Redis source code analysis: AOF strategy and time trigger task

Time periodic tasks and AOF strategy

When analyzing the startup process and server-side processing process of periodic tasks, the processing process of time tasks has been described. In the process of event driven in Redis, there is a mechanism to trigger and process events through time. This paper will first analyze what the main time driven tasks will accomplish. After analyzing the time task, continue to analyze Redis's AOF mechanism. This mechanism provides a more secure way for the data in memory to ensure that the data in the running Redis can be recovered in case of sudden power failure or server exception. This mode usually uses an AOF file to save the relevant operation records. If When Redis service crashes and restarts, you only need to read the AOF file to replay the relevant operations, and then you can recover the data before the crash as much as possible.

Time cycle task

The time event analyzed this time is in the main cycle event. As long as it is awakened from IO reuse, it will execute the callback function of expiration time. In Redis, there is a relatively important time callback function. The main function of this function is to timeout the client, process the expired data, update various statistical information of the server, and check whether the AOF or RDB persistence operation is required, If the cluster mode is used to check whether data synchronization and other operations are needed for the slave server, the function is serverCron, which is created in the initServer process;

    /* Create the timer callback, this is our way to process many background
     * operations incrementally, like clients timeout, eviction of unaccessed
     * expired keys and so forth. */
    if (aeCreateTimeEvent(server.el, 1, serverCron, NULL, NULL) == AE_ERR) {           // Create event event
        serverPanic("Can't create event loop timers.");

The registration time is to execute the callback function every second, and the function needs to perform many functions;

/* This is our timer interrupt, called server.hz times per second.
 * Here is where we do a number of things that need to be done asynchronously.
 * For instance:
 * - Active expired keys collection (it is also performed in a lazy way on
 *   lookup).
 * - Software watchdog.
 * - Update some statistic.
 * - Incremental rehashing of the DBs hash tables.  
 * - Triggering BGSAVE / AOF rewrite, and handling of terminated children.
 * - Clients timeout of different kinds.
 * - Replication reconnection.
 * - Many more...
 * Everything directly called here will be called server.hz times per second,
 * so in order to throttle execution of things we want to do less frequently
 * a macro is used: run_with_period(milliseconds) { .... }

int serverCron(struct aeEventLoop *eventLoop, long long id, void *clientData) {
    int j;

    /* Software watchdog: deliver the SIGALRM that will reach the signal
     * handler if we don't return here fast enough. */
    if (server.watchdog_period) watchdogScheduleSignal(server.watchdog_period);         // Setting timing signal mainly uses setitimer to send timing signal

    /* Update the time cache. */
    updateCachedTime();                     // Update cache time

    server.hz = server.config_hz;               // Get cycle time of server configuration
    /* Adapt the server.hz value to the number of configured clients. If we have
     * many clients, we want to call serverCron() with an higher frequency. */
    if (server.dynamic_hz) {                                                        // Whether to configure the dynamic frequency. If the frequency is dynamic, the sleep time will be retrieved by calculation every time
        while (listLength(server.clients) / server.hz >
               MAX_CLIENTS_PER_CLOCK_TICK)                                          // The number of currently connected clients except whether the frequency is greater than each clock value
            server.hz *= 2;                                                         // Increase frequency if idle
            if (server.hz > CONFIG_MAX_HZ) {                                        // Check that the maximum setting cannot be exceeded
                server.hz = CONFIG_MAX_HZ;

    run_with_period(100) {                                                         // The number of times the loop executes is used to determine whether it is executed or less than a hz execution time, so as to extend to the specified time length and callback
        trackInstantaneousMetric(STATS_METRIC_COMMAND,server.stat_numcommands);    // Run sampling data every time and save

    /* We have just LRU_BITS bits per object for LRU information.
     * So we use an (eventually wrapping) LRU clock.
     * Note that even if the counter wraps it's not a big problem,
     * everything will still work but some object will appear younger
     * to Redis. However for this to happen a given object should never be
     * touched for all the time needed to the counter to wrap, which is
     * not likely.
     * Note that you can change the resolution altering the
     * LRU_CLOCK_RESOLUTION define. */
    unsigned long lruclock = getLRUClock();                                     // Get LRU time

    /* Record the max memory used since the server was started. */
    if (zmalloc_used_memory() > server.stat_peak_memory)                        // Record memory used above peak
        server.stat_peak_memory = zmalloc_used_memory();                        

    run_with_period(100) {                                                      // Once per execution
        /* Sample the RSS and other metrics here since this is a relatively slow call.
         * We must sample the zmalloc_used at the same time we take the rss, otherwise
         * the frag ratio calculate may be off (ratio of two samples at different times) */
        server.cron_malloc_stats.process_rss = zmalloc_get_rss();               // Record RSS data
        server.cron_malloc_stats.zmalloc_used = zmalloc_used_memory();          // Record memory data used
        /* Sampling the allcator info can be slow too.
         * The fragmentation ratio it'll show is potentically more accurate
         * it excludes other RSS pages such as: shared libraries, LUA and other non-zmalloc
         * allocations, and allocator reserved pages that can be pursed (all not actual frag) */
        /* in case the allocator isn't providing these stats, fake them so that
         * fragmention info still shows some (inaccurate metrics) */
        if (!server.cron_malloc_stats.allocator_resident) {
            /* LUA memory isn't part of zmalloc_used, but it is part of the process RSS,
             * so we must desuct it in order to be able to calculate correct
             * "allocator fragmentation" ratio */
            size_t lua_memory = lua_gc(server.lua,LUA_GCCOUNT,0)*1024LL;
            server.cron_malloc_stats.allocator_resident = server.cron_malloc_stats.process_rss - lua_memory;
        if (!server.cron_malloc_stats.allocator_active)
            server.cron_malloc_stats.allocator_active = server.cron_malloc_stats.allocator_resident;
        if (!server.cron_malloc_stats.allocator_allocated)
            server.cron_malloc_stats.allocator_allocated = server.cron_malloc_stats.zmalloc_used;

    /* We received a SIGTERM, shutting down here in a safe way, as it is
     * not ok doing so inside the signal handler. */
    if (server.shutdown_asap) {                             // Safe shutdown if SIGTERM signal is received
        if (prepareForShutdown(SHUTDOWN_NOFLAGS) == C_OK) exit(0);          // Call prepareForShutdown to close
        serverLog(LL_WARNING,"SIGTERM received but errors trying to shut down the server, check the logs for more information");
        server.shutdown_asap = 0;

    /* Show some info about non-empty databases */
    run_with_period(5000) {                                         // Once every 50 loop s
        for (j = 0; j < server.dbnum; j++) {
            long long size, used, vkeys;

            size = dictSlots(server.db[j].dict);                    // It mainly displays the size and idle number of currently saved data of each db
            used = dictSize(server.db[j].dict);
            vkeys = dictSize(server.db[j].expires);
            if (used || vkeys) {
                serverLog(LL_VERBOSE,"DB %d: %lld keys (%lld volatile) in %lld slots HT.",j,used,vkeys,size);
                /* dictPrintStats(server.dict); */

    /* Show information about connected clients */
    if (!server.sentinel_mode) {                                    // Cluster mode or not
        run_with_period(5000) {                                     // Show the connection information, the number of slave nodes and the memory used every 50 times
                "%lu clients connected (%lu replicas), %zu bytes in use",

    /* We need to do a few operations on clients asynchronously. */
    clientsCron();                                                  // Expect to be able to update connected clients such as timeout

    /* Handle background operations on Redis databases. */
    databasesCron();                                                // Perform background operation commands of database, such as key expiration, expansion size, rehash, etc

    /* 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 &&
        rewriteAppendOnlyFileBackground();                          // Check if the AOF file needs to be rewritten in the background

      /* Check if a background saving or AOF rewrite in progress terminated. */
    if (server.rdb_child_pid != -1 || server.aof_child_pid != -1 ||
        ldbPendingChildren())                                       // Check whether there is currently a background rdb or AOF rewritten task process
        int statloc;
        pid_t pid;

        if ((pid = wait3(&statloc,WNOHANG,NULL)) != 0) {            // Check whether the task is completed
            int exitcode = WEXITSTATUS(statloc);
            int bysignal = 0;

            if (WIFSIGNALED(statloc)) bysignal = WTERMSIG(statloc);

            if (pid == -1) {                                            // Execution error if - 1
                serverLog(LL_WARNING,"wait3() returned an error: %s. "
                    "rdb_child_pid = %d, aof_child_pid = %d",
                    (int) server.rdb_child_pid,
                    (int) server.aof_child_pid);
            } else if (pid == server.rdb_child_pid) {
                backgroundSaveDoneHandler(exitcode,bysignal);           // Mission accomplished
                if (!bysignal && exitcode == 0) receiveChildInfo();
            } else if (pid == server.aof_child_pid) {
                if (!bysignal && exitcode == 0) receiveChildInfo();
            } else {
                if (!ldbRemoveChild(pid)) {
                        "Warning, detected child with unmatched pid: %ld",
    } else {
        /* If there is not a background saving/rewrite in progress check if
         * we have to save/rewrite now. */
        for (j = 0; j < server.saveparamslen; j++) {                    // Get parameters to execute task
            struct saveparam *sp = server.saveparams+j;

            /* Save if we reached the given amount of changes,
             * the given amount of seconds, and if the latest bgsave was
             * successful or if, in case of an error, at least
             * CONFIG_BGSAVE_RETRY_DELAY seconds already elapsed. */
            if (server.dirty >= sp->changes &&
                server.unixtime-server.lastsave > sp->seconds &&
                (server.unixtime-server.lastbgsave_try >
                 CONFIG_BGSAVE_RETRY_DELAY ||
                 server.lastbgsave_status == C_OK))
                serverLog(LL_NOTICE,"%d changes in %d seconds. Saving...",
                    sp->changes, (int)sp->seconds);
                rdbSaveInfo rsi, *rsiptr;
                rsiptr = rdbPopulateSaveInfo(&rsi);
                rdbSaveBackground(server.rdb_filename,rsiptr);              // Start rdb background save task

        /* 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();                          // Check if aof flushing is needed and start aof rewrite if the task needs to be executed

    /* AOF postponed flush: Try at every cron cycle if the slow fsync
     * completed. */
    if (server.aof_flush_postponed_start) flushAppendOnlyFile(0);

    /* AOF write errors: in this case we have a buffer to flush as well and
     * clear the AOF error in case of success to make the DB writable again,
     * however to try every second is enough in case of 'hz' is set to
     * an higher frequency. */
    run_with_period(1000) {                                 // Check whether there is an aof error at the end of every 10 loop s. If there is an error, swipe the aof file
        if (server.aof_last_write_status == C_ERR)

    /* Close clients that need to be closed asynchronous */
    freeClientsInAsyncFreeQueue();                              // Release clients for asynchronous operations to close

    /* Clear the paused clients flag if needed. */
    clientsArePaused(); /* Don't check return value, just use the side effect.*/

    /* Replication cron function -- used to reconnect to master,
     * detect transfer failures, start background RDB transfers and so forth. */
    run_with_period(1000) replicationCron();                    // Check whether replication is needed ten times per loop

    /* Run the Redis Cluster cron. */
    run_with_period(100) {
        if (server.cluster_enabled) clusterCron();              // Check the connectivity of the cluster

    /* Run the Sentinel timer if we are in sentinel mode. */
    if (server.sentinel_mode) sentinelTimer();                  // Is it sentinel mode? If so, start sentinel timer

    /* Cleanup expired MIGRATE cached sockets. */
    run_with_period(1000) {                                     // Check whether the timeout client needs to be closed every 10 times of loop

    /* Start a scheduled BGSAVE if the corresponding flag is set. This is
     * useful when we are forced to postpone a BGSAVE because an AOF
     * rewrite is in progress.
     * Note: this code must be after the replicationCron() call above so
     * make sure when refactoring this file to keep this order. This is useful
     * because we want to give priority to RDB savings for replication. */
    if (server.rdb_child_pid == -1 && server.aof_child_pid == -1 &&
        server.rdb_bgsave_scheduled &&
        (server.unixtime-server.lastbgsave_try > CONFIG_BGSAVE_RETRY_DELAY ||
         server.lastbgsave_status == C_OK))
        rdbSaveInfo rsi, *rsiptr;
        rsiptr = rdbPopulateSaveInfo(&rsi);                                 // If the flag bit of BGSAVE is set, rdb saving is performed
        if (rdbSaveBackground(server.rdb_filename,rsiptr) == C_OK)
            server.rdb_bgsave_scheduled = 0;

    server.cronloops++;                                                     // Add execution time
    return 1000/server.hz;                                                  // Next run time

It can be seen from the execution process of this function that the functions of updating all kinds of statistical information of the server, clearing the expiration time in the database, processing the timeout clients, and whether to perform AOF or RDB operations are all completed in this timed task function.

Deletion of overdue value of database and expansion of hash table
/* This function handles 'background' operations we are required to do
 * incrementally in Redis databases, such as active key expiring, resizing,
 * rehashing. */
void databasesCron(void) {
    /* Expire keys by random sampling. Not required for slaves
     * as master will synthesize DELs for us. */
    if (server.active_expire_enabled && server.masterhost == NULL) {
        activeExpireCycle(ACTIVE_EXPIRE_CYCLE_SLOW);            // Random expiration of some key s
    } else if (server.masterhost != NULL) {
        expireSlaveKeys();                                      // Expired key from

    /* Defrag keys gradually. */
    if (server.active_defrag_enabled)           // Is it a gradual arrangement

    /* Perform hash tables rehashing if needed, but only if there are no
     * other processes saving the DB on disk. Otherwise rehashing is bad
     * as will cause a lot of copy-on-write of memory pages. */
    if (server.rdb_child_pid == -1 && server.aof_child_pid == -1) {         // Processing hash table to rehash hash
        /* We use global counters so if we stop the computation at a given
         * DB we'll be able to start from the successive in the next
         * cron loop iteration. */
        static unsigned int resize_db = 0;
        static unsigned int rehash_db = 0;
        int dbs_per_call = CRON_DBS_PER_CALL;
        int j;

        /* Don't test more DBs than we have. */
        if (dbs_per_call > server.dbnum) dbs_per_call = server.dbnum;

        /* Resize */
        for (j = 0; j < dbs_per_call; j++) {
            tryResizeHashTables(resize_db % server.dbnum);                 // Try to resize the database

        /* Rehash */
        if (server.activerehashing) {                                       // Try to rehash the table because rehash is required after resizing
            for (j = 0; j < dbs_per_call; j++) {
                int work_done = incrementallyRehash(rehash_db);
                if (work_done) {
                    /* If the function did some work, stop here, we'll do
                     * more at the next cron loop. */
                } else {
                    /* If this db didn't need rehash, we'll try the next one. */
                    rehash_db %= server.dbnum;

This function is to check whether the expired key needs to be deleted, and whether the hash table in each database needs to be expanded and rehash.

The specific implementation process of other operations will be learned and understood later. Then we'll look at the AOF mechanism.

AOF strategy

The AOF mechanism is that Redis records the database status by saving the write related commands executed by the server. The purpose of this file is to recover the lost data as much as possible when the server is abnormal or the Redis server is abnormal. If the server receives the write command during the execution of set a b to the Redis server, it will be saved to the server.aof_buf, and then the AOF file will be written through the configured AOF write policy. Aof has three file writing strategies;

appendfsync always   # Always write the contents of AOF buf to a file
appendfsync everysec # Write the data of AOF buf to the file every second. If the interval is less than one second, do not write
appendfsync no 			 # It is to drop the data through the drop mechanism of the operating system itself, with high writing efficiency but uncontrollable drop time

At this time, it can be seen from these three parameters that in each loop cycle check, it is necessary to check whether the current aof data writing is needed. In the previous article, the beforeSleep function will be executed for each loop execution;

void beforeSleep(struct aeEventLoop *eventLoop) {
    /* Write the AOF buffer on disk */

Every time, the flushAppendOnlyFile function is called to check whether the data of aof file needs to be dropped. Before that, we can understand that the set command should be saved to aof buf after it is executed by the server, so as to simply browse the process together.

Write command record to AOF

In the process analysis of previous server processing, we can know that ordinary commands will call the call function for the final callback processing;

void call(client *c, int flags) {
    long long dirty, start, duration;
    int client_old_flags = c->flags;
    struct redisCommand *real_cmd = c->cmd;                     // Get current command

    /* Propagate the command into the AOF and replication link */
    if (flags & CMD_CALL_PROPAGATE &&
        int propagate_flags = PROPAGATE_NONE;

        /* Check if the command operated changes in the data set. If so
         * set for replication / AOF propagation. */
        if (dirty) propagate_flags |= (PROPAGATE_AOF|PROPAGATE_REPL);

        /* If the client forced AOF / replication of the command, set
         * the flags regardless of the command effects on the data set. */
        if (c->flags & CLIENT_FORCE_REPL) propagate_flags |= PROPAGATE_REPL;
        if (c->flags & CLIENT_FORCE_AOF) propagate_flags |= PROPAGATE_AOF;

        /* However prevent AOF / replication propagation if the command
         * implementations called preventCommandPropagation() or similar,
         * or if we don't have the call() flags to do so. */
        if (c->flags & CLIENT_PREVENT_REPL_PROP ||
            !(flags & CMD_CALL_PROPAGATE_REPL))
                propagate_flags &= ~PROPAGATE_REPL;
        if (c->flags & CLIENT_PREVENT_AOF_PROP ||
            !(flags & CMD_CALL_PROPAGATE_AOF))
                propagate_flags &= ~PROPAGATE_AOF;

        /* Call propagate() only if at least one of AOF / replication
         * propagation is needed. Note that modules commands handle replication
         * in an explicit way, so we never replicate them automatically. */
        if (propagate_flags != PROPAGATE_NONE && !(c->cmd->flags & CMD_MODULE))


The propagate function is used to record whether the command needs to be written to AOF buf;

/* Propagate the specified command (in the context of the specified database id)
 * to AOF and Slaves.
 * flags are an xor between:
 * + PROPAGATE_NONE (no propagation of command at all)
 * + PROPAGATE_AOF (propagate into the AOF file if is enabled)
 * + PROPAGATE_REPL (propagate into the replication link)
 * This should not be used inside commands implementation. Use instead
 * alsoPropagate(), preventCommandPropagation(), forceCommandPropagation().
void propagate(struct redisCommand *cmd, int dbid, robj **argv, int argc,
               int flags)
    if (server.aof_state != AOF_OFF && flags & PROPAGATE_AOF)       // Whether to record aof
    if (flags & PROPAGATE_REPL)                                     // If there is a slave, it needs to be sent to the slave

Add the command to AOF buf through the feedAppendOnlyFile function;

void feedAppendOnlyFile(struct redisCommand *cmd, int dictid, robj **argv, int argc) {
    sds buf = sdsempty();
    robj *tmpargv[3];

    /* The DB this command was targeting is not the same as the last command
     * we appended. To issue a SELECT command is needed. */
    if (dictid != server.aof_selected_db) {             // Set db first if db command is not selected
        char seldb[64];

        buf = sdscatprintf(buf,"*2\r\n$6\r\nSELECT\r\n$%lu\r\n%s\r\n",
            (unsigned long)strlen(seldb),seldb);
        server.aof_selected_db = dictid;

    if (cmd->proc == expireCommand || cmd->proc == pexpireCommand ||
        cmd->proc == expireatCommand) {                                     // Determine what command is expired or not
        /* Translate EXPIRE/PEXPIRE/EXPIREAT into PEXPIREAT */
        buf = catAppendOnlyExpireAtCommand(buf,cmd,argv[1],argv[2]);        // Add expired command
    } else if (cmd->proc == setexCommand || cmd->proc == psetexCommand) {   // If it's a set command
        /* Translate SETEX/PSETEX to SET and PEXPIREAT */
        tmpargv[0] = createStringObject("SET",3);
        tmpargv[1] = argv[1];
        tmpargv[2] = argv[3];
        buf = catAppendOnlyGenericCommand(buf,3,tmpargv);                   // Add expired commands to cache
        buf = catAppendOnlyExpireAtCommand(buf,cmd,argv[1],argv[2]);      
    } else if (cmd->proc == setCommand && argc > 3) {
        int i;
        robj *exarg = NULL, *pxarg = NULL;
        /* Translate SET [EX seconds][PX milliseconds] to SET and PEXPIREAT */
        buf = catAppendOnlyGenericCommand(buf,3,argv);                      // If it is a set command with multiple parameters
        for (i = 3; i < argc; i ++) {
            if (!strcasecmp(argv[i]->ptr, "ex")) exarg = argv[i+1];
            if (!strcasecmp(argv[i]->ptr, "px")) pxarg = argv[i+1];
        serverAssert(!(exarg && pxarg));
        if (exarg)
            buf = catAppendOnlyExpireAtCommand(buf,server.expireCommand,argv[1],   // All added to buffer
        if (pxarg)
            buf = catAppendOnlyExpireAtCommand(buf,server.pexpireCommand,argv[1],
    } else {
        /* All the other commands don't need translation or need the
         * same translation already operated in the command vector
         * for the replication itself. */
        buf = catAppendOnlyGenericCommand(buf,argc,argv);

    /* Append to the AOF buffer. This will be flushed on disk just before
     * of re-entering the event loop, so before the client will get a
     * positive reply about the operation performed. */
    if (server.aof_state == AOF_ON)                                             //  Whether the AOF on flag is turned on and the data is copied to AOF buf
        server.aof_buf = sdscatlen(server.aof_buf,buf,sdslen(buf));

    /* If a background append only file rewriting is in progress we want to
     * accumulate the differences between the child DB and the current one
     * in a buffer, so that when the child process will do its work we
     * can append the differences to the new append only file. */
    if (server.aof_child_pid != -1)
        aofRewriteBufferAppend((unsigned char*)buf,sdslen(buf));                    

    sdsfree(buf);                                                               // Free memory

This function is to determine whether the executed command is a write command, and save the corresponding parameters to aof buf, so as to save the executed command every time. Next let's look at the write policy of each aof file.

Write policy of AOF

In the flushAppendOnlyFile function, every time it is executed, first check the execution of the function;

/* Write the append only file buffer on disk.
 * Since we are required to write the AOF before replying to the client,
 * and the only way the client socket can get a write is entering when the
 * the event loop, we accumulate all the AOF writes in a memory
 * buffer and write it on disk using this function just before entering
 * the event loop again.
 * About the 'force' argument:
 * When the fsync policy is set to 'everysec' we may delay the flush if there
 * is still an fsync() going on in the background thread, since for instance
 * on Linux write(2) will be blocked by the background fsync anyway.
 * When this happens we remember that there is some aof buffer to be
 * flushed ASAP, and will try to do that in the serverCron() function.
 * However if force is set to 1 we'll write regardless of the background
 * fsync. */
#define AOF_WRITE_LOG_ERROR_RATE 30 /* Seconds between errors logging. */
void flushAppendOnlyFile(int force) {
    ssize_t nwritten;
    int sync_in_progress = 0;
    mstime_t latency;

    if (sdslen(server.aof_buf) == 0) return;                        // If the buffer to be written is 0, no execution is returned

    if (server.aof_fsync == AOF_FSYNC_EVERYSEC)                      // Whether the configured policy is executed every second
        sync_in_progress = bioPendingJobsOfType(BIO_AOF_FSYNC) != 0;   // Returns whether a task is executing for a specific synchronization flag

    if (server.aof_fsync == AOF_FSYNC_EVERYSEC && !force) {           // Is it per second and not enforced
        /* With this append fsync policy we do background fsyncing.
         * If the fsync is still in progress we can try to delay
         * the write for a couple of seconds. */
        if (sync_in_progress) {
            if (server.aof_flush_postponed_start == 0) {                 // If execution is complete
                /* No previous write postponing, remember that we are
                 * postponing the flush and return. */
                server.aof_flush_postponed_start = server.unixtime;      // Record the current IDE start time
            } else if (server.unixtime - server.aof_flush_postponed_start < 2) {   // If less than 2, wait for fsync to complete and return
                /* We were already waiting for fsync to finish, but for less
                 * than two seconds this is still ok. Postpone again. */
            /* Otherwise fall trough, and go write since we can't wait
             * over two seconds. */
            serverLog(LL_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.");
    /* 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.
     * 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 */

    nwritten = aofWrite(server.aof_fd,server.aof_buf,sdslen(server.aof_buf));     // Write data
    /* We want to capture different events for delayed writes:
     * when the delay happens with a pending fsync, or with a saving child
     * active, and when the above two conditions are missing.
     * We also use an additional event name to save all samples which is
     * useful for graphing / monitoring purposes. */
    if (sync_in_progress) {                                                      
    } else if (server.aof_child_pid != -1 || server.rdb_child_pid != -1) {
    } else {

    /* We performed the write so reset the postponed flush sentinel to zero. */
    server.aof_flush_postponed_start = 0;

    if (nwritten != (ssize_t)sdslen(server.aof_buf)) {                      // If the length of the write is not the same as the AOF buf length
        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. */
        if ((server.unixtime - last_write_error_log) > AOF_WRITE_LOG_ERROR_RATE) {
            can_log = 1;
            last_write_error_log = server.unixtime;

        /* Log the AOF write error and record the error code. */
        if (nwritten == -1) {                                                   // Logging fails if write fails
            if (can_log) {
                serverLog(LL_WARNING,"Error writing to the AOF file: %s",
                server.aof_last_write_errno = errno;
        } else {
            if (can_log) {
                serverLog(LL_WARNING,"Short write while writing to "
                                       "the AOF file: (nwritten=%lld, "
                                       (long long)nwritten,
                                       (long long)sdslen(server.aof_buf));

            if (ftruncate(server.aof_fd, server.aof_current_size) == -1) {   // Truncates the current buffer size
                if (can_log) {
                    serverLog(LL_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 ftruncate() succeeded we can set nwritten to
                 * -1 since there is no longer partial data into the AOF. */
                nwritten = -1;
            server.aof_last_write_errno = ENOSPC;

        /* Handle the AOF write error. */
        if (server.aof_fsync == AOF_FSYNC_ALWAYS) {                             // If it is always dropping, it will exit if this dropping fails
            /* 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 synced on disk. */
            serverLog(LL_WARNING,"Can't recover from AOF write error when the AOF fsync policy is 'always'. Exiting...");
        } 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 = C_ERR;                               // Record last write failed

            /* 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;
            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. */
        if (server.aof_last_write_status == C_ERR) {                            // Record failure
                "AOF write error looks solved, Redis can write again.");
            server.aof_last_write_status = C_OK;
    server.aof_current_size += nwritten;                                        // Save buffer size written

    /* 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). */
    if ((sdslen(server.aof_buf)+sdsavail(server.aof_buf)) < 4000) {
    } else {
        server.aof_buf = sdsempty();

    /* Don't fsync if no-appendfsync-on-rewrite is set to yes and there are
     * children doing I/O in the background. */
    if (server.aof_no_fsync_on_rewrite &&
        (server.aof_child_pid != -1 || server.rdb_child_pid != -1))                 // Return if the operation is in the background

    /* Perform the fsync if needed. */
    if (server.aof_fsync == AOF_FSYNC_ALWAYS) {                                     // If fsync is always
        /* redis_fsync is defined as fdatasync() for Linux in order to avoid
         * flushing metadata. */
        redis_fsync(server.aof_fd); /* Let's try to get this data on the disk */   // Calling fsync function to drop the disk directly
        server.aof_last_fsync = server.unixtime;                                    // Save last time
    } else if ((server.aof_fsync == AOF_FSYNC_EVERYSEC &&
                server.unixtime > server.aof_last_fsync)) {                         // If the disk drops every second and the server time is greater than the last time
        if (!sync_in_progress) aof_background_fsync(server.aof_fd);                 // Check whether background dropping is required
        server.aof_last_fsync = server.unixtime;                                    // Set the last fsyn time

This is a complete implementation of the configuration strategy. The last paragraph is different strategies for different operations. If it is AOF ﹣ fsync ﹣ always, call redis ﹣ fsync (fsync in essence) directly to drop the newly written data. If it is AOF ﹣ fsync ﹣ everysec, call AOF ﹣ background ﹣ fsync to drop the data asynchronously, if it is NO , nothing to do, waiting for the operating system to write the written data to the hard disk. Aof background fsync is distributed to background tasks asynchronously.

/* Starts a background task that performs fsync() against the specified
 * file descriptor (the one of the AOF file) in another thread. */
void aof_background_fsync(int fd) {
    bioCreateBackgroundJob(BIO_AOF_FSYNC,(void*)(long)fd,NULL,NULL);  // Create a bio task
void bioCreateBackgroundJob(int type, void *arg1, void *arg2, void *arg3) {
    struct bio_job *job = zmalloc(sizeof(*job));

    job->time = time(NULL);                         // Acquisition time
    job->arg1 = arg1;                               // Save parameters
    job->arg2 = arg2;
    job->arg3 = arg3;
    pthread_mutex_lock(&bio_mutex[type]);           // Get lock
    listAddNodeTail(bio_jobs[type],job);            // Add task to list
    pthread_cond_signal(&bio_newjob_cond[type]);    // Wake up the thread waiting to execute the task
    pthread_mutex_unlock(&bio_mutex[type]);         // Release this type of lock

In fact, in the bioInit function, several task threads are opened for task processing. The processing function is bioProcessBackgroundJobs;

void *bioProcessBackgroundJobs(void *arg) {
    struct bio_job *job;
    unsigned long type = (unsigned long) arg;
    sigset_t sigset;

    /* Check that the type is within the right interval. */
    if (type >= BIO_NUM_OPS) {
            "Warning: bio thread started with wrong type %lu",type);
        return NULL;

    /* Make the thread killable at any time, so that bioKillThreads()
     * can work reliably. */
    pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);    
    pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS, NULL);

    /* Block SIGALRM so we are sure that only the main thread will
     * receive the watchdog signal. */
    sigaddset(&sigset, SIGALRM);
    if (pthread_sigmask(SIG_BLOCK, &sigset, NULL))
            "Warning: can't mask SIGALRM in bio.c thread: %s", strerror(errno));

    while(1) {
        listNode *ln;               

        /* The loop always starts with the lock hold. */
        if (listLength(bio_jobs[type]) == 0) {                      // Sleep if the current type has no tasks
        /* Pop the job from the queue. */
        ln = listFirst(bio_jobs[type]);                             // Get the first one in the queue
        job = ln->value;                                            // Get task type
        /* It is now possible to unlock the background system as we know have
         * a stand alone job structure to process.*/
        pthread_mutex_unlock(&bio_mutex[type]);                     // Get the lock for this task

        /* Process the job accordingly to its type. */
        if (type == BIO_CLOSE_FILE) {
        } else if (type == BIO_AOF_FSYNC) {                         // If it's AOF? Fsync
            redis_fsync((long)job->arg1);                           // Call redis? Fsync directly to drop the data
        } else if (type == BIO_LAZY_FREE) {
            /* What we free changes depending on what arguments are set:
             * arg1 -> free the object at pointer.
             * arg2 & arg3 -> free two dictionaries (a Redis DB).
             * only arg3 -> free the skiplist. */
            if (job->arg1)
            else if (job->arg2 && job->arg3)
            else if (job->arg3)
        } else {
            serverPanic("Wrong job type in bioProcessBackgroundJobs().");

        /* Lock again before reiterating the loop, if there are no longer
         * jobs to process we'll block again in pthread_cond_wait(). */

        /* Unblock threads blocked on bioWaitStepOfType() if any. */

So far, the implementation of the three disk dropping strategies is basically as shown above. With the help of queue and thread pool, disk dropping per second can improve the efficiency of disk dropping.


This paper mainly describes the work of periodic tasks, which is only a preliminary understanding without detailed analysis. Then, it introduces the AOF strategy and looks at the implementation process of three disk dropping strategies in turn. In the process of disk dropping per second, the common mechanism also uses thread pool and queue to improve the performance of fsync Rate to improve the efficiency of the main loop execution. Due to the lack of knowledge, please criticize and correct any mistakes.

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Tags: Redis Database less Linux

Posted on Thu, 06 Feb 2020 05:25:26 -0500 by carylson