An idle curiosity, is it ever possible to replicate something that has been written to disk before a header is flushed?
On Mon, Apr 12, 2010 at 8:46 AM, Adam Kocoloski <[email protected]> wrote: > Yep, your analysis is dead-on, and is a more complete solution than what I > propose. Best, > > Adam > > On Apr 12, 2010, at 4:51 AM, Robert Newson wrote: > >> Would it be safer to have a low- and high- watermark for the >> update_seq in memory? What I mean is that the db writer will never >> write out an update_seq that is N higher than the last committed one; >> if it is forced to do so, to permit a write, it then fsync's and >> resets high_seq to last_committed_seq. This way you can genuinely >> ensure that you don't reuse an update_seq. In practice we could allow >> a large delta, one that is larger than the number of fsyncs we expect >> to manage in the commit interval. >> >> Your idea to just bump the update_seq "significantly" mostly pans out >> (I know a system that does precisely this) but it would be a data loss >> scenario if when it doesn't pan out. >> >> B. >> >> On Mon, Apr 12, 2010 at 3:54 AM, Adam Kocoloski >> <[email protected]> wrote: >>> Currently a DB update_seq can be reused if there's a power failure before >>> the header is sync'ed to disk. This adds some extra complexity and >>> overhead to the replicator, which must confirm before saving a checkpoint >>> that the source update_seq it is recording will not be reused later. It >>> does this by issuing an ensure_full_commit call to the source DB, which may >>> be a pretty expensive operation if the source has a constant write load. >>> >>> Should we try to fix that? One way to do so would be start at a >>> significantly higher update_seq than the committed one whenever the DB is >>> opened after an "unclean" shutdown; that is, one where the DB header is not >>> the last term stored in the file. Although, I suppose that's not an >>> ironclad test for data loss -- it might be the case that none of the lost >>> updates were written to the file. I suppose we could "bump" the update_seq >>> on every startup. >>> >>> Adam >>> >>> > >
