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Displaying posts with tag: delay (reset)

How to troubleshoot MySQL replication issues?
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In MySQL a big portion of the problems you’re facing is different replication delays so there’s no surprise this is one of the most common interview questions on DBA interviews and I still found people having problems with explaining what they would do in a certain situation. This is why I decided to write a bit about the subject.

1. IO

99% of times the reason is IO. The slave cannot keep up with the amount of writes it gets from the binary logs while in parallel it has to return results of queries as well. In spite of the common belief in MySQL it’s much easier to saturate the …

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MySQL Slave Lag (Delay) Explained And 7 Ways To Battle It
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Slave delay can be a nightmare. I battle it every day and know plenty of people who curse the serialization problem of replication. For those who are not familiar with it, replication on MySQL slaves runs commands in series – one by one, while the master may run them in parallel. This fact usually causes bottlenecks. Consider these 2 examples:

  • Between 1 and 100 UPDATE queries are constantly running on the master in parallel. If the slave IO is only fast enough to handle 50 of them without lagging, as soon as 51 start running, the slaves starts to lag.
  • A more common problem is when one query takes an hour to run …
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Introducing MySQL Slave Delay
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MySQL Slave Delay implements an oft-requested replication feature: the ability to make a slave lag its master, a.k.a scheduling binlog events for some time in the future. This will probably be built into MySQL replication someday, but in the meantime, you can use this tool. A delayed slave is great if disaster strikes and propagates through replication, because it lets you stop the delayed slave before it gets corrupted, restart the slave until just before the offending statement, and recover quickly. This is a lot faster than restoring last night's full backup and replaying a day's worth of binary logs on the master.

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