The journal has a number of tunables that affect filesystem performance. Journal commits are fairly expensive operations as they require issuing FLUSH and FUA operations to the underlying devices. By default, we issue a journal flush one second after a filesystem update has been done; this is controlled with the journal_flush_delay option, which takes a parameter in milliseconds.

Filesystem sync and fsync operations issue journal flushes; this can be disabled with the journal_flush_disabled option - the journal_flush_delay option will still apply, and in the event of a system crash we will never lose more than (by default) one second of work. This option may be useful on a personal workstation or laptop, and perhaps less appropriate on a server.

The journal reclaim thread runs in the background, kicking off btree node writes and btree key cache flushes to free up space in the journal. Even in the absence of space pressure it will run slowly in the background: this is controlled by the journal_reclaim_delay parameter, with a default of 100 milliseconds.

The journal should be sized sufficiently that bursts of activity do not fill up the journal too quickly; also, a larger journal mean that we can queue up larger btree writes. The bcachefs device resize-journal can be used for resizing the journal on disk on a particular device - it can be used on a mounted or unmounted filesystem.

In the future, we should implement a method to see how much space is currently utilized in the journal.