Common Log File System (CLFS) is a general-purpose logging subsystem that is accessible to bothkernel-mode as well asuser-mode applications for building high-performancetransaction logs. It was introduced withWindows Server 2003 R2 and included in laterWindows operating systems. CLFS can be used for bothdata logging as well as forevent logging. CLFS is used byTxF andTxR to store transactional state changes before they commit a transaction. Binary Log File(s) created from CLFS can not be viewed by any integrated Windows tool.
The job of CLFS, like any other transactional logging system, is to record a series of steps required for some action so that they can be either played back accurately in the future to commit the transaction to secondary storage or undone if required. CLFS firstmarshals logs records to in-memory buffers and then writes them to log-files onsecondary storage (stable media in CLFS terminology) for permanent persistence. When the data will be flushed to stable media is controlled by built-in policies, but a CLFS client application can override that and force a flush. CLFS allows for customizable log formats, expansion and truncation of logs according to defined policies, as well as simultaneous use by multiple client applications. CLFS is able to store log files anywhere on the file system.[1]
CLFS defines adevice driverinterface (DDI), via which physical storage system specific drivers plug into the CLFS API. The CLFS driver implements theARIES recovery algorithm; other algorithms can be supported by using custom drivers.[1]
CLFS supports both dedicated logs, as well as multiplexed logs. A dedicated log contains a single stream of log records whereas multiplexed log contain multiple streams, each stream for a different application. Even though a multiplexed log has multiple streams, logs are flushed to the streams sequentially, in a single batch. CLFS can allocate space for a set of log records ahead-of-time (before the logs are actually generated) to make sure the operation does not fail due to lack of storage space.[1]
A log record in a CLFS stream is first placed toLog I/O Block in a buffer in system memory. Periodically blocks are flushed to stable storage devices. On the storage device, a log contains a set ofContainers, which are allocated contiguously, each containing multiple Log I/O Blocks. New log records are appended to the present set. Each record is identified by aLog Sequence Number (LSN), an increasing 32-bit sequence number. The LSN and other metadata are stored in the record header. The LSN encodes the identifier of the container, the offset to the record and the identifier of the record - this information is used to access the log record subsequently. However, the container identifiers are logical identifiers, they must be mapped to physical containers. The mapping is done by CLFS itself.[2]
