wal | 易学教程

Why Hbase need WAL？

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问题 I'm new to Hbase, and I found that Hbase will write all the operations to WAL and memstore. Q1: I wonder why Hbase need WAL? Q2 : Hbase must write to WAL every time when I put or delete data, why don't operate it just in its data file? 回答1: HBase has is its own ACID semantics : http://hbase.apache.org/acid-semantics.html It needs a WAL so that it can replay edits in case of Failure of a RegionServer. WAL plays an important to provide durability guarantee. WAL is optional. You can disable WAL

Redis AOF fsync (ALWAYS) vs. LSM tree

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问题 My understanding of log structured merge trees (LSM trees) is that it takes advantage of the fact that appending to disk is very fast (since it requires no seeks) by just appending the update to a write-ahead log and returning to the client. My understanding was that this still provides immediate persistence, while still being extremely fast. Redis, which I don't think uses LSM trees, seems to have a mode where you can AOF+fsync on every write. https://redis.io/topics/latency . The

SQLite: Is there any way for use of WAL without mmap() function?

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问题 I want to use WAL mode for the reason of performance and reliability. However my environment doesn't have mmap() function, so I can't compile SQLite with WAL (WAL needs mmap().). Although setting PRAGMA locking_mode=EXCLUSIVE allow use of WAL without mmap() (In this case, WAL-index is created on heap memory not shared file), it is not good solution for the applications that manage multiple database connections. I use SQLite with multiple database connections (for inter-thread concurrency) in

Disabling sqlite Write-Ahead logging in Android Pie

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In Android Pie sqlite Write-Ahead logging (WAL) has been enabled by default. This is causing errors for my existing code only in Pie devices. I have been unable to turn off WAL successfully using SQLiteDatabase.disableWriteAheadLogging() or PRAGMA journal_mode due to the way I access the database. I would like to disable WAL completely with an Android setting called db_compatibility_wal_supported : Compatibility WAL (Write-Ahead Logging) for Apps Does anyone know how to configure this? I don't know if this file can be altered programmatically at startup or if it is changed manually. Further

How to merge contents of SQLite 3.7 WAL file into main database file

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With WAL (Write-Ahead-Logging) enabled in SQLite 3.7 (which is the default for Core Data on iOS 7), how do I merge/commit the content from the -wal file back into the main database file? Do a checkpoint , i.e., execute PRAGMA wal_checkpoint . From the command line, do this: sqlite3 MyDatabase.sqlite VACUUM; CTRL-D to exit the sqlite console. Done! The -wal file should now have a size of 0 and everything should be in your main database file. 来源： https://stackoverflow.com/questions/19574286/how-to-merge-contents-of-sqlite-3-7-wal-file-into-main-database-file

How to disable WAL journal mode

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https://developer.apple.com/library/ios/releasenotes/DataManagement/WhatsNew_CoreData_iOS/ I am having trouble in disabling journal mode. My code is: static NSManagedObjectContext *managedObjectContext(){ static NSManagedObjectContext *context = nil; if (context != nil) { return context; } NSString * const NSSQLitePragmasOption; NSSQLitePragmasOption : @{ @"journal_mode" : @"DELETE" }; @autoreleasepool { context = [[NSManagedObjectContext alloc] init]; NSPersistentStoreCoordinator *coordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:managedObjectModel()]; [context

How to disable WAL journal mode

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问题 https://developer.apple.com/library/ios/releasenotes/DataManagement/WhatsNew_CoreData_iOS/ I am having trouble in disabling journal mode. My code is: static NSManagedObjectContext *managedObjectContext(){ static NSManagedObjectContext *context = nil; if (context != nil) { return context; } NSString * const NSSQLitePragmasOption; NSSQLitePragmasOption : @{ @\"journal_mode\" : @\"DELETE\" }; @autoreleasepool { context = [[NSManagedObjectContext alloc] init]; NSPersistentStoreCoordinator