Scaling writes in MySQL

We use MySQL on most of our projects. One of these projects has a an access pattern unlike any other I've worked on. Several million records a day need to be written to a table. These records are then read out once at the end of the day, summarised and then very rarely touched again. Each record is about 104 bytes long (thre's one VARCHAR column, everything else is fixed), and that's after squeezing out every byte possible. The average number of records that we write in a day is 40 million, but this could go up.

A little bit about the set up. We have fairly powerful boxes with large disks using RAID1/0 and 16GB RAM, however at the time they only had 4GB. For BCP, we have a multi-master set up in two colos with statement level replication. We used MySQL 5.1.

My initial tests with various parameters that affect writes showed that while MyISAM performed slightly better than InnoDB while the tables were small, it quickly deteriorated as the table size crossed a certain point. InnoDB performance deteriorated as well, but at a higher table size. The table size turned out to be related to the innodb_buffer_pool_size, and that in turn was capped by the amount of RAM we had on the system.

I decided to go with InnoDB since we also needed transactions for the summary tables and I preferred not to divide my RAM between two different engines. I stripped out all indexes, and retained only the primary key. Since InnoDB stores the table in the primary key, I decided that rather than use an auto_increment column, I'd cover several columns with the primary key to guarantee uniqueness. This had the added advantage that if the same record was inserted more than once, it would not result in duplicates. This small point was crucial for BCP, because it meant that we did not have to keep track of which records had already been inserted. If something crashed, we could just reinsert the last 30 minutes worth of data, possibly into the secondary master, and not have any duplicates at the end of it. I used INSERT IGNORE to get this done automatically.

Read more: The other side of the moon

Posted via email from jasper22's posterous