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The output indicates the maximum size of the change journal on the volume and its current state. As a simple experiment to see how NTFS records changes in the journal, create a file called Usn.txt in the current directory, rename it to UsnNew.txt, and then dump the journal with Usndump, as shown here: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
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C:\>echo hello > Usn.txt C:\>ren Usn.txt UsnNew.txt C:\>Usndump.exe ... File Ref# : 0x00670000000006BF ParentFile Ref# : 0x0005000000000005 Usn : 0x0000000000000000 SecurityId : 0x00000000 Reason : 0x00000100 (USN_REASON_FILE_CREATE) Name (014) : usn.txt File Ref# : 0x00670000000006BF ParentFile Ref# : 0x0005000000000005 Usn : 0x0000000000000050 SecurityId : 0x00000000
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Name (014) : usn.txt File Ref# : 0x00670000000006BF ParentFile Ref# : 0x0005000000000005 Usn : 0x00000000000000A0 SecurityId : 0x00000000
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Name (014) : usn.txt File Ref# : 0x00670000000006BF ParentFile Ref# : 0x0005000000000005 Usn : 0x0000000000000138 SecurityId : 0x00000000 Reason : 0x00001000 (USN_REASON_RENAME_OLD_NAME) Name (014) : usn.txt File Ref# : 0x00670000000006BF ParentFile Ref# : 0x0005000000000005 Usn : 0x0000000000000188 SecurityId : 0x00000000 Reason : 0x00002000 (USN_REASON_RENAME_NEW_NAME) Name (020) : UsnNew.txt File Ref# : 0x00670000000006BF ParentFile Ref# : 0x0005000000000005 Usn : 0x00000000000001D8 SecurityId : 0x00000000
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Name (020) : UsnNew.txt
The entries reflect the individual modification operations involved in the operations underlying the command-line operations. The journal is sparse so that it never overflows; when the journal s on-disk size exceeds the maximum defined for the file, NTFS simply begins zeroing the file data that precedes the window of change information having a size equal to the maximum journal size, as shown in Figure 11-42. To prevent constant resizing when an application is continuously exceeding the journal s size, NTFS shrinks the journal only when its size is twice an application-defined value over the maximum configured size.
Indexing In NTFS, a file directory is simply an index of file names that is, a collection of file names (along with their file references) organized in a particular way for quick access. To create a directory, NTFS indexes the filename attributes of the files in the directory. The MFT record for the root directory of a volume is shown in Figure 11-43.
Conceptually, an MFT entry for a directory contains in its index root attribute a sorted list of the files in the directory. For large directories, however, the file names are actually stored in 4-KB, fixed-size index buffers that contain and organize the file names. Index buffers implement a b-tree data structure, which minimizes the number of disk accesses needed to find a particular file, especially for large directories. The index root attribute contains the first level of the b-tree (root subdirectories) and points to index buffers containing the next level (more subdirectories, perhaps, or files). Figure 11-43 shows only file names in the index root attribute and the index buffers (file6, for example), but each entry in an index also contains the file reference in the MFT where the file is described and time stamp and file size information for the file. NTFS duplicates the time stamp and file size information from the file s MFT record. This technique, which is used by FAT and NTFS, requires updated information to be written in two places. Even so, it s a significant speed optimization for directory browsing because it enables the file system to display each file s time stamps and size without opening every file in the directory. The index allocation attribute maps the VCNs of the index buffer runs to the LCNs that indicate where the index buffers reside on the disk, and the bitmap attribute keeps track of which VCNs in the index buffers are in use and which are free. Figure 11-43 shows one file entry per VCN (that is, per cluster), but file name entries are actually packed into each cluster. Each 4-KB index buffer can contain about 20 to 30 file name entries. The b-tree data structure is a type of balanced tree that is ideal for organizing sorted data stored on a disk because it minimizes the number of disk accesses needed to find an entry. In the MFT, a directory s index root attribute contains several file names that act as indexes into the second level of the b-tree. Each file name in the index root attribute has an optional pointer associated with it that points to an index buffer. The index buffer it points to contains file names with lexicographic values less than its own. In Figure 11-43, for example, file4 is a first-level entry in the b-tree. It points to an index buffer containing file names that are (lexicographically) less than itself the file names file0, file1, and file3. Note that the names file1, file3, and so on that are used in this example are not literal file names but names intended to show the relative placement of files that are lexicographically ordered according to the displayed sequence. Storing the file names in b-trees provides several benefits. Directory lookups are fast because the file names are stored in a sorted order. And when higher-level software enumerates the files in a directory, NTFS returns already-sorted names. Finally, because b-trees tend to grow wide rather than deep, NTFS s fast lookup times don t degrade as directories grow. NTFS also provides general support for indexing data besides file names, and several NTFS features including object IDs, quota tracking, and consolidated security use indexing to manage internal data. Object IDs In addition to storing the object ID assigned to a file or directory in the $OBJECT_ID attribute of its MFT record, NTFS also keeps the correspondence between object IDs and their file reference numbers in the $O index of the \$Extend\$ObjId metadata file. The index collates entries by object ID, making it easy for NTFS to quickly locate a file based on its ID. This feature allows applications, using undocumented native API functionality, to open a file or directory using
its object ID. Figure 11-44 demonstrates the correspondence of the $ObjId metadata file and $OBJECT_ID attributes in MFT records.
Quota Tracking NTFS stores quota information in the \$Extend\$Quota metadata file, which consists of the indexes $O and $Q. Figure 11-45 shows the organization of these indexes. Just as NTFS assigns each security descriptor a unique internal security ID, NTFS assigns each user a unique user ID. When an administrator defines quota information for a user, NTFS allocates a user ID that corresponds to the user s SID. In the $O index, NTFS creates an entry that maps an SID to a user ID and sorts the index by user ID; in the $Q index, NTFS creates a quota control entry. A quota control entry contains the value of the user s quota limits, as well as the amount of disk space the user consumes on the volume.
When an application creates a file or directory, NTFS obtains the application user s SID and looks up the associated user ID in the $O index. NTFS records the user ID in the new file or directory s $STANDARD_INFORMATION attribute, which counts all disk space allocated to the file or directory against that user s quota. Then NTFS looks up the quota entry in the $Q index and determines whether the new allocation causes the user to exceed his or her warning or limit
threshold. When a new allocation causes the user to exceed a threshold, NTFS takes appropriate steps, such as logging an event to the System event log or not letting the user create the file or directory. As a file or directory changes size, NTFS updates the quota control entry associated with the user ID stored in the $STANDARD_INFORMATION attribute. NTFS uses general indexing to efficiently correlate user IDs with account SIDs, and, given a user ID, to efficiently look up a user s quota control information. Consolidated Security NTFS has always supported security, which lets an administrator specify which users can and can t access individual files and directories. NTFS optimizes disk utilization for security descriptors by using a central metadata file named $Secure to store only one instance of each security descriptor on a volume. The $Secure file contains two index attributes $SDH and $SII and a data-stream attribute named $SDS, as Figure 11-46 shows. NTFS assigns every unique security descriptor on a volume an internal NTFS security ID (not to be confused with an SID, which uniquely identifies computers and user accounts) and hashes the security descriptor according to a simple hash algorithm. A hash is a potentially nonunique shorthand representation of a descriptor. Entries in the $SDH index map the security descriptor hashes to the security descriptor s storage location within the $SDS data attribute, and the $SII index entries map NTFS security IDs to the security descriptor s location in the $SDS data attribute. When you apply a security descriptor to a file or directory, NTFS obtains a hash of the descriptor and looks through the $SDH index for a match. NTFS sorts the $SDH index entries according to the hash of their corresponding security descriptor and stores the entries in a b-tree. If NTFS finds a match for the descriptor in the $SDH index, NTFS locates the offset of the entry s security descriptor from the entry s offset value and reads the security descriptor from the $SDS attribute. If the hashes match but the security descriptors don t, NTFS looks for another matching entry in the $SDH index. When NTFS finds a precise match, the file or directory to which you re applying the security descriptor can reference the existing security descriptor in the $SDS attribute. NTFS makes the reference by reading the NTFS security identifier from the $SDH entry and storing it in the file or directory s $STANDARD_INFORMATION attribute. The NTFS $STANDARD_INFORMATION attribute, which all files and directories have, stores basic information about a file, including its attributes, time stamp information, and security identifier.
If NTFS doesn t find in the $SDH index an entry that has a security descriptor that matches the descriptor you re applying, the descriptor you re applying is unique to the volume and NTFS assigns the descriptor a new internal security ID. NTFS internal security IDs are 32-bit values, whereas SIDs are typically several times larger, so representing SIDs with NTFS security IDs saves space in the $STANDARD_INFORMATION attribute. NTFS then adds the security descriptor to the $SDS attribute, which is sorted in a b-tree by the NTFS security ID, and it adds to the $SDH and $SII indexes entries that reference the descriptor s offset in the $SDS data. When an application attempts to open a file or directory, NTFS uses the $SII index to look up the file or directory s security descriptor. NTFS reads the file or directory s internal security ID from the MFT entry s $STANDARD_INFORMATION attribute. It then uses the $Secure file s $SII index to locate the ID s entry in the $SDS attribute. The offset into the $SDS attribute lets NTFS read the security descriptor and complete the security check. NTFS stores the 32 most recently accessed security descriptors with their $SII index entries in a cache so that it will access the $Secure file only when the $SII isn t cached. NTFS doesn t delete entries in the $Secure file, even if no file or directory on a volume references the entry. Not deleting these entries doesn t significantly decrease disk space because most volumes, even those used for long periods, have relatively few unique security descriptors. NTFS s use of general indexing lets files and directories that have the same security settings efficiently share security descriptors. The $SII index lets NTFS quickly look up a security descriptor in the $Secure file while performing security checks, and the $SDH index lets NTFS quickly determine whether a security descriptor being applied to a file or directory is already stored in the $Secure file and can be shared. Reparse Points As described earlier in the chapter, a reparse point is a block of up to 16 KB of applicationdefined reparse data and a 32-bit reparse tag that are stored in the $REPARSE_POINT attribute of a file or directory. Whenever an application creates or deletes a reparse point, NTFS updates the \$Extend\$Reparse metadata file, in which NTFS stores entries that identify the file record numbers of files and directories that contain reparse points. Storing the records in a central location enables NTFS to provide interfaces for applications to enumerate all a volume s reparse
points or just specific types of reparse points, such as mount points. (See 8 for more information on mount points.) The \$Extend\$Reparse file uses the general indexing facility of NTFS by collating the file s entries (in an index named $R) by reparse point tags. Transaction Support By leveraging the Kernel Transaction Manager (KTM) support in the kernel, as well as the facilities provided by the Common Log File System that were described earlier, NTFS implements a transactional model called Transactional NTFS or TxF. TxF provides a set of usermode APIs that applications can use for transacted operations on their files and directories and also a file system control (FSCTL) interface for managing its resource managers. Note Support for TxF was added to the NTFS driver starting with Windows Vista and Windows Server 2008 without actually changing the format of the NTFS data structures, which is why the NTFS format version number, 3.1, is the same as for Windows XP and Windows Server 2003. TxF achieves backward compatibility by reusing the attribute type that was previously used only for EFS support instead of adding a new one. The overall architecture for TxF, shown in Figure 11-47, uses several components: Transacted APIs implemented in the Kernel32.dll library A library for reading TxF logs (%SystemRoot%\System32\Txfw32.dll) A COM component for TxF logging functionality (%SystemRoot\System32\Txflog.dll) The transactional NTFS library inside the NTFS driver The CLFS infrastructure for reading and writing log records
Isolation Although transactional file operations are opt-in, just like the transactional registry (TxR) operations described in 4, TxF has an impact on regular applications that are not transaction-aware because it ensures that the transactional operations are isolated. For example, if an antivirus program is scanning a file that s currently being modified by another application via a transacted operation, TxF must make sure that the scanner reads the pretransaction data, while applications that access the file within the transaction work with the modified data. This model is called read-committed isolation.
Read-committed isolation involves the concept of transacted writers and transacted readers. The former always view the most up-to-date version of a file, including all changes made by the transaction that is currently associated with the file. At any given time, there can be only one transacted writer for a file, which means that its write access is exclusive. Transacted readers, on the other hand, only have access to the committed version of the file at the time they open the file. They are therefore isolated from changes made by transacted writers. This allows for readers to have a consistent view of a file, even when a transacted writer commits its changes. To see the updated data, the transacted reader must open a new handle to the modified file. Nontransacted writers, on the other hand, are prevented from opening the file by both transacted writers and transacted readers, so they cannot make changes to the file without being part of the transaction. Nontransacted readers act similarly to transacted readers in that they see only the file contents that were last committed when the file handle was open. Unlike transacted readers, however, they do not receive read-committed isolation, and as such they always receive the updated view of the latest committed version of a transacted file without having to open a new file handle. This allows non-transaction-aware applications to behave as expected. To summarize, TxF s read-committed isolation model has the following characteristics: Changes are isolated from transacted readers Changes are rolled back (undone) if the associated transaction is rolled back, if the machine crashes, or if the volume is forcibly dismounted. Changes are flushed to disk if the associated transaction is committed. eXPeriMeNT: understanding and Managing Transactions In this experiment we ll use the Transactdemo.exe tool to create a new file, add some data to it as part of a transaction, and see how nontransacted clients interact with the file while the transaction is active. First, open a Command Prompt window and run Transactdemo.exe: 1. 2. 3. 4. 5. 6. 7. C:\>Transactdemo.exe Transaction Demo v1.0 by Mark Russinovich Transaction created: {5CD5E900-9DA8-11DD-8379-005056C00008} Created C:\TransactionDemo.txt. Pass TransDemo the GUID listed above to see the transacted file. Rollback or commit transaction (r/c):
Transactdemo creates c:\TransactionDemo.txt within a transaction that it has not committed. Open a second Command Prompt window, and use the dir command to look for the presence of the TransactionDemo.txt file: 1. 2. 3. 4. 5. C:\>dir transactiondemo.txt Volume in drive C is OS Volume Serial Number is 0C30-686E Directory of C:\ File Not Found
According to this second command prompt, the file doesn t even exist. Now simulate a nontransacted writer by trying to add data to the file via the echo command: 1. 2. C:\>echo Hello > TransactionDemo.txt The function attempted to use a name that is reserved for use by another transaction. As expected, nontransacted writers are blocked from modifying the file. The Ktmutil and Fsutil built-in applications can be very useful for dealing with transactional operations on the file system. For example, you can get a list of all current transactions on the system with the following command: 1. 2. 3.
C:\>ktmutil list transactions TxGuid Description -------------------------------------- ----------------------------------------------{5cd5e900-9da8-11dd-8379-005056c00008} Demo Transaction
Note that the GUID matches what Transactdemo returned. With the GUID, you can now use the Fsutil command to query information about the transaction and to commit it or roll it back. For example, here s how to list the files part of the transaction and the owner account: 1. 2. 3. 4. 5. 6. 7. 8. C:\>fsutil transaction query all {5cd5e900-9da8-11dd-8379-005056c00008} dwOutcome: 1 dwIsolationLevel: 0 dwIsolationFlags: 0 dwTimeout: -1 Owner: BUILTIN\Administrators Number of Files: 1 ---- \TransactionDemo.txt
Although the Transactdemo tool presents you with the option to roll back or commit the current transaction, the Fsutil utility allows commits or rollbacks to any ongoing transaction your account has access to. Go back to the command prompt where you ran Transactdemo and press C to commit the transaction, after which the file becomes a standard nontransacted file. Transactional APIs TxF implements transacted versions of the Windows file I/O APIs, which use the suffix Transacted: Create APIs CreateDirectoryTransacted, CreateFileTransacted, CreateHardLinkTransacted, CreateSymbolicLinkTransacted Find APIs FindFirstStreamTransacted FindFirstFileNameTransacted, FindFirstFileTransacted,
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