What are odds MySQL table can be recovered?

This is the most asked question. Every single customer asks if their MySQL table can be recovered. Although it’s not possible to answer that with 100% confidence there are ways to estimate recovery chances. I will describe few tricks.

Generally speaking, if data is on media there are high odds TwinDB data recovery toolkit can fetch it. Where to look for depends on accident type.

Online MySQL data recovery toolkit

On our Data Recovery portal you can upload an .ibd file and check if the InnoDB tablespace contains any good records. The table space may be corrupt. The tool should handle that.

DROP TABLE or DATABASE with innodb_file_per_table=OFF

If …

Benchmarking is a popular topic. People love drawing graphs as much as watching how X is 10% faster than Y; there must be something special in measurements.

For a DBA, however, more tangible improvements come from less popular area of database maintenance. While MariaDB spreads FUD around InnoDB (nonetheless still uses it) I have to admit InnoDB gets more friendly to DBAs.

In MySQL 5.6 new temporary table naming scheme was introduced – one of improvements. Invisible, yet important.

Temporary table names became more random and should not ever be reused.

Some time ago I wrote a post about how to remove …

Sometimes corruption is not the true corruption. Corruption in compressed InnoDB tables may be a false positive.

Compressed InnoDB table may hit false checksum verification failure. The bug (http://bugs.mysql.com/bug.php?id=73689) reveals itself in the error log as follows:

2014-10-18 08:26:31 7fb114254700 InnoDB: Compressed page type (17855); stored checksum in field1 0; calculated checksums for field1: crc32 4289414559, innodb 0, none 3735928559; page LSN 24332465308430; page number (if stored to page already) 60727; space id (if stored to page already) 448
InnoDB: Page may be an index page where index id is 516

InnoDB complains that a stored checksum is zero. If you look closely it’s suspicious that calculated checksum is zero too.

Every InnoDB page stores a checksum in first four bytes. When InnoDB reads a page it compares the checksum, stored in …

The unDROP for InnoDB tool can be used to recover corrupt MySQL database. In this post we will show how to repair MySQL database if its files became corrupted and even innodb_force_recovery=6 doesn’t help.

The corruption of InnoDB tablespace may be caused by many reasons. A dying hard drive can write garbage, thus page checksum will be wrong. InnoDB then reports to the error log:

InnoDB: Database page corruption on disk or a failed
InnoDB: file read of page 4.

MySQL is well know for poor start-up script. A simple upgrade procedure may end up with two mysqld processes writing to the same tablespace. That leads to the corruption too. Sometimes power reset corrupts not only InnoDB files, but file system becomes unusable for the operating system.

InnoDB …

InnoDB provides no means to repair corrupted table space. Once a table got corrupt the only way to repair MySQL files is to start it with innodb_force_recovery={4,5,6} in hope that you can dump the table, so you can rebuild the table space from scratch. At least this is what the manual says. But let’s take a closer look at InnoDB corruption. In some case you can repair InnoDB table space much faster.

UPDATE: If corruption is in PRIMARY index check post Recover Corrupt MySQL Database

What InnoDB provides to repair tablespace corruption

InnoDB doesn’t let you repair the table space, but you can rebuild secondary indexes with ALTER TABLE DROP/ADD KEY. That may be very useful in case corruption malformed pages where secondary index is stored.

So, before starting a whole story with innodb_force_recovery, dump, drop, create and reload …