Fixing a corrupt InnoDB database

Recently I was asked to help fixing a mysql server issue. The mysql server couldn’t start on a somewhat older Ubuntu (=Trusty).

I checked /var/log/mysql/error.log, and it said something like that it might be a mysql bug or even the mysql binaries (or libraries?) may not be for this platform. WTF?

Finally the customer explained that there had been a power outage, and even the UPS had been failing, resulting a corrupted database. So the innodb database was in a pretty bad shape. OK, let’s try to bring it back to life by healing it:

mysqld –user=mysql –datadir=/var/lib/mysql –innodb-force-recovery=1

No cigar. I tried up to 4 which is the highest recommended or safe(?) value (a higher than 4 value may permanently corrupt data files) according to the official mysql docs. Still no luck. Because at this point I had nothing (more) to lose (there was no backup of the database, and customer couldn’t start mysqld), I took a deep breadth, and told the customer to prepare for even the worst (ie. data loss), and tried –innodb-force-recovery=5, and then –innodb-force-recovery=6.

The last attempt was successful in a sense that at least mysqld started, but it was logging the following message in every second:

InnoDB: Waiting for the background threads to start.

OK, then I fixed the command, and managed to start mysqld finally:

mysqld –user=mysql –datadir=/var/lib/mysql –innodb-force-recovery=6 –innodb-purge-threads=0

After that it was possible to make it work with the usual: service mysql start command. Fortunately (after a quick sanity check on the piler database) it seemed that no data were lost, despite the dreaded –innodb-force-recovery=6 settings.

The moral of the story:

  • always have a working power supply backed up with a working UPS
  • be sure to backup the piler mysql database at least daily
  • additionally you may setup a master-slave mysql replication as well

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