What is [jbd2/dm-3-8] and [kdmflush]? And why are they constantly on iotop?

Question

Using Fedora for a small Samba and development server.

Solution

They're kernel threads.

[jbd2/%s] are used by JBD2 (the journal manager for ext4) to periodically flush journal commits and other changes to disk.

[kdmflush] is used by Device Mapper to process deferred work that it has queued up from other contexts where doing immediately so would be problematic.

Comments

[Iwan Aucamp]

I gave some details on how I tracked down what jbd2 was doing when I had it showing up at the top of iotop all the time here: serverfault.com/a/801223/181704