Windows® Internals, Sixth Edition, Part 2

To free up space in the log file, NTFS must momentarily prevent further transactions on files. To do so, NTFS blocks file creation and deletion and then requests exclusive access to all system files and shared access to all user files. Gradually, active transactions either are completed successfully or receive the “log file full” exception. NTFS rolls back and queues the transactions that receive the exception.

Once it has blocked transaction activity on files as just described, NTFS calls the cache manager to flush unwritten data to disk, including unwritten log file data. After everything is safely flushed to disk, NTFS no longer needs the data in the log file. It resets the beginning of the log file to the current position, making the log file “empty.” Then it restarts the queued transactions. Beyond the short pause in I/O processing, the “log file full” error has no effect on executing programs.

This scenario is one example of how NTFS uses the log file not only for file system recovery but also for error recovery during normal operation. You’ll find out more about error recovery in the following section.

Recovery

NTFS automatically performs a disk recovery the first time a program accesses an NTFS volume after the system has been booted. (If no recovery is needed, the process is trivial.) Recovery depends on two tables NTFS maintains in memory: a transaction table, which behaves just like the one TxF maintains, and a dirty page table, which records which pages in the cache contain modifications to the file system structure that haven’t yet been written to disk. This data must be flushed to disk during recovery.

NTFS writes a checkpoint record to the log file once every 5 seconds. Just before it does, it calls the LFS to store a current copy of the transaction table and of the dirty page table in the log file. NTFS then records in the checkpoint record the LSNs of the log records containing the copied tables. When recovery begins after a system failure, NTFS calls the LFS to locate the log records containing the most recent checkpoint record and the most recent copies of the transaction and dirty page tables. It then copies the tables to memory.

The log file usually contains more update records following the last checkpoint record. These update records represent volume modifications that occurred after the last checkpoint record was written. NTFS must update the transaction and dirty page tables to include these operations. After updating the tables, NTFS uses the tables and the contents of the log file to update the volume itself.

To perform its volume recovery, NTFS scans the log file three times, loading the file into memory during the first pass to minimize disk I/O. Each pass has a particular purpose:

Analysis

Redoing transactions

Undoing transactions

Analysis Pass

During the analysis pass, as shown in Figure 12-55, NTFS scans forward in the log file from the beginning of the last checkpoint operation to find update records and use them to update the transaction and dirty page tables it copied to memory. Notice in the figure that the checkpoint operation stores three records in the log file and that update records might be interspersed among these records. NTFS therefore must start its scan at the beginning of the checkpoint operation.

Figure 12-55. Analysis pass

Most update records that appear in the log file after the checkpoint operation begins represent a modification to either the transaction table or the dirty page table. If an update record is a “transaction committed” record, for example, the transaction the record represents must be removed from the transaction table. Similarly, if the update record is a “page update” record that modifies a file system data structure, the dirty page table must be updated to reflect that change.

Once the tables are up to date in memory, NTFS scans the tables to determine the LSN of the oldest update record that logs an operation that hasn’t been carried out on disk. The transaction table contains the LSNs of the noncommitted (incomplete) transactions, and the dirty page table contains the LSNs of records in the cache that haven’t been flushed to disk. The LSN of the oldest update record that NTFS finds in these two tables determines where the redo pass will begin. If the last checkpoint record is older, however, NTFS will start the redo pass there instead.

Note

In the TxF recovery model, there is no distinct analysis pass. Instead, as described in the TxF recovery section, TxF performs the equivalent work in the redo pass.

Redo Pass