--On Tuesday, February 15, 2011 01:07:50 AM -0500 Derrick Brashear
<[email protected]> wrote:
I'm not clear on how snapshotting interacts with GetFile/SendFile and
active operations. I think in practice the mechanism you need is one
that allows you to "freeze" the target's databases so that active
transactions read from the frozen copy, while sendfile prepares a "new"
copy; note that there can be no write transactions, since writes happen
only on the sync site and these calls are made only by the sync site and
never to itself. Having done a snapshot and sent some new files, it must
be possible to either commit the new files or discard them; recovery
should only do the commit operation if it is still sync site.
the original intent of getfilediff was for some future use, not at this
time.
sendfilediff is an optimization. just because you're recovering
doesn't mean the extant quorum can't continue taking writes. so i take
writes and when sendfile to you finishes, i stop taking writes, send
*only* a diff, and then commit and resume taking writes, not unlike a
volume release.
First, properly, "recovering" is something that only the sync site does.
Other sites don't "recover"; they simply do what they're told. Still, your
point is taken -- the sync site can send the bulk of the database while
still handling write transactions, and then do an incremental update of
some sort at the end.
However, I think you will discover you need an operation which throws away
changes since the snapshot, because as soon as you allow not only for
multiple files but also for the sync site to keep taking updates during
sendfile, there is the possibility that the sync site will stop being sync
site, and need to abort any sends it has in progress. Previously this was
not an issue, because even though the SendFile took time to run, it was an
atomic operation with respect to anything that might modify the database on
either side.
-- Jeff
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