On 2015-07-24 14:21:39 -0400, Robert Haas wrote: > - Mixing synchronous_commit=on and synchronous_commit=off won't work > as well, because if the LSN ordering of commit records matches the > order in which transactions become visible, then an async-commit > transaction can't become visible before a later sync-commit > transaction. I expect we might just decide we can live with this, but > it's worth discussing in case people feel otherwise.
I'm not following this anymore. Even though a couple months back I apparently did. Heikki's description of the problem was: On 2014-05-30 17:59:23 +0300, Heikki Linnakangas wrote: > One thorny issue came up in discussions with other hackers on this in PGCon: > > When a transaction is committed asynchronously, it becomes visible to other > backends before the commit WAL record is flushed. With CSN-based snapshots, > the order that transactions become visible is always based on the LSNs of > the WAL records. This is a problem when there is a mix of synchronous and > asynchronous commits: > > If transaction A commits synchronously with commit LSN 1, and transaction B > commits asynchronously with commit LSN 2, B cannot become visible before A. > And we cannot acknowledge B as committed to the client until it's visible to > other transactions. That means that B will have to wait for A's commit > record to be flushed to disk, before it can return, even though it was an > asynchronous commit. Afacs the xlog insertion order isn't changed relevantly by the patch? Right now the order for sync commit = on is: XactLogCommitRecord(); XLogFlush(); TransactionIdCommitTree(); ProcArrayEndTransaction(); and for sync commit = off it's: XactLogCommitRecord(); TransactionIdAsyncCommitTree(); ProcArrayEndTransaction(); with the CSN patch it's: sync on: TransactionIdSetCommitting(); XactLogCommitRecord(); XLogFlush(XactLastRecEnd); TransactionIdCommitTree(); sync off: TransactionIdSetCommitting(); XactLogCommitRecord(); TransactionIdCommitTree(); How does this cause the above described problem? Yes, mixing sync/async commits can give somewhat weird visibility semantics, but that's not really new, is it? Regards, Andres -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
