In my previous post we had a look at the general storage architecture of HBase.
I think it is better to rephrase: Why does new distributed VoltDB use a command log over write-ahead log? Undoubtedly you are advanced enough to abstract a file system and use block storage along with some additional optimizations.
Next step is to execute some command: Please note several important aspects: A command may affect many stored entities, so many blocks will get dirty Next state is a function of the current state and the command Some intermediate states can be skipped, because it is enough to have a chain of commands instead.
Finally, you need to guarantee data integrity.
Write-Ahead Logging - central concept is that State changes should be logged before any heavy update to permanent storage. Following our idea we can log incremental changes for each block.
Command Logging - central concept is to log only Command, which is used to produce the state. There are Pros and Cons for both approaches. Write-Ahead log contains all changed data, Command log will require addition processing, but fast and lightweight.
Command Logging and Recovery The key to command logging is that it logs the invocations, not the consequences, of the transactions.
Write-Ahead Logging The traditional rollback journal works by writing a copy of the original unchanged database content into a separate rollback journal file and then writing changes directly into the database file.
Thus a COMMIT can happen without ever writing to the original database, which allows readers to continue operating from the original unaltered database while changes are simultaneously being committed into the WAL.
Write-Ahead Logging WAL Using WAL results in a significantly reduced number of disk writes, because only the log file needs to be flushed to disk to guarantee that a transaction is committed, rather than every data file changed by the transaction. The log file is written sequentially, and so the cost of syncing the log is much less than the cost of flushing the data pages.
This is especially true for servers handling many small transactions touching different parts of the data store. Furthermore, when the server is processing many small concurrent transactions, one fsync of the log file may suffice to commit many transactions.Write-Ahead Logging (WAL) protocol The term protocol is an excellent way to describe WAL.
It is a specific and defined set of implementation steps necessary to make sure that data is stored and exchanged correctly and can be recovered to a known state in the event of a failure. Another way to think about the difference between rollback and write-ahead log is that in the rollback-journal approach, there are two primitive operations, reading and writing, whereas with a write-ahead log there are now three primitive operations: reading, writing, and checkpointing.
What is the Write-ahead-Log you ask? In my previous post we had a look at the general storage architecture of HBase. One thing that was mentioned is the Write-ahead-Log, or WAL. Feb 22, · If a write-ahead log is used, the program can check this log and compare what it was supposed to be doing when it unexpectedly lost power to what was actually done.
Stable-Storage Implementation. We introduced the write-ahead log, which requires the availability of stable storage. By definition, information residing in stable storage is never lost.
To implement such storage, we need to replicate the needed information on multiple storage devices (usually disks) with independent failure modes.
For database systems which guarantee consistency this seems to be important, since in WAL (Write Ahead Logging) recovery, you'd need your logs to be persisted on disk before actually changing your data, so that in the event of an application/system failure .