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11.8.2 Metadata Logging
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NTFS provides file system recoverability by using the same logging technique used by TxF, which consists of recording all operations that modify file system metadata to a log file. Unlike TxF, however, NTFS s built-in file system recovery support doesn t make use of CLFS but uses an internal logging implementation called the log file service. Another difference is that while TxF is used only when callers opt-in for transacted operations, NTFS records all metadata changes so that the file system can be made consistent in the face of a system failure. Log File Service The log file service (LFS) is a series of kernel-mode routines inside the NTFS driver that NTFS uses to access the log file. NTFS passes the LFS a pointer to an open file object, which specifies a log file to be accessed. The LFS either initializes a new log file or calls the Windows cache manager to access the existing log file through the cache, as shown in Figure 11-49. Note that although LFS and CLFS have similar sounding names, they are separate logging implementations used for different reasons, although their operation is similar in many ways.
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The LFS divides the log file into two regions: a restart area and an infinite logging area, as shown in Figure 11-50.
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NTFS calls the LFS to read and write the restart area. NTFS uses the restart area to store context information such as the location in the logging area at which NTFS will begin to read during recovery after a system failure. The LFS maintains a second copy of the restart data in case the first becomes corrupted or otherwise inaccessible. The remainder of the log file is the logging area, which contains transaction records NTFS writes to recover a volume in the event of a system failure. The LFS makes the log file appear infinite by reusing it circularly (while guaranteeing that it doesn t overwrite information it needs). Just like CLFS, the LFS uses LSNs to identify records written to the log file. As the LFS cycles through the file, it increases the values of the LSNs. NTFS uses 64 bits to represent LSNs, so the number of possible LSNs is so large as to be virtually infinite. NTFS never reads transactions from or writes transactions to the log file directly. The LFS provides services that NTFS calls to open the log file, write log records, read log records in forward or backward order, flush log records up to a particular LSN, or set the beginning of the log file to a higher LSN. During recovery, NTFS calls the LFS to perform the same actions as described in the TxF recovery section: a redo pass for nonflushed committed changes, followed by an undo pass for noncommitted changes. Here s how the system guarantees that the volume can be recovered: 1. NTFS first calls the LFS to record in the (cached) log file any transactions that will modify the volume structure. 2. NTFS modifies the volume (also in the cache). 3. The cache manager prompts the LFS to flush the log file to disk. (The LFS implements the flush by calling the cache manager back, telling it which pages of memory to flush. Refer back to the calling sequence shown in Figure 11-49.) 4. After the cache manager flushes the log file to disk, it flushes the volume changes (the metadata operations themselves) to disk. These steps ensure that if the file system modifications are ultimately unsuccessful, the corresponding transactions can be retrieved from the log file and can be either redone or undone as part of the file system recovery procedure. File system recovery begins automatically the first time the volume is used after the system is rebooted. NTFS checks whether the transactions that were recorded in the log file before the crash were applied to the volume, and if they weren t, it redoes them. NTFS also guarantees that transactions not completely logged before the crash are undone so that they don t appear on the volume.
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