Unexplained writes
There are several situations where you can find yourself executing statements that only read against the database, yet find significant write volume happening on the database disk.
Parts of the buffer cache must be dirty before writes happen, so if any of that work is left hanging around it could be involved. Flushing everything out with a manual checkpoint is one way to assure that's not the cause of the writes.
A second small source of writes are the access time updates many operating systems do every time you read from a file. It's suggested these get turned off in Chapter 4, Disk Setup, and the actual volume of writes from that is pretty low anyway.
If the volume of writes is substantial, and this data was recently created, what you're mostly likely running into are updates to the hint bits in the database. Hint bits are two bits stored in each row that indicate whether that row's associated transaction has committed or rolled back. If they...
