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Berkeley DB Reference Guide:
  Transaction Protected Applications

# Berkeley DB recoverability

Berkeley DB recovery is based on write-ahead logging. What this means is that, when a change is made to a database page, a description of the change is written into a log file. This description in the log file is guaranteed to be written to stable storage before the database pages that were changed are written to stable storage. This is the fundamental feature of the logging system that makes durability and rollback work.

If the application or system crashes, the log is reviewed during recovery. Any database changes described in the log that were part of committed transactions, and that were never written to the actual database itself, are written to the database as part of recovery. Any database changes described in the log that were never committed, and that were written to the actual database itself, are backed-out of the the database as part of recovery. This design allows the database to be written lazily, and only blocks from the log file have to be forced to disk as part of transaction commit.

There are two interfaces that are a concern when considering Berkeley DB recoverability:

1. The interface between Berkeley DB and the operating system/filesystem.
2. The interface between the operating system/filesystem and the underlying stable storage hardware.

Berkeley DB uses the operating system interfaces and its underlying filesystem when writing its files. This means that Berkeley DB can fail if the underlying filesystem fails in some unrecoverable way. Otherwise, the interface requirements here are simple: the system call that Berkeley DB uses to flush data to disk (normally fsync(2)), must guarantee that all the information necessary for a file's recoverability has been written to stable storage before it returns to Berkeley DB, and that no possible application or system crash can cause that file to be unrecoverable.

In addition, Berkeley DB implicitly uses the interface between the operating system and the underlying hardware. The interface requirements here are not as simple.

First, it is necessary to consider the underlying page size of the Berkeley DB databases. The Berkeley DB library performs all database writes using the page size specified by the application. These pages are not checksummed and Berkeley DB assumes that they are written atomically. This means that if the operating system performs filesystem I/O in different sized blocks than the database page size, it may increase the possibility for database corruption. For example, assume that Berkeley DB is writing 32KB pages for a database and the operating system does filesystem I/O in 16KB blocks. If the operating system writes the first 16KB of the database page successfully, but crashes before being able to write the second 16KB of the database, the database has been corrupted and this corruption will not be detected during recovery. For this reason, it may be important to select database page sizes that will be written as single block transfers by the underlying operating system.

Second, it is necessary to consider the behavior of the system's underlying stable storage hardware. For example, consider a SCSI controller that has been configured to cache data and return to the operating system that the data has been written to stable storage, when, in fact, it has only been written into the controller RAM cache. If power is lost before the controller is able to flush its cache to disk, and the controller cache is not stable (i.e., the writes will not be flushed to disk when power returns), the writes will be lost. If the writes include database blocks, there is no loss as recovery will correctly update the database. If the writes include log file blocks, it is possible that transactions that were already committed may not appear in the recovered database, although the recovered database will be coherent after a crash.

If the underlying hardware can fail in any way such that only part of the block was written, the failure conditions are the same as those described above for an operating system failure that only writes part of a logical database block.

For these reasons, it is important to select hardware that does not do partial writes and does not cache data writes (or does not return that the data has been written to stable storage until it either has been written to stable storage or the actual writing of all of the data is guaranteed barring catastrophic hardware failure, e.g., your disk drive exploding). You should also be aware that Berkeley DB does not protect against all cases of stable storage hardware failure, nor does it protect against hardware misbehavior.

If the disk drive on which you are storing your databases explodes, you can perform normal Berkeley DB catastrophic recovery, as that requires only a snapshot of your databases plus all of the log files you have archived since those snapshots were taken. In this case, you will lose no database changes at all. If the disk drive on which you are storing your log files explodes, you can still perform catastrophic recovery, but you will lose any database changes that were part of transactions committed since your last archival of the log files. For this reason, storing your databases and log files on different disks should be considered a safety measure as well as a performance enhancement.

Finally, if your hardware misbehaves, for example, a SCSI controller writes incorrect data to the disk, Berkeley DB will not detect this and your data may be corrupted.

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