We are using AWS-RDS. The following is the screenshot of how the binary log disk usage has varied over the last 6 weeks.

enter image description here

The log retention period is 3 days. Tables are optimized and archived once a week. We were expecting the BinLogDiskUsage to be almost constant(assuming periodic purging of logs by AWS ) with spikes during archival tasks.

Can anyone explain this wavy nature of the graph?

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    SHOW VARIABLES LIKE 'max_binlog_size'; – Rick James Jul 4 '19 at 16:37
  • @RickJames The value of max_binlog_size is 134217728. SHOW BINARY LOGS showed 1447 files with the average file size being 1396626. However, there was one file with size 70067577. – Subrata Das Jul 5 '19 at 7:37

(Long-winded, but incomplete, answer)

The graph moves up and down rather smoothly. This puzzled me. With your latest numbers, I can explain some of what is going on.

Data is written to the binlog until it exceeds max_binlog_size (128M, in your case). At that point, a new binlog file is created.

Slave(s) pull the data from the binlogs continually, and usually keep up. There is, however, no feedback to let the Master know when all the Slaves are finished with a binlog file. So, instead, there is some other mechanism for deleting old binlogs.

With expire_logs_days (3 in your case), binlog files older than that many days will be deleted.

Normally your 2GB (1447*1396626) would occupy about 16 binlog files (that /134217728). And the graph would show distinct drops of 128MB every so often.

The only thing I can think of for leading to 1447 binlogs instead of 16 is spurious PURGE commands. Please run these:


The quotient will say how many purges per second. Normally, this will be close to zero. But your system seems to have several per hour. So, I have converted the mysterious graph into "Why is PURGE being performed so often."

(See http://mysql.rjweb.org/doc.php/mysql_analysis#tuning if you would like deeper analysis of STATUS and VARIABLES.)


1447 binlogs with purge_logs_days_seconds = 500 (5 minutes). This says there are usually 1 or 2 binlogs being purged each time. The graph shows a weekly pattern. One part of the week is quite busy -- adding a lot to the binlogs while purging 3-day-old files that have much less. This makes the graph go up. Ditto for down. Flat says that you are adding at the same rate you are deleting.

I would guess there was a lull in traffic, or a national holiday, during the last week of May?

There are still contradictions in what little data you have provided. 1447 and purge after 3 days would say 3 minute interval, not 12/hour. Or 5 days, not 3. Also, max binlog size of 70MB (out of 128MB max) would imply that a purge cycle never purges more than 1 file.

The system has a rhythm; nothing seems terribly wrong.

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  • Yes, the instance is purging almost 12 times an hour. As far as I know, PURGE command deletes binary logs. Should not it decrease the binary log count? Kindly correct me if I am wrong. – Subrata Das Jul 8 '19 at 7:31
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    @SubrataDas - I'm still missing enough clues to fit the pieces together. I added to my Answer. PURGE should delete file(s) each time it is run; based on your data, Some of your data says there is never more than 1 file deleted; some says more than one. Normally, binlogs grow to the max, but that is not happening; still no clue of why. – Rick James Jul 8 '19 at 13:28
  • Is it normal to have such regular purging? Can it have any adverse effect? – Subrata Das Jul 9 '19 at 12:45
  • @SubrataDas - The frequency of purging should not matter. Perhaps there is frequent FLUSHing. Frequent flushing should not matter, either. Both are unusual. – Rick James Jul 9 '19 at 15:41

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