前些天一台redis机器硬盘写满了，主要是由于程序bug导致备份量激增，而恰好监控程序的通知机制也罢工了，于是第一次体验到了redis的罢工（只读不写）。 现在我们来看下在磁盘写满后redis的处理机制： save流程：serverCron-rdbSaveBackground-rdbSave save后
前些天一台redis机器硬盘写满了，主要是由于程序bug导致备份量激增，而恰好监控程序的通知机制也罢工了，于是第一次体验到了redis的罢工（只读不写）。
现在我们来看下在磁盘写满后redis的处理机制：受其影响，processCommand和luaRedisGenericCommand中判断如果是写操作，则直接返回REDIS_OK，而没有实际写入
1.rdbSave所有的写出错都会返回REDIS_ERR
int rdbSave(char *filename) {
dictIterator *di = NULL;
dictEntry *de;
char tmpfile[256];
char magic[10];
int j;
long long now = mstime();
FILE *fp;
rio rdb;
uint64_t cksum;
snprintf(tmpfile,256,"temp-%d.rdb", (int) getpid());
fp = fopen(tmpfile,"w");
if (!fp) {
redisLog(REDIS_WARNING, "Failed opening .rdb for saving: %s",
strerror(errno));
return REDIS_ERR;
}
rioInitWithFile(&rdb,fp);
if (server.rdb_checksum)
rdb.update_cksum = rioGenericUpdateChecksum;
snprintf(magic,sizeof(magic),"REDIS%04d",REDIS_RDB_VERSION);
if (rdbWriteRaw(&rdb,magic,9) == -1) 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(&rdb,REDIS_RDB_OPCODE_SELECTDB) == -1) goto werr;
if (rdbSaveLen(&rdb,j) == -1) goto werr;
/* Iterate this DB writing every entry */
while((de = dictNext(di)) != NULL) {
sds keystr = dictGetKey(de);
robj key, *o = dictGetVal(de);
long long expire;
initStaticStringObject(key,keystr);
expire = getExpire(db,&key);
if (rdbSaveKeyValuePair(&rdb,&key,o,expire,now) == -1) goto werr;
}
dictReleaseIterator(di);
}
di = NULL; /* So that we don't release it again on error. */
/* EOF opcode */
if (rdbSaveType(&rdb,REDIS_RDB_OPCODE_EOF) == -1) goto werr;
/* CRC64 checksum. It will be zero if checksum computation is disabled, the
* loading code skips the check in this case. */
cksum = rdb.cksum;
memrev64ifbe(&cksum);
if (rioWrite(&rdb,&cksum,8) == 0) goto werr;
/* 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 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);
server.lastbgsave_status = REDIS_OK;
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;
}2.rdbSaveBackground中，如果子进程调用rdbsave返回REDIS_ERR，那么子进程exit(1)
int rdbSaveBackground(char *filename) {
pid_t childpid;
long long start;
if (server.rdb_child_pid != -1) return REDIS_ERR;
server.dirty_before_bgsave = server.dirty;
server.lastbgsave_try = time(NULL);
start = ustime();
if ((childpid = fork()) == 0) {
int retval;
/* Child */
closeListeningSockets(0);
redisSetProcTitle("redis-rdb-bgsave");
retval = rdbSave(filename);
if (retval == REDIS_OK) {
size_t private_dirty = zmalloc_get_private_dirty();
if (private_dirty) {
redisLog(REDIS_NOTICE,
"RDB: %zu MB of memory used by copy-on-write",
private_dirty/(1024*1024));
}
}
exitFromChild((retval == REDIS_OK) ? 0 : 1); //进程退出时返回0/1
} else {
/* Parent */
server.stat_fork_time = ustime()-start;
if (childpid == -1) {
server.lastbgsave_status = REDIS_ERR;
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.rdb_save_time_start = time(NULL);
server.rdb_child_pid = childpid;
updateDictResizePolicy();
return REDIS_OK;
}
return REDIS_OK; /* unreached */
}
/* 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); //根据bgsave子进程的exitcode以及是否由信号结束的标签进行后续处理
} else if (pid == server.aof_child_pid) {
backgroundRewriteDoneHandler(exitcode,bysignal);
} else {
redisLog(REDIS_WARNING,
"Warning, detected child with unmatched pid: %ld",
(long)pid);
}
updateDictResizePolicy();
}
}
void backgroundSaveDoneHandler(int exitcode, int bysignal) {
if (!bysignal && exitcode == 0) {
redisLog(REDIS_NOTICE,
"Background saving terminated with success");
server.dirty = server.dirty - server.dirty_before_bgsave;
server.lastsave = time(NULL);
server.lastbgsave_status = REDIS_OK;
} else if (!bysignal && exitcode != 0) {
redisLog(REDIS_WARNING, "Background saving error");
server.lastbgsave_status = REDIS_ERR; //状态转换
} else {
mstime_t latency;
redisLog(REDIS_WARNING,
"Background saving terminated by signal %d", bysignal);
latencyStartMonitor(latency);
rdbRemoveTempFile(server.rdb_child_pid);
latencyEndMonitor(latency);
latencyAddSampleIfNeeded("rdb-unlink-temp-file",latency);
/* SIGUSR1 is whitelisted, so we have a way to kill a child without
* tirggering an error conditon. */
if (bysignal != SIGUSR1)
server.lastbgsave_status = REDIS_ERR;
}
server.rdb_child_pid = -1;
server.rdb_save_time_last = time(NULL)-server.rdb_save_time_start;
server.rdb_save_time_start = -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((!bysignal && exitcode == 0) ? REDIS_OK : REDIS_ERR);
}
/* Don't accept write commands if there are problems persisting on disk
* and if this is a master instance. */
if (((server.stop_writes_on_bgsave_err &&
server.saveparamslen > 0 &&
server.lastbgsave_status == REDIS_ERR) ||
server.aof_last_write_status == REDIS_ERR) &&
server.masterhost == NULL &&
(c->cmd->flags & REDIS_CMD_WRITE ||
c->cmd->proc == pingCommand))
{
flagTransaction(c);
if (server.aof_last_write_status == REDIS_OK)
addReply(c, shared.bgsaveerr);
else
addReplySds(c,
sdscatprintf(sdsempty(),
"-MISCONF Errors writing to the AOF file: %s\r\n",
strerror(server.aof_last_write_errno)));
return REDIS_OK;
}
/* Write commands are forbidden against read-only slaves, or if a
* command marked as non-deterministic was already called in the context
* of this script. */
if (cmd->flags & REDIS_CMD_WRITE) {
if (server.lua_random_dirty) {
luaPushError(lua,
"Write commands not allowed after non deterministic commands");
goto cleanup;
} else if (server.masterhost && server.repl_slave_ro &&
!server.loading &&
!(server.lua_caller->flags & REDIS_MASTER))
{
luaPushError(lua, shared.roslaveerr->ptr);
goto cleanup;
} else if (server.stop_writes_on_bgsave_err &&
server.saveparamslen > 0 &&
server.lastbgsave_status == REDIS_ERR)
{
luaPushError(lua, shared.bgsaveerr->ptr);
goto cleanup;
}
}
cleanup:
/* Clean up. Command code may have changed argv/argc so we use the
* argv/argc of the client instead of the local variables. */
for (j = 0; j < c->argc; j++) {
robj *o = c->argv[j];
/* Try to cache the object in the cached_objects array.
* The object must be small, SDS-encoded, and with refcount = 1
* (we must be the only owner) for us to cache it. */
if (j < LUA_CMD_OBJCACHE_SIZE &&
o->refcount == 1 &&
o->encoding == REDIS_ENCODING_RAW &&
sdslen(o->ptr) <= LUA_CMD_OBJCACHE_MAX_LEN)
{
struct sdshdr *sh = (void*)(((char*)(o->ptr))-(sizeof(struct sdshdr)));
if (cached_objects[j]) decrRefCount(cached_objects[j]);
cached_objects[j] = o;
cached_objects_len[j] = sh->free + sh->len;
} else {
decrRefCount(o);
}
}
if (c->argv != argv) {
zfree(c->argv);
argv = NULL;
}
if (raise_error) {
/* If we are here we should have an error in the stack, in the
* form of a table with an "err" field. Extract the string to
* return the plain error. */
lua_pushstring(lua,"err");
lua_gettable(lua,-2);
return lua_error(lua);
}
return 1; 
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