On Wed, Feb 5, 2014 at 7:50 AM, Brett Mcdonald <aheavensbantert...@gmail.com > wrote:
> What are the recovery consequence(s) to a database (journal mode wal) > should a master-journal file be lost? > > Suppose a flash sensitive embedded product had to update its database every > second. To save flash wear the database (journal mode delete) is created > in ram (e.g. /tmp). Suppose every 8 seconds there had to be a transaction > across the ram database to not only update it but also to sync with another > database in flash (journal mode wal). Both databases must be updated and > sync'd atomically, a.k.a one transaction. Flash cannot afford to have an > mj file created/deleted every 8 seconds. So, assuming... > > -The ram db connection attaches the flash db > -Transaction is started and mj file created in ram > -Ram and flash databases are modified > -On power failure the flash db's wal file is flushed (persists reboot) > however the mj transaction never commits and the ram db and mj file are > gone (they're in ram after all). > > Ideally, on reboot the flash database will recover via its wal file, the > ram db will be recreated and back in business. > > Obviously the product can afford to lose the ram db and the previous 8 > seconds of data, but does losing the ram db with the corresponding mj file > have recovery consequences for the flash db and its wal file? > The on-disk database file will be unharmed. The only consequence of losing -mjXXXX file is that in a multi-database transaction (which can only be in rollback mode, not in WAL mode) if you lose power and come back up without the -mjXXXX file, then the transaction might commit to some of the database files, but not to others. In other words you use cross-database atomicity. But updates to each individual database file are all still atomic. In your case, since one of the two databases is in RAM and is lost completely, it doesn't really matter if it is in sync with the on-disk database. The change to the on-disk database is atomic. -- D. Richard Hipp d...@sqlite.org _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
