Setting correct innodb_log_file_size in mysql

Question

We ran an alter table today today that took down the DB. We failed over to the slave, and in the post-mortem, we discovered this in the mysql error.log

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Answer 1

0. Preliminary information

Your settings:

innodb_log_file_size = 50331648
innodb_log_files_in_group = 2

Therefore your "log group capacity" = 2 x 50331648 = 96 MB

1. How to determine the largest row

There is no direct method. But one can easily calculate the size of one given row based on these tables (compression should not matter to us here, if, as I assume, rows are not compressed in the log files).

2. Impact of innodb_log_file_size

Reference manual:

The larger the value, the less checkpoint flush activity is needed in the buffer pool, saving disk I/O. Larger log files also make crash recovery slower, although improvements to recovery performance in MySQL 5.5 and higher make the log file size less of a consideration.

3. Anything else to worry about

6169.8 GB / 650k rows = about 10 MB per row on average This is a serious problem per se if you intend to use your database in a transactional, multi-user situation. Consider storing your BLOB's as files outside of the database. Or, at least, store them in a separate MyISAM (non-transactional) table.