Matthew D. Fuller wrote:

You can easily save the stream of updates as a redo log (well,
that's the idea I've been running around with).

Isn't that what the gjournal SoC thing was about?

No, not exactly. The idea was to make a journal of a GEOM device I/O requests on a separate device in the attempt to solve filesystem journaling, but that didn't work out (actually, gjournal works more or less fine, but as far as I understand it, there's a problem similar as with SoftUpdates - even if you journal writes on device level with filesystem mounted "sync", UFS keeps references to inodes in not-entirely-consistent way, so a fsck is always needed after unclean shutdown; in other words, UFS journaling must be done on UFS/VFS level, not GEOM in order to keep track of metadata semantics).

Something like a filesystem that doesn't do any read caching, and keeps ALL data ALWAYS in sync with on-disk state could work with gjournal and also with gmirror+geom_gate combination. Such filesystem would probably be similar to GFS, AFAIK.
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