On Fri, May 08, 2009 at 05:32:45PM +0700, Dan wrote: > > On May 8, 2009, at 5:21 PM, Christian Smith wrote: > > > On Wed, May 06, 2009 at 10:36:50PM -0400, D. Richard Hipp wrote: > >> > >> A new optional extension is included that implements an > >> asynchronous I/ > >> O backend for SQLite on either windows or unix. The asynchronous I/O > >> backend processes all writes using a background thread. This gives > >> the appearance of faster response time at the cost of durability and > >> additional memory usage. See http://www.sqlite.org/asyncvfs.html for > >> additional information. > > > > > > What are the benefits of using async I/O over "PRAGMA synchronous = > > OFF"? > > If AIO is used for the rollback journal as well, you've lost your ACID > > properties already, so you may as well just use "PRAGMA synchronous > > = OFF" > > anyway and keep the code simpler. > > That's not the case. You lose the Durability property, in that a COMMIT > statement may return before a transaction is stored on the persistent > media, > but transactions are still Atomic, Consistent and Isolated. > > When using the "PRAGMA synchronous=off" your database might be corrupted > by a power failure or OS crash. When using asynchronous IO this should > not > be possible (assuming the hardware is not being untruthful - just as > when > using regular "PRAGMA synchronous=full" mode without the async IO VFS).
Ah, the bulb has lit. Because the writes and syncs are processed by the single queue in order, journal writes are guaranteed to be synced and consistent before main in-place updates to the db file. Might be worth mentioning this in the documentation, as this is not clear without examining the source. In that case, I like it :) Is this something that might be made the default in the future, with the addition of some synchronization between foreground and background threads on the xSync messages to emulate the existing "PRAGMA synchronous=full" behaviour? > > Dan. Christian _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
