migrating from postgres to mysql causing strange errors Mysql2::Error: MySQL server has gone away

I'm switching the database of a rails 3.0.3 app I have developed from postgres to mysql so that I can avail of amazon's rds. Before I make the change I have been running my test code using mysql on my dev machine with the mysql2 adaptor . My test code is throwing up some errors that I haven't quite been able to get to the bottom of yet. Basically I have a model that is used to store large xml uploads. My test code looks something like this

test "xml upload for large file" do 
  file = File.new("test/files/lib/upload_sample.xml") 
  upload = XmlUpload.create(:xml_contents => contents = file.read) 
  ..... 
  ..... 
end 

The create line is throwing up the following error

ActiveRecord::StatementInvalid: Mysql2::Error: SAVEPOINT active_record_1 does not exist:     ROLLBACK TO SAVEPOINT active_record_1 
/Users/conor/.rvm/gems/ruby-1.9.2-p136/gems/activerecord-3.0.3/lib/ active_record/connection_adapters/abstract_adapter.rb:202:in `rescue in log' 
/Users/conor/.rvm/gems/ruby-1.9.2-p136/gems/activerecord-3.0.3/lib/active_record/connection_adapters/abstract_adapter.rb:194:in `log' 
/Users/conor/.rvm/gems/ruby-1.9.2-p136/gems/mysql2-0.2.6/lib/ active_record/connection_adapters/mysql2_adapter.rb:314:in `execute' 
/Users/conor/.rvm/gems/ruby-1.9.2-p136/gems/mysql2-0.2.6/lib/ active_record/connection_adapters/mysql2_adapter.rb:358:in `rollback_to_savepoint' 
/Users/conor/.rvm/gems/ruby-1.9.2-p136/gems/activerecord-3.0.3/lib/ active_record/connection_adapters/abstract/database_statements.rb: 149:in `rescue in transaction' 
/Users/conor/.rvm/gems/ruby-1.9.2-p136/gems/activerecord-3.0.3/lib/  active_record/connection_adapters/abstract/database_statements.rb: 127:in `transaction' 
/Users/conor/.rvm/gems/ruby-1.9.2-p136/gems/activerecord-3.0.3/lib/ active_record/transactions.rb:204:in `transaction' 
/Users/conor/.rvm/gems/ruby-1.9.2-p136/gems/activerecord-3.0.3/lib/ active_record/transactions.rb:287:in `with_transaction_returning_status' 
/Users/conor/.rvm/gems/ruby-1.9.2-p136/gems/activerecord-3.0.3/lib/ active_record/transactions.rb:237:in `block in save' 
/Users/conor/.rvm/gems/ruby-1.9.2-p136/gems/activerecord-3.0.3/lib/ active_record/transactions.rb:248:in `rollback_active_record_state!' 
/Users/conor/.rvm/gems/ruby-1.9.2-p136/gems/activerecord-3.0.3/lib/ active_record/transactions.rb:236:in `save'  
....

I have been storing the file contents in a text开发者_如何学Python field. I realise that I should seriously look at storing the files in s3 but this is the setup that I have at the moment. In postgres everything worked fine but in order to get things to work with mysql I had to set the :limit variable so that LONGTEXT was used instead of the standard text field. The files can be quite large but when I test using small files there are no problems

I could be barking up the wrong tree entirely but I suspect that the problem may be caused by the database connection being dropped based on the errors thrown up when I try uploading a file in the development mode. I did some checking on this error and I'm not sure what could be dropping the connection, the file isn't taking 8 hrs (the default connection drop time) to insert

Mysql2::Error: MySQL server has gone away: INSERT INTO xml_uploads ........

My database.yaml settings are the following.

test: 
   adapter: mysql2 
   encoding: utf8 
   reconnect: true 
   database: app_test 
   username: username 
   password: password 
   host: localhost 

Does anyone have any clues as to what the problem is and how it can fixed? Any help with this would be greatly appreciated.

I decided to go with the storing the data in S3 anyway but a friend did point me in the direction of the solution to this issue, I tested it and it worked, so I thought I should post it here in case anyone else runs into the same problem.

Basically the problem is caused by the max_allowed_packet variable being set to something smaller than the blog/text field size. The query can't be executed so the connection gets dropped. Here are some details about the max_allowed_packet variable

http://dev.mysql.com/doc/refman/5.0/en/server-system-variables.html#sysvar_max_allowed_packet

and also some info on adjusting it on rds instances

http://www.henrybaxter.ca/?p=111