# Strange Seconds\_Behind\_Master behavior **URL:** \ **Category:** Percona Server for MySQL 5.7\ **Created:** [July 2, 2024, 10:51am UTC](https://forums.percona.com/t/strange-seconds-behind-master-behavior/31349 "2024-07-02T10:51:44Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Deniska80](https://avatars.discourse-cdn.com/v4/letter/d/db5fbb/32.png) [@Deniska80](https://forums.percona.com/u/Deniska80)\ **Post date:** [July 2, 2024, 10:51am UTC](https://forums.percona.com/t/strange-seconds-behind-master-behavior/31349/1 "2024-07-02T10:51:44Z")
Hello, i’ve got one master and two replics from this master (all the same versions 5.7.44-48-log), it’s GTID replication. One replica works fine, and the second one has issue. It’s “Seconds\_Behind\_Master” value constantly changes: i mean it’s 0 for several ‘show slave status’ command, and then, after several seconds it’s become much more - 3836 for example. Next call show slave status immediatly shows 0 again. No errors in log-files, “Slave\_IO\_Running” and “Slave\_SQL\_Running” always Yes, time on all servers are the same and ntp synced. In fact - this replica is far beyond master, but why then it almost always show “Seconds\_Behind\_Master”: 0? And when it’s 0 “Retrieved\_Gtid\_Set” and “Executed\_Gtid\_Set” are the same. From monitoring tool it’s look like this ![Screenshot 2024-07-02 135103](https://us1.discourse-cdn.com/flex019/uploads/percona1/original/3X/0/2/0207e4b1ba8cb4fb8e7f64ab2a3bedd0d2babffa.png) Anyone has some clue? ---
**Author:** ![Deniska80](https://avatars.discourse-cdn.com/v4/letter/d/db5fbb/32.png) [@Deniska80](https://forums.percona.com/u/Deniska80)\ **Post date:** [July 3, 2024, 6:03am UTC](https://forums.percona.com/t/strange-seconds-behind-master-behavior/31349/2 "2024-07-03T06:03:03Z")
Ok, i stoped this strange replica, made xtrabackup from master (as always), transfer files snd start replica again. It’s all takes about 14 hours. Ok, i started replica again and this strange behavior sill there ^( One seconds ‘show slave status’ shows … Seconds\_Behind\_Master: 51211 Slave\_SQL\_Running\_State: Reading event from the relay log … another seconds it show … Seconds\_Behind\_Master: 0 Slave\_SQL\_Running\_State: Slave has read all relay log; waiting for more updates … Don’t have any idea, why is ths ---
**Author:** ![kedarpercona](https://sea1.discourse-cdn.com/flex019/user_avatar/forums.percona.com/kedarpercona/32/809_2.png) [@kedarpercona](https://forums.percona.com/u/kedarpercona)\ **Post date:** [July 3, 2024, 6:21am UTC](https://forums.percona.com/t/strange-seconds-behind-master-behavior/31349/3 "2024-07-03T06:21:13Z")
Hi @Deniska80, What was the replica executing while you saw the increased lag? Did you see while your lag was increasing did you have exec\_master\_log\_pos moving? Is the binlog size too large (or larger than usual)? Check contents of binary log using `mysqlbinlog` command and see what the replica is trying to execute. ( `mysqlbinlog --base64-output=decode-rows -vv --start-position= ` ) Ref: [How to Read Simplified SHOW REPLICA STATUS Output](https://www.percona.com/blog/how-to-read-simplified-show-replica-status/) Thanks, K ---
**Author:** ![Deniska80](https://avatars.discourse-cdn.com/v4/letter/d/db5fbb/32.png) [@Deniska80](https://forums.percona.com/u/Deniska80)\ **Post date:** [July 3, 2024, 7:55am UTC](https://forums.percona.com/t/strange-seconds-behind-master-behavior/31349/4 "2024-07-03T07:55:42Z")
> What was the replica executing while you saw the increased \>lag? > Not sure, what i understand you clearly. It executes commands from relay-bin.log, what it fetch from master. No other load on this server, it’s dedicated DB. By the way - it’s hard to catch moment, when it’s not null. I just make 10 consecutive calls of ‘show slave status’ in 2 seconds and only once i saw ‘Seconds behind master’ defferent from 0. > Did you see while your lag was increasing did you have \>exec\_master\_log\_pos moving? > Yes, it’s moving allways > Is the binlog size too large (or larger than usual)? > Bin log on master? No, usually size > see what the replica is trying to execute. > looks like it normally executes commands from master, about 500 op/s One thing, i noted. May be it’s important. At this time this replica has “Master\_Log\_File: mysql-bin.016728”, while master allready has mysql-bin.017213. That’s OK, replica behind master. But at the same time replica has only two relay-bin.xxxx files (as i understand one is some kind of index, and other constantly gowing, while reach max\_binlog\_size). Then this files instantly removes and next two files appears. As i remeber from past, when replica try to reach master, there are a lot of relay-bin.xxxx files, coz fetching binlog is faster, than executes it. May be that’s the reason? Low network speed? ---
**Author:** ![kedarpercona](https://sea1.discourse-cdn.com/flex019/user_avatar/forums.percona.com/kedarpercona/32/809_2.png) [@kedarpercona](https://forums.percona.com/u/kedarpercona)\ **Post date:** [July 4, 2024, 8:28am UTC](https://forums.percona.com/t/strange-seconds-behind-master-behavior/31349/5 "2024-07-04T08:28:36Z")
hmmm! 400+ binlogs behind?! Well then I think you surely need to figureout why do we have this case. May be it is low network… not sure if your replication down for long… ---
**Author:** ![Deniska80](https://avatars.discourse-cdn.com/v4/letter/d/db5fbb/32.png) [@Deniska80](https://forums.percona.com/u/Deniska80)\ **Post date:** [July 4, 2024, 11:40am UTC](https://forums.percona.com/t/strange-seconds-behind-master-behavior/31349/6 "2024-07-04T11:40:33Z")
[quote=“kedarpercona, post:5, topic:31349”] 400+ binlogs behind?! my binlog just 100Mb size, so 400 files is just about 8 hours main question for me is “why sometimes Seconds behind master is 0”. May be it’s 0 in time, while next binlog not fully fetched from master, due to network perfomance… but it’s strange. ---
**Author:** ![Deniska80](https://avatars.discourse-cdn.com/v4/letter/d/db5fbb/32.png) [@Deniska80](https://forums.percona.com/u/Deniska80)\ **Post date:** [July 4, 2024, 11:52am UTC](https://forums.percona.com/t/strange-seconds-behind-master-behavior/31349/7 "2024-07-04T11:52:43Z")
Ok, i iperf network perfomance between this replica and master, results are about 20…40Mbit/s. But speed of growing current relay-bin.log is far slower, it’s transfer one 100Mb file about 1 minute. That’s really strange, i think replica must fetch bin logs on full speed.