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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