The amount of persistent space used by the journal seems to be determined in some way other than as described in journald.conf(5). In particular, if the partition on which the persistent log resides contains a small proportion of free space relative to the partition's full size, the journal can be kept significantly smaller than SystemMaxUse, even when SystemKeepFree is too small to apply. With all journald.conf settings at their defaults except for SystemMaxUse=50M, I noticed that my journal was not growing larger than about 2.5M on a 22G filesystem with about 700M free. According to the description in journald.conf(5), there should have been enough space for the jornal to grow to the maximal 50M. See https://bbs.archlinux.org/viewtopic.php?id=166049 for the Arch forum post where I discussed this.
Quoting the arch topic: >> Strictly speaking this seems inaccurate, though. You have more than the value of SystemMaxUse available so SystemKeepFree should not apply. However, it does. > I thought so too. But anyway, clearing up some space did work. It seems that only the documentation was unclear. Tweaked in http://cgit.freedesktop.org/systemd/systemd/commit/?id=65b571f.
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