Karl Tomlinson <[EMAIL PROTECTED]> wrote: > Thank you for your comments. > >>> Karl Tomlinson wrote: >>> >>>> One thing I'm trying to understand is whether there was a reason >>>> for SQLite choosing to use a rollback journal (of the steps to >>>> undo a transaction) rather than a replay journal (of the steps to >>>> perform a transaction). > > I didn't make this quite clear. I really meant, as a first > possibility, that writing the pages to the database itself would > be performed during the commit process after syncing the replay > journal (but the database need not be synced until the journal is > about to be removed).
So if I insert large amounts of data, this data would be written to disk twice: once into a replay journal, and then again into the database file on commit. With rollback journal, it is written once into the database (with rollback journal merely noting which new pages have been created), and commit is nearly instantaneous. I'd rather optimize for the common case (transactions successfully committed) than the exceptional one. >> SQLite would have to keep track of where in the replay journal each >> new page was written, so that if it needed to reload that page for >> any reason, it could. As long as the transaction (and thus the >> replay journal) is not too big, this is not a problem. A hash table >> in memory would suffice. But sometimes transactions do get very >> large. For example, with the VACUUM command. And in those cases, >> the amount of memory needed to record the mapping from pages to >> offsets into the replay journal could become too large to fit into >> memory, especially in memory-constrained devices such as cellphones. > > The size of the transaction would be something to consider in the > decision on when to sync the journal and write its pages to the > database. So presumably, as the transaction grows, at some point you may decide to dump the replay journal into the database file even while the transaction is still open. What if the user decides to roll it back soon afterwards? Wouldn't you need both a replay and a rollback journals then? Igor Tandetnik _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
