aof文件。
下面看具体代码，首先是rewriteAppendOnlyFileBackground。
pid_t childpid;
long long start;
// <MM>
// 避免同时多个进程进行rewrite
// </MM>
if (server.aof_child_pid != -1) return REDIS_ERR;
如果有其他aof rewrite进程正在进行，直接返回错误。
start = ustime();
if ((childpid = fork()) == 0) {
char tmpfile[256];
/* Child */
// <MM>
// 子进程不能接受连接
// </MM>
closeListeningSockets(0);
redisSetProcTitle("redis-aof-rewrite");
// <MM>
// 生成临时aof文件名
// </MM>
snprintf(tmpfile,256,"temp-rewriteaof-bg-%d.aof", (int) getpid());
if (rewriteAppendOnlyFile(tmpfile) == REDIS_OK) {
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));
}
exitFromChild(0);
} else {
exitFromChild(1);
}
去当前时间，用于统计fork耗时。然后调用fork，进入子进程的流程。子进程首先关闭监听socket，避免接收客户端连接。同时设置进程的title。然后，生成rewirte要写入的临时文件名。接下来调用rewriteAppendOnlyFile进行rewrite。如果rewrite成功，统计copy-on-write的脏页并记录日志，然后以退出码0退出进程。如果rewrite失败，则退出进程并返回1作为退出码。
下面看一下父进程的流程：
} else {
/* Parent */
server.stat_fork_time = ustime()-start;
server.stat_fork_rate = (double) zmalloc_used_memory() * 1000000 / server.stat_fork_time / (1024*1024*1024); /* GB per second. */
latencyAddSampleIfNeeded("fork",server.stat_fork_time/1000);
if (childpid == -1) {
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);
server.aof_rewrite_scheduled = 0;
server.aof_rewrite_time_start = time(NULL);
server.aof_child_pid = childpid;
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. */
server.aof_selected_db = -1;
replicationScriptCacheFlush();
return REDIS_OK;
}
父进程首先统计fork耗时并采样。如果fork失败，记录日志并返回错误。如果fork成功，对aof_rewrite_scheduled清零，记录rewrite开始时间以及aof_child_pid（redis通过这个属性判断是否有aof rewrite在进行）。调用updateDictResizePolicy调整db的key space的rehash策略，由于创建了子进程，避免copy-on-write复制大量内存页，这里会禁止dict的rehash。
将aof_selected_db置为-1，目的是，下一条aof会首先生成一条select db的日志，同时会写到aof_rewrite_buf中，这样就可以将aof_rewrite_buf正常的追加到rewrite之后的文件。replicationScriptCacheFlush暂时没看到这，之后再补。
下面看一下子进程进行aof rewrite的过程，进入rewriteAppendOnlyFile函数。大体上，就是遍历所有key，进行序列化，然后记录到aof文件中。
dictIterator *di = NULL;
dictEntry *de;
rio aof;
FILE *fp;
char tmpfile[256];
int j;
long long now = mstime();
/* Note that we have to use a different temp name here compared to the
* one used by rewriteAppendOnlyFileBackground() function. */
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;
}
获取当前时间，生成临时文件名并创建该文件。
rioInitWithFile(&aof,fp);
if (server.aof_rewrite_incremental_fsync)
rioSetAutoSync(&aof,REDIS_AOF_AUTOSYNC_BYTES);
rio就是面向流的I/O接口，底层可以有不同实现，目前提供了文件和内存buffer的实现。这里对rio进行初始化。如果配置了server.aof_rewrite_incremental_fsync，则在写aof时会增量地进行fsync，这里配置的是每写入32M就sync一次。避免集中sync导致磁盘跑满。
接下来是一个循环，用于遍历redis的每个db，对其进行rewirte。直接看循环内部：
char selectcmd[] = "*2
$6
SELECT
";
redisDb *db = server.db+j;
dict *d = db->dict;
if (dictSize(d) == 0) continue;
di = dictGetSafeIterator(d);
if (!di) {
fclose(fp);
return REDIS_ERR;
}
/* SELECT the new DB */
if (rioWrite(&aof,selectcmd,sizeof(selectcmd)-1) == 0) goto werr;
if (rioWriteBulkLongLong(&aof,j) == 0) goto werr;
首先，生成对应db的select命令，然后查看如果db为空的话，就跳过，rewrite下一个db。然后获取该db的迭代器，如果获取失败，直接返回错误。最后将select db的命令写入文件。
接下来还是一个循环，用于遍历db的每一个key，生成相应的命令。
while ((de = dictNext(di)) != NULL) {
// ...
}
dictReleaseIterator(di);
继续看循环内部：
sds keystr;
robj key, *o;
long long expiretime;
keystr = dictGetKey(de);
o = dictGetVal(de);
initStaticStringObject(key,keystr);
expiretime = getExpire(db,&key);
/* If this key is already expired skip it */
if (expiretime != -1 && expiretime < now) continue;
de是dict的一个entry，包含了key和value。这里，首先获取key和value，并将key转换成robj类型。然后，获取key对应的超时时间。如果已经超时，则跳过这个key。
/* Save the key and associated value */
if (o->type == REDIS_STRING) {
/* Emit a SET command */
char cmd[]="*3
$3
SET
";
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");
}
接下来，根据对象的类型，序列化成相应的命令。并将命令写入aof文件中。具体各个对象的序列化，这里不再详述。
/* Save the expire time */
if (expiretime != -1) {
char cmd[]="*3
$9
PEXPIREAT
";
if (rioWrite(&aof,cmd,sizeof(cmd)-1) == 0) goto werr;
if (rioWriteBulkObject(&aof,&key) == 0) goto werr;
if (rioWriteBulkLongLong(&aof,expiretime) == 0) goto werr;
}
如果有超时时间，同样序列化成命令记录到aof文件。
所有db的rewrite结束后，进行清理工作。
/* Make sure data will not remain on the OS's output buffers */
if (fflush(fp) == EOF) goto werr;
if (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. */
if (rename(tmpfile,filename) == -1) {
redisLog(REDIS_WARNING,"Error moving temp append only file on the final destination: %s", strerror(errno));
unlink(tmpfile);
return REDIS_ERR;
}
redisLog(REDIS_NOTICE,"SYNC append only file rewrite performed");
return REDIS_OK;
调用fflush，fsync将数据落地到磁盘，最后close文件。将临时文件重命名，确保生成的aof文件完全ok，避免出现aof不完整的情况。最后，打印日志并返回。
werr:
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;
在打开文件后，任何一个步出错，都会跳到werr，进行错误处理。这里，需要将文件close，删除临时文件，如果dict的迭代器没有释放的话，需要进行释放。最后，返回error。
到这，子进程的aof rewrite任务就完成了，现在rewrite后的文件已经生成，但是在rewrite过程中得日志并没有记录到aof文件，所以还需部分收尾工作，这是在主进程中完成的。
3. AOF Rewrite Buffer追加
多进程编程中，子进程退出后，父进程需要对其进行清理，否则子进程会编程僵尸进程。同样是在serverCron函数中，主进程完成对rewrite进程的清理。
/* Check if a background saving or AOF rewrite in progress terminated. */
if (server.rdb_child_pid != -1 || server.aof_child_pid != -1) {
int statloc;
pid_t pid;
if ((pid = wait3(&statloc,WNOHANG,NULL)) != 0) {
int exitcode = WEXITSTATUS(statloc);
int bysignal = 0;
if (WIFSIGNALED(statloc)) bysignal = WTERMSIG(statloc);
if (pid == server.rdb_child_pid) {
backgroundSaveDoneHandler(exitcode,bysignal);
} else if (pid == server.aof_child_pid) {
backgroundRewriteDoneHandler(exitcode,bysignal);
} else {
redisLog(REDIS_WARNING,
"Warning, detected child with unmatched pid: %ld",
(long)pid);
}
updateDictResizePolicy();
}
} else {
如果正在进程rdb dump或者aof rewrite，主进程会非阻塞的调用wait3函数，以便在子进程退出后，获取其退出状态。如果退出的进程是aof rewrite进程的话，会调用backgroundRewriteDoneHandler函数进行最后的收尾工作。下面看一下这个函数。
如果正常退出的情况下，就是没有被信号kill，并且退出码等于0。
int newfd, oldfd;
char tmpfile[256];
long long now = ustime();
mstime_t latency;
redisLog(REDIS_NOTICE,
"Background AOF rewrite terminated with success");
/* Flush the differences accumulated by the parent to the
* rewritten AOF. */
latencyStartMonitor(latency);
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;
}
首先是记录日志，然后打开临时写入的rewrite文件。
// <MM>
// 将rewrite buf追加到文件
// </MM>
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;
}
latencyEndMonitor(latency);
latencyAddSampleIfNeeded("aof-rewrite-diff-write",latency);
redisLog(REDIS_NOTICE,
"Parent diff successfully flushed to the rewritten AOF (%lu bytes)", aofRewriteBufferSize());
接下来，将aof rewrite buffer追加到文件。
/* 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) {
// <MM>
// 没有开启AOF，由命令触发的aof rewrite
// </MM>
/* 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". */
latencyStartMonitor(latency);
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;
}
latencyEndMonitor(latency);
latencyAddSampleIfNeeded("aof-rename",latency);
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;
server.aof_fd = newfd;
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. */
sdsfree(server.aof_buf);
server.aof_buf = sdsempty();
}
然后，将临时文件重命名为最终的aof文件。
server.aof_lastbgrewrite_status = REDIS_OK;
redisLog(REDIS_NOTICE, "Background AOF rewrite finished successfully");
/* Change state from WAIT_REWRITE to ON if needed */
if (server.aof_state == REDIS_AOF_WAIT_REWRITE)
server.aof_state = REDIS_AOF_ON;
/* Asynchronously close the overwritten AOF. */
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);
最后，更新状态，异步关闭之前的aof文件。
如果rewrite子进程异常退出，由信号kill或者退出码非0，则只是记录 日志。
} 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);
}
在追加rewrite buffer或者重命名文件失败时，需要进行清理工作，有cleanup分支处理：
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;
主要就是重置状态，以便进行下一次rewrite。
上面就是aof rewrite的整体流程，下面会介绍rdb相关部分。