Core Data and iOS 7: Different behavior of persistent store
I'm preparing an update for a Core Data based app for fixes with iOS 7. I use Xcode 5 and iOS 7 SDK GM. However I realized a different behavior of the persistent store (which is a UIManagedDocument): Prior to iOS 7 builds there was only one file persistentStore in the documents folder (sometimes there was a second one persistentStore-journal).
In iOS 7 builds (clean installation) there are now three files for the persistent store:
persistentStorepersistentStore-walandpersistentStore-shm
Did Apple change the journal mode by default to WAL now? I wonder if there is an impact on my app (think of users how update from the last version)? Would it be best to disable WAL - and if so, how can I do this with iOS 7/UIManagedDocument?
Answer
Yes, Apple have changed the default journal mode to WAL for iOS7. You can specify the journal mode by adding the NSSQLitePragmasOption to the options when calling addPersistentStoreWithType:configuration:url:options:error. E.g. to set the previous default mode of DELETE:
NSDictionary *options = @{ NSSQLitePragmasOption : @{@"journal_mode" : @"DELETE"} };
In my experience WAL gives better performance, but also see this post:
iOS CoreData - are there any disadvantages to enabling sqlite WAL / Write-Ahead Logging