Showing posts from October, 2019

Tuning checkpoints

We recently had the chance to help a customer with some IO related issues that ended up being unconfigured checkpoints. Something that may not always be obvious but can actually be somewhat common. Let's start with how things roughly work. Postgres smallest IO unit is a disk block that is 8kb (by default). Each time postgres needs a new block it will fetch it from the disks and load it to an area in RAM called shared_buffers. When postgres needs to write, it does it in the same manner: Fetches the block(s) from the disks if the block is not in shared_buffers Changes the page in shared buffers. Marks the page as changed (dirty) in shared buffers. It writes the change in  a "sequential ledger of changes" called WAL to ensure durability. This basically means that the writes are not yet "on disk". This operation is taken care of by a postgres process called checkpointer. Checkpoints are how postgres guarantees that data files and index files will be upda