Last week I posted in-depth about how checkpoints work and what exactly goes on (see How do checkpoints work and what gets logged). About a year ago I posted about why the buffer pool on a busy system may seem to have an inordinate amount of dirty tempdb pages in it, and now I want to clarify a bit more why this is the case and how checkpoints work for tempdb. To see the buffer pool contents, see my post Inside the Storage Engine: What’s in the buffer pool?.
A checkpoint is only done for tempdb when the tempdb log file reaches 70% full – this is to prevent the tempdb log from growing if at all possible (note that a long-running transaction can still essentially hold the log hostage and prevent it from clearing, just like in a user database).
If you read the first post I reference above, you’ll see that when a checkpoint occurs for a user database, all dirty pages for that database are flushed to disk (as well as other operations). This does not happen for tempdb. Tempdb is not recovered in the event of a crash, and so there is no need to force dirty tempdb pages to disk, except in the case where the lazywriter process (part of the buffer pool) has to make space for pages from other databases. Of course, when you issue a *manual* CHECKPOINT, all the dirty pages are flushed, but for automatic checkpoints they’re not. You can easily prove this to yourself with a large transaction inserting into a user table on tempdb, and watch the Databases/Log File(s) Used Size (KB) and Databases/Log Truncations for tempdb, when you see them go up, check to see if the number of dirty pages in tempdb has reduced – it won’t have.
The other operation that occurs during a checkpoint of databases in the SIMPLE recovery model is that the VLFs in the log are examined to see if they can be made inactive (see TechNet Magazine: feature article on understanding logging and recovery and Inside the Storage Engine: More on the circular nature of the log). This process allows the log to wrap-around and overwrite itself without having to grow – and this process *IS* required by tempdb, for proper management of it’s log size.
This is all that happens for a checkpoint of tempdb – no data pages are flushed to disk except for a manual CHECKPOINT.