08 Oct

ProxySQL Configuration File & Startup Process Explained

When learning ProxySQL, we have seen some confusion around the configuration and especially around the usage of the configuration file. In this post, we will explain how ProxySQL treats its internal configuration and its configuration file.

The Proxysql.Cnf

For the initial startup, the configuration is loaded from the /etc/proxysql.cnf file. At this time, the configuration file will be parsed and the settings will be stored in the embedded SQLite database. From this point onwards, the settings will be accessed from the SQLite database.

However, there are four configuration settings which are always parsed from the config file, even when the SQLite database is present. This places a requirement for the configuration file to always be in the place, if missing, ProxySQL terminates.

datadir: The path of ProxySQL datadir which stores database file, logs.
restart_on_missing_heartbeats (new in 1.4.4): If MySQL threads miss restart_on_missing_heartbeats (number of) heartbeats, proxysql will raise a SIGABRT signal and restart. Default is 10.
execute_on_exit_failure (new in 1.4.4): If set, ProxySQL’s parent process will execute the defined script every time ProxySQL crashes. It is recommended to use this setting to generate an alert or log the event.
errorlog (new in 2.0.0): The file to be used as error-log. Defaults to ${datadir}/proxysql.log

07 Sep

MySQL 8 Resource Group – introduction and dynamic allocation

MySQL 8 is GA and it has a variety of new features. Recently, we happened to work with resource groups to restrict resource utilization for a MySQL thread.

That’s why I thought of writing this blog post: to give you a quick introduction to resource groups, an idea of how to dynamically allocate them and to discuss a related bug report.

Introduction To Resource Groups:

A resource group, a new feature in MySQL 8, is the entity that defines the allowed resources consumption for threads pertaining to that group. Group attributes enable control over resources and hence the database threads get restricted with those limits.

Resource groups introduce the following associated components:
1. SQL to manage the resource groups (create, alter, assign, drop).
2. Resource groups related privileges.
3. The information_schema.resource_groups table, which holds groups details.
4. A new column was added to the performance_schema.threads table to show us the assignments of threads to respective resource groups.

29 Jun

Restore A Table / Database From Full Backup – Yet Another Way

Restore requests are common and so are the restores of specific entities: a database, or one or more table(s). This has been discussed a lot and we have plenty of tools and solutions already available.
In this blog post we will cover an interesting solution that I came across when I received a restoration request from a client with a specific scenario.

The scenario? Well, the client was on a Windows server with 400GB of mysqldump and wanted to restore a table.

As Linux players we already know of some tools and techniques to export a table or database from mysqldump – for example, using sed command or using the script mysqldumpsplitter (based on sed itself). But on Windows we are powerless by not being able to use sed (we’re sad without sed.) Also, there was no cygwin to ease up the pain.

We had to come-up with a solution that works on Windows as well. During this discussion, my Pythian colleague, Romuller, suggested a cool but simple trick which enlightens us and offers one more way of exporting or recovering a table from a full mysqldump.

So the trick here is as follows:

18 Jan

A tale of Corrupt InnoDB table, MySQL crash & recovery

I’m going to narrate you a story that happened around a crashing MyQL, Corrupted InnoDB table and finally the recovery by table restore. We will see how our database administrator detected the issue and what he did to resolve it.

A day in MySQL Database Consultant’s day was taking its shape while a friend called for help.

Friend: Hey, my mysql is crashing and website isn't functioning well. Everything is down. Can you help me?

Our database admin quickly jumps in and checks for the MySQL error log.

2018-01-01T07:39:03.173398Z 0 [ERROR] InnoDB: Database page corruption on disk or a failed file read of page [page id: space=13701, page number=4603]. You may have to recover from a backup.
2018-01-01T07:39:03.173428Z 0 [Note] InnoDB: Page dump in ascii and hex (16384 bytes):
...
InnoDB: End of page dump
2018-01-01T07:39:03.265864Z 0 [Note] InnoDB: Uncompressed page, stored checksum in field1 4187651462, calculated checksums for field1: crc32 4128877936/1194051977, innodb 680941878, none 3735928559, stored checksum in field2 3735928559, calculated checksums for field2: crc32 4128877936/1194051977, innodb 1675940203, none 3735928559, page LSN 400 3284265879, low 4 bytes of LSN at page end 3284252104, page number (if stored to page already) 4603, space id (if created with >= MySQL-4.1.1 and stored already) 13701
InnoDB: Page may be an index page where index id is 33515
2018-01-01T07:39:03.265911Z 0 [Note] InnoDB: Index 33515 is `PRIMARY` in table `nitty-witty`.`flat_address`
2018-01-01T07:39:03.265919Z 0 [Note] InnoDB: It is also possible that your operating system has corrupted its own file cache and rebooting your computer removes the error. If the corrupt page is an index page. You can also try to fix the corruption by dumping, dropping, and reimporting the corrupt table. You can use CHECK TABLE to scan your table for corruption. Please refer to http://dev.mysql.com/doc/refman/5.7/en/forcing-innodb-recovery.html for information about forcing recovery.
2018-01-01T07:39:03.265944Z 0 [ERROR] [FATAL] InnoDB: Aborting because of a corrupt database page in the system tablespace. Or, there was a failure in tagging the tablespace as corrupt.
2018-01-01 07:39:03 0x7f5fa0466700 InnoDB: Assertion failure in thread 140048687589120 in file ut0ut.cc line 916
InnoDB: We intentionally generate a memory trap.
InnoDB: Submit a detailed bug report to http://bugs.mysql.com.
InnoDB: If you get repeated assertion failures or crashes, even
InnoDB: immediately after the mysqld startup, there may be
InnoDB: corruption in the InnoDB tablespace. Please refer to
InnoDB: http://dev.mysql.com/doc/refman/5.7/en/forcing-innodb-recovery.html
InnoDB: about forcing recovery.
07:39:03 UTC - mysqld got signal 6 ;
This could be because you hit a bug. It is also possible that this binary
or one of the libraries it was linked against is corrupt, improperly built,
or misconfigured. This error can also be caused by malfunctioning hardware.
Attempting to collect some information that could help diagnose the problem.
As this is a crash and something is definitely wrong, the information
collection process might fail.

key_buffer_size=536870912
read_buffer_size=16777216
max_used_connections=6
max_threads=214
thread_count=6
connection_count=6
It is possible that mysqld could use up to
key_buffer_size + (read_buffer_size + sort_buffer_size)*max_threads = 7539498 K bytes of memory
Hope that's ok; if not, decrease some variables in the equation.

Thread pointer: 0x0
Attempting backtrace. You can use the following information to find out
where mysqld died. If you see no messages after this, something went
terribly wrong...

02 Mar

MySQL to Galera Cluster Migration, Deadlock, Back to basics

This post is a lab experiment learning from migration to the Percona Xtradb Cluster (Galera) and a very unexpected DEADLOCK scenario which took me back to basics.

(root@localhost) [test]>insert into app values (1);
ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction

Little background:

We needed to work on migrating the standard MySQL replication to a Galera (Percona Xtradb Cluster).

Previous configuration being one Master with two slaves detailed as follows:
server-1: MySQL community 5.6 serving as Master
server-2: MySQL community 5.6 serving as Slave
server-3: MySQL community 5.6 serving as Slave

We wanted to migrate above MySQL standard replication setup to Percona Xtradb Cluster (PXC 5.6) as following 3 nodes:
galera-1
galera-2
galera-3