Alex Stetsenko

Apparently some web applications do store uploaded files in the database as blob field by default (for instance it is true for Mantis bug tracker). While it is acceptable (but not too common?) to store files in the database, it may cause certain problems like uploading large files (particularly 1Mb+).

Normally the first thing to look at would be the actual application configuration. OK, Mantis is configured to accept files up to 5Mb by default. I.e. no problems here. Wait… but PHP is configured to accept files up to 2Mb by default… So where does 1Mb restriction come from?

The answer is that it comes from the database. MySQL has limits on packet size. The largest possible packet that can be transmitted to or from a MySQL 5.1 server or client is 1Gb. However by default it is 1Mb! As result any packets larger than 1Mb will cause packet too large error.

There is max_allowed_packet option in the configuration which allows to set the packet size. For example:

[mysqld]
max_allowed_packet=16M