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Displaying posts with tag: bugs (reset)
Fun with Bugs #48 - Group Replication Bugs and Missing Features as of MySQL 5.7.17

It seems recent post on Group Replication by Vadim caused an interesting discussion on Facebook. I am NOT going to continue it here, but decided to present some facts, specifically, list of public bug reports and feature requests for Group Replication (mostly "Verified", that is, accepted by Oracle engineers as valid) as of MySQL 5.7.17 (where the feature is promoted as GA), with just few comments to some of the bugs.

The goal is to double check this post when next Oracle MySQL 5.7.x release appears, to find out how much Oracle carews to fix the problems already identified by MySQL Community.

So, here are the bugs and feature requests in …

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MySQL super_read_only Bugs

This blog we describe an issue with MySQL 5.7’s super_read_only feature when used alongside with GTID in chained slave instances.


In MySQL 5.7.5 and onward introduced the gtid_executed table in the MySQL database to store every GTID. This allows slave instances to use the GTID feature regardless whether the binlog option is set or not. Here is an example of the rows in the gtid_executed table:

mysql> SELECT * FROM mysql.gtid_executed;
| source_uuid                          | interval_start | interval_end | …
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Profiling MyRocks with perf: Good Old Bug #68079 Use Case

Almost a year ago I've got really interested in MyRocks and built MySQL from Facebook that provides it from source. Since that time I build it from fresh sources few times per week (as I've described in that post) and once in a while try to work with it and study some details or use cases. Today I'd like to discuss one of them that I've recently studied with perf profiler.

This is not only because I am going to talk about applying profilers to all kinds and forks of MySQL at FOSDEM 2017 MySQL & Friends …

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perf Basics for MySQL Profiling

Oprofile was widely used for MySQL profiling on Linux in the past. But since 2010 and 2.6.31 Linux kernels another profiler, perf, gets increasing popularity. It uses performance counters (CPU hardware registers that count hardware events such as instructions executed) subsystem in Linux. perf is capable of lightweight profiling. It is included in the Linux kernel, under tools/perf (so features available depends on kernel version), and is frequently updated and enhanced.

So, probably perf is the future of profiling on Linux and it makes sense to discuss its basic usage for profiling MySQL servers. For detailed discussions of features provided, numerous examples (not related to MySQL) and links I suggest to read …

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When MySQL Lies: Wrong seconds_behind_master with slave_parallel_workers > 0

In today’s blog, I will show an issue with seconds_behind_master that one of our clients faced when running slave_parallel_works > 0. We found out that the reported seconds_behind_master from SHOW SLAVE STATUS was lying. To be more specific, I’m talking about bugs #84415 and #1654091.

The Issue

MySQL will not report the correct slave lag if you have slave_parallel_workers> 0. Let’s show it in practice.

I’ll use MySQL Sandbox to speed up one master and two slaves on MySQL version …

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MySQL Support Engineer's Chronicles, Issue #4

This week I had to deal with some unusual problems. But let me start with Percona's xtrabackup, software that I consider a key component of many current production MySQL setups and use regularly. Recently new minor versions of XtraBackup were released, check the details on 2.4.5, for example. It made a step towards support of MariaDB 10.2, but it's still a long way to go, see this pull request #200.

My main problem with xtrabackup, though, is not with lack of support of MariaDB 10,2-specific features. Why should they care, after all... The problem is that old well known bugs and problems are not resolved, those that may affect all MySQL versions, forks and environments. Check  …

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MySQL Support Engineer's Chronicles, Issue #3

The original idea of this series was to publish one post per week, but it seems every other week I have some special topic that well deserves a dedicated post. Last week I had no time to complete my writing because of long (and, I hope, useful) Howto post on replacing corrupted partition using non-corrupted one from other server in replication setup. But I had links and notes collected in a draft that I am going to complete now.

First of all, during the previous week I had time to submit two more talks for the  …

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RocksDB doesn't support large transactions very well

So I tried to do my first set of benchmarks and testing on RocksDB today, but I ran into a problem and had to file a bug:

MySQL @ Facebook RocksDB appears to store at least 2x the size of the volume of changes in a transaction. I don't know how much space for the row + overhead there is in each transcation, so I'm just going to say 2x the raw size of the data changed in the transaction, as approximation. I am not sure how this works for updates either, that is whether old/new row information is maintained. If old/row data is maintained, then a pure update workload you would need 4x the ram for the given transactional changes. My bulk load was 12GB of raw data, so it failed as I have only 12GB of RAM in my test system.

The workaround (as suggested in the bug) is to set two configuration …

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MySQL Support Engineer's Chronicles, Issue #1

I have a habit to keep a log of everything I work on (or even just read about while working) every day. It's just a plain text file, with date labels, where most lines are URLs of issues and bugs I contributed to somehow, and my comments on what I did and why. But there are also code fragments, texts, copy-pasted outputs and links to interesting sites, articles and documentation.

Some of lines from this log I post on Facebook separately as I add them, these are mostly related to MySQL bugs and other things that annoy me. I know that some of my Facebook friends actually do not like this kind of posts at all... When I used LiveJornal actively, I've posted digests of my log from time to time (you can check what was there if you read Russian). But that digests …

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Visualizing the MySQL Bug Tide

On the MySQL Bugs website there are some tide stats available. These show rate of bug creation.

I've put them in a graph:
I made these with this IPython Notebook. There are more detailed graphs per version in the notebook.

Update: The version in the notebook now uses the same range for the Y axis and has a marker for the GA dates of each release.

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