AOFRewrite触发场景
AOFRewrite触发场景：
- 执行BGREWRITEAOF命令重写AOF日志时触发
- 使用CONFIG SET命令开启AOF日志时触发
- 当AOF日志超过基准大小的特定百分比时(参数auto-aof-rewrite-percentage控制)时触发。
执行BGREWRITEAOF命令时触发
当执行BGREWRITEAOF命令时，会调用bgrewriteaofCommand来进行AOF日志重写。
void bgrewriteaofCommand(client *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() == C_OK) {
addReplyStatus(c,"Background append only file rewriting started");
} else {
addReply(c,shared.err);
}
}
使用CONFIG SET命令开启AOF日志时触发
当使用config set appendonly yes命令来开启AOF日志时，会调用startAppendOnly来进行AOF日志重写。
/* Called when the user switches from "appendonly no" to "appendonly yes"
* at runtime using the CONFIG command. */
int startAppendOnly(void) {
char cwd[MAXPATHLEN]; /* Current working dir path for error messages. */
int newfd;
newfd = open(server.aof_filename,O_WRONLY|O_APPEND|O_CREAT,0644);
serverAssert(server.aof_state == AOF_OFF);
if (newfd == -1) {
char *cwdp = getcwd(cwd,MAXPATHLEN);
serverLog(LL_WARNING,
"Redis needs to enable the AOF but can‘t open the "
"append only file %s (in server root dir %s): %s",
server.aof_filename,
cwdp ? cwdp : "unknown",
strerror(errno));
return C_ERR;
}
if (server.rdb_child_pid != -1) {
server.aof_rewrite_scheduled = 1;
serverLog(LL_WARNING,"AOF was enabled but there is already a child process saving an RDB file on disk. An AOF background was scheduled to start when possible.");
} else {
/* If there is a pending AOF rewrite, we need to switch it off and
* start a new one: the old one cannot be reused because it is not
* accumulating the AOF buffer. */
if (server.aof_child_pid != -1) {
serverLog(LL_WARNING,"AOF was enabled but there is already an AOF rewriting in background. Stopping background AOF and starting a rewrite now.");
killAppendOnlyChild();
}
if (rewriteAppendOnlyFileBackground() == C_ERR) {
close(newfd);
serverLog(LL_WARNING,"Redis needs to enable the AOF but can‘t trigger a background AOF rewrite operation. Check the above logs for more info about the error.");
return C_ERR;
}
}
/* We correctly switched on AOF, now wait for the rewrite to be complete
* in order to append data on disk. */
server.aof_state = AOF_WAIT_REWRITE;
server.aof_last_fsync = server.unixtime;
server.aof_fd = newfd;
return C_OK;
}
当Redis实例故障恢复时，如果非sentinel模式，会尝试从本地存储上加载数据。
int main(int argc, char **argv) {
if (!server.sentinel_mode) {
moduleLoadFromQueue();
InitServerLast();
loadDataFromDisk();
} else {
InitServerLast();
sentinelIsRunning();
}
}
在加载过程中，如果开启AOF日志，只会加载AOF文件。如果未开启AOF日志，则会尝试加载RDB文件。
/* Function called at startup to load RDB or AOF file in memory. */
void loadDataFromDisk(void) {
long long start = ustime();
if (server.aof_state == AOF_ON) {
if (loadAppendOnlyFile(server.aof_filename) == C_OK)
serverLog(LL_NOTICE,
"DB loaded from append only file: %.3f seconds",
(float)(ustime()-start)/1000000);
} else {
rdbSaveInfo rsi = RDB_SAVE_INFO_INIT;
if (rdbLoad(server.rdb_filename,&rsi) == C_OK) {
serverLog(LL_NOTICE,"DB loaded from disk: %.3f seconds",
(float)(ustime()-start)/1000000);
/* Restore the replication ID / offset from the RDB file. */
if ((server.masterhost ||
(server.cluster_enabled &&
nodeIsSlave(server.cluster->myself))) &&
rsi.repl_id_is_set &&
rsi.repl_offset != -1 &&
/* Note that older implementations may save a repl_stream_db
* of -1 inside the RDB file in a wrong way, see more
* information in function rdbPopulateSaveInfo. */
rsi.repl_stream_db != -1)
{
memcpy(server.replid,rsi.repl_id,sizeof(server.replid));
server.master_repl_offset = rsi.repl_offset;
/* If we are a slave, create a cached master from this
* information, in order to allow partial resynchronizations
* with masters. */
replicationCacheMasterUsingMyself();
selectDb(server.cached_master,rsi.repl_stream_db);
}
} else if (errno != ENOENT) {
serverLog(LL_WARNING,"Fatal error loading the DB: %s. Exiting.",strerror(errno));
exit(1);
}
}
}
在Redis实例运行过程中，如果开启AOF日志且AOF日志中仅存放开启AOF日志后的AOF日志，那么无法保证Redis故障恢复时AOF文件中包含所有数据，因此需要在开启AOF日志时进行一次AOF Rewrite操作。
PS：如果开启AOF日志后但AOF Rewrite操作未完成前发生故障恢复会丢失数据。
AOF日志增长率超过阈值时触发
在定期执行的serverCron函数中，会检查AOF日志文件与基准文件的增长比例，当增长比例超过参数auto-aof-rewrite-percentage阈值后触发AOF日志重写：
int serverCron(struct aeEventLoop *eventLoop, long long id, void *clientData) {
/* Trigger an AOF rewrite if needed. */
if (server.aof_state == AOF_ON &&
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) {
serverLog(LL_NOTICE,"Starting automatic rewriting of AOF on %lld%% growth",growth);
rewriteAppendOnlyFileBackground();
}
}
}
参数auto-aof-rewrite-percentage初始化：
## 设置AOF日志重写的触发阈值,默认为100，即当前AOF日志是基准AOF日志大小的两倍时触发重写。
#define AOF_REWRITE_PERC 100
## 通过参数auto-aof-rewrite-percentage或预定义变量AOF_REWRITE_PERC来初始化server.aof_rewrite_perc
rewriteConfigNumericalOption(
state,
"auto-aof-rewrite-percentage",
server.aof_rewrite_perc,AOF_REWRITE_PERC);
变量server.aof_rewrite_base_size初始化：
- 当Redis实例故障恢复时，使用loadAppendOnlyFile来恢复数据，并用当前的AOF日志文件作为基准文件。
- 当Redis实例AOF日志重写完成后，使用backgroundRewriteDoneHandler来清理操作，并用当前AOF日志文件作为基准文件。
/* Replay the append log file. On success C_OK is returned. On non fatal
* error (the append only file is zero-length) C_ERR is returned. On
* fatal error an error message is logged and the program exists. */
int loadAppendOnlyFile(char *filename) {
loaded_ok: /* DB loaded, cleanup and return C_OK to the caller. */
fclose(fp);
freeFakeClient(fakeClient);
server.aof_state = old_aof_state;
stopLoading();
aofUpdateCurrentSize();
server.aof_rewrite_base_size = server.aof_current_size;
server.aof_fsync_offset = server.aof_current_size;
return C_OK;
}
/* A background append only file rewriting (BGREWRITEAOF) terminated its work.
* Handle this. */
void backgroundRewriteDoneHandler(int exitcode, int bysignal) {
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)
redis_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;
server.aof_fsync_offset = 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();
}
}