Windows® Internals, Sixth Edition, Part 2

The $TXF_DATA attribute also stores internal flags that describe the state information to TxF and the index of the USN record that was applied to the file on commit. A TxF transaction can span multiple USN records that may have been partly updated by NTFS’s recovery mechanism (described shortly), so the index tells TxF how many more USN records must be applied after a recovery.

Logging Implementation

As mentioned earlier, each time a change is made to the disk because of an ongoing transaction, TxF writes a record of the change to its log. TxF uses a variety of log record types to keep track of transactional changes, but regardless of the record type, all TxF log records have a generic header that contains information identifying the type of the record, the action related to the record, the TxID that the record applies to, and the GUID of the KTM transaction that the record is associated with.

A redo record specifies how to reapply a change part of a transaction that’s already been committed to the volume if the transaction has actually never been flushed from cache to disk. An undo record, on the other hand, specifies how to reverse a change part of a transaction that hasn’t been committed at the time of a rollback. Some records are redo-only, meaning they don’t contain any equivalent undo data, while other records contain both redo and undo information.

Through the TOPS file, TxF maintains two critical pieces of data, the base LSN and the restart LSN. The base LSN determines the LSN of the first valid record in the log, while the restart LSN indicates at which LSN recovery should begin when starting the resource manager. When TxF writes a restart rec-ord, it updates these two values, indicating that changes have been made to the volume and flushed out to disk—meaning that the file system is fully consistent up to the new restart LSN.

TxF also writes compensating log records, or CLRs. These records store the actions that are being performed during transaction rollback (explained next). They’re primarily used to store the undo-next LSN, which allows the recovery process to avoid repeated undo operations by bypassing undo records that have already been processed, a situation that can happen if the system fails during the recovery phase and has already performed part of the undo pass. Finally, TxF also deals with prepare records, abort records, and commit records, which describe the state of the KTM transactions related to TxF.

Recovery Implementation

When a resource manager starts because of an FSCTL_TXFS_START_RM call (or, for the default resource manager, as soon as the volume is mounted), TxF runs the recovery process. It reads the TOPS file to determine the restart LSN, where the recovery process should start, and then reads each record forward through the log (called the redo pass). As each record is being processed, TxF opens the file referenced by the record and compares the LSN in the $TXF_DATA attribute with the LSN in the record. If the LSN stored in the attribute is greater than or equal to the LSN of the log record, the action is not applied because the on-disk copy of the file is as new or newer than that of the log record action. If the LSN is not greater than or equal to the LSN in the record, the log contains information about the file that was never written to the file itself. In this case, TxF applies whichever action was recorded in the log record and updates the LSN in the $TXF_DATA attribute with the LSN from the record.

As TxF is processing its redo pass, it builds its transaction table, which describes the operations that it has completed; if it encounters an abort or commit record along the way, TxF discards the related transactions. By the end of the redo pass, TxF parses the final transaction table and connects to the KTM to see whether the KTM recorded a commit or an abort for the transactions. (KTM stores this information in the KtmLog stream of the TxF multiplexed log, as explained earlier.)

After TxF has finished communicating with the KTM, it looks at any leftover transactions in the transaction table and begins the undo pass. In the undo pass, TxF aborts all the remaining transactions in the transaction table by traversing each transaction’s undo LSN chain and applying the undo action for each log record. At the end of the undo pass, the resource manager is consistent and initialized.

This process is very similar to the log file service’s recovery procedure, which is described later in more detail. You should refer to this description for a complete picture of the standard transactional recovery mechanisms.

NTFS Recovery Support