On Fri, 2 Feb 2007, Torrey McMahon wrote:
Jason J. W. Williams wrote:
Hi Jim,
Thank you very much for the heads up. Unfortunately, we need the
write-cache enabled for the application I was thinking of combining
this with. Sounds like SNDR and ZFS need some more soak time together
before you can use both to their full potential together?
Well...there is the fact that SNDR works with other FS other then ZFS. (Yes,
I know this is the ZFS list.) Working around architectural issues for ZFS and
ZFS alone might cause issues for others.
SNDR has some issues with logging UFS as well. If you start a SNDR live
copy on an active logging UFS (not _writelocked_), the UFS log state may
not be copied consistently.
If you want a live remote replication facility, it _NEEDS_ to talk to the
filesystem somehow. There must be a callback mechanism that the filesystem
could use to tell the replicator "and from exactly now on you start
replicating". The only entity which can truly give this signal is the
filesystem itself.
And no, that _not_ when the filesystem does a "flush write cache" ioctl.
Or when the user has just issued a "sync" command or similar.
For ZFS, it'd be when a ZIL transaction is closed (as I understand it),
for UFS it'd be when the UFS log is fully rolled. There's no notification
to external entities when these two events happen.
SNDR tries its best to achieve this detection, but without actually
_stopping_ all I/O (on UFS: writelocking), there's a window of
vulnerability still open.
And SNDR/II don't stop filesystem I/O - by basic principle. That's how
they're sold/advertised/intended to be used.
I'm all willing to see SNDR/II go open - we could finally work these
issues !
FrankH.
I think the best of both worlds approach would be to let SNDR plug-in to ZFS
along the same lines the crypto stuff will be able to plug in, different
compression types, etc. There once was a slide that showed how that
worked....or I'm hallucinating again.
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