File Systems

1. File Systems

2. NTFS • New Technology File System (NTFS) was built to provide features like: • Reliability: introduced ideas like “transactions” (grouping certain updates together to maintain integrity) • Security and Access Control: built-in features to manage who can access files and what type of access they have

3. NTFS Features (Cont.) • Large-capacity partitions: allows large partitions and even RAID (Redundant Array of Inexpensive Disks, treating multiple disk as one large disk) • Slack reduction: allocates space differently from FAT • Allows for long file names (not limited to 8-character names with 3 character extensions) • Networking: built with networking in mind

4. A more structured file system • In NTFS files are more than just pools of data, they have structure • The difference between FAT and NTFS is somewhat analogous to the difference between a flat file and a database. • This file system as database idea is taken further in WinFS • Just as in databases where one has data and metadata (the data about the data), NTFS has metadata files (files that contain data about other files).

5. Partition/Volume Boot Sector/Record • One of the first things made when an NTFS partition is created is the volume boot sector, which contains: • BIOS parameter block: identifies the partition, how big it is, etc. • Volume boot code: code that starts to load the operating system

6. All else is files • After the volume boot sector, just about everything else is a file. There are • metadata files: files about files • Created automatically when the partition is formatted • Placed at the beginning • (Actual or real) Data files

7. MFT • Think of the Master File Table (MFT) as a database containing records about all of the files (both data and metadata, including itself). • Each file’s record holds the values of its attributes. • The actual data in a data file is simply one of its attributes.

8. The first several records • The first several records in the MFT are about other important metadata files, including • MFT itself • MFT Mirror (1st 16 records) • Log file (keeps account of transactions) • Attribution Definition Table (names file properties and says what they are) • Root Directory Folder • Bad cluster file • Etc.

9. MFT Zone • There will be a record in the MFT for every file on the partition. • Thus the MFT needs room to grow. • Some space in the partition, called the MFT Zone, is reserved for this purpose. • If one needs part of the MFT zone for storage, it will eventually be used. • On the other hand, the MFT can grow to be larger than the MFT zone. It is then fragmented which could affect performance.

10. Resident vs. Non-Resident Attributes • The MFT’s record size is fixed (between 1KB and 4KB), but the attributes may be of any size (especially since a data file’s data is an attribute). • Attributes that are contained in the MFT are called resident. • A small file may be entirely resident. • Attributes that are linked to but not actually contained in the MFT are called non-resident.

11. Some File Attributes • File name: (can be up to 255 characters, allows a file to have aliases) • Standard Information: read-only, hidden, archived, time stamps, etc. • Security Descriptor: Access Control Lists (ACLs) who owns the file, who has what privilege, etc. • Data: the actual data

12. Security • NTFS was designed with the idea of multiple users and security in mind. • The features necessary to implement a security policy are built directly into the file system. • In FAT32 a file may be hidden or read-only, but in NTFS a file can be hidden from user1, read-only to user2 and fully accessible to user3.

13. Security Concepts • Ownership: some user owns a file/folder and he or she grants permissions to other users. • Permissions: what a user can do with a file/folder (read, read-write, delete, etc.) • Users are placed in groups (possibly more than one) and permissions are assigned to groups • Permissions can be inherited, e.g. new files gets permissions of folder it was created in • Auditing: tracking information about users’ access to and modification of files

16. ACLs • An important security attribute of a file is its Access Control List (ACL). • The ACL specifies which users can access the file and in what way they can access the file • There are two types of ACL: • System ACL: used for auditing purposes • Discretionary ACL: explicit assigning of permissions to users or groups

17. Permissions

18. Reparse points • Reparse Points: One can associate an action or actions with a file. So that if the file is accessed, the action is performed. • Analogous to a trigger in a database • Reparse points is very flexible, one example is redirection sending one to another file or directory, it may be on another drive or even have been archived.

19. Other features • Improved Security and Permissions: one change is from static to dynamic permission inheritance. • Static: a child inherits the parent’s permissions when it is created but is unaffected by subsequent changes in the parent’s permissions • Dynamic: a change to the parent’s permission will affect the child’s permissions • Change Journals: improved auditing (journaling) of file/folder access activity. • Encryption: Automatic encryption/decryption of files (when accessed by users with the appropriate permissions).

20. Improvements • Disk Quotas: Users or groups of users can be limited in the amount of disk space they can use. • Sparse File Support: A sparse file is one that may be big but hold very little data (relative to its size). NTFS has utilities to help store sparse files more efficiently. • Disk Defragmenter: Strictly speaking part of the operating system, it affects the file system.

21. Transactions • Don’t forget NTFS is database like. • Almost any activity involving the drive in anyway is going to affect a number of files. • NTFS introduces the notion of a transaction – the grouping together of various operations to form an “atomic” unit. • In other words these operations should be viewed as “all or nothing” in order to maintain the file system’s integrity. • Recall the “ACID test” from databases?

22. Logging and Committing • There is a special metafile for logging all activity. • When all of the components of a transaction are complete, this completion is indicated in the log file and the transaction is said to be committed. • If something goes wrong (e.g. power failure) before a transaction is completed, the file system can undo the partially enacted transaction to return the file system to a consistent state. Doing so is said to be rolling back the transaction. • It is also called transaction recovery.

23. Effect on Performance • Logging each activity which is great for security and integrity of the file system but does have some negative effects on performance. • Each file access now requires another file access (writing to the log file). • One way to save on performance but risk somewhat integrity is to cache the activity log changes rather than write to disk every time. • The cached log results are written to disk periodically but not continuously.

24. Recovery • Recovery then involves three passes over the log file: • Analysis pass: determine the part of the disk affected • Redo pass: perform any transaction that was completed since the last “checkpoint” • Undo pass: roll back any incomplete transactions

25. Change Journal • NTFS can record changes to files, these are kept in the Change Journal. • Each change is assigned an ID, an Update Sequence Number (USN). • It will record that a file was written to but not what was written. Otherwise it would be gargantuan.

26. Fault Tolerance • NTFS has a fault tolerance disk driver known as FTDISK. • That’s where one can find the transaction recovery features. • Also where one finds support for RAID (redundant array of inexpensive (or is that independent) disks). • And where you’ll find dynamic bad cluster remapping. • Basically the drive reads immediately after writing to ensure that the cluster written to was OK. If it was not, it writes it somewhere else and marks the cluster as bad.

27. Compression • NTFS has build-in utilities for file compression • File compression takes advantage of patterns in data to reduce the amount of space required to store it. • E.g. instead of ASCII code for text (each character 8 bits) one might use a variable length code with short codes for common letters like e and longer codes for uncommon letters like q or j. On average the files are much smaller. • In NTFS one can compress any part of the partition.

28. POSIX support • NTFS offers POSIX support. • POSIX stands for Portable Operating System Interface for UNIX • It allows software developers to make sure that their code can be ported to a POSIX-compliant operating system, which includes most versions of UNIX.

29. Supports Encryption • NTFS supports Encrypting File System (EFS). • EFS is really part of the operating system (Windows 2000). But the operating system works with the file system to make this feature easy to use.

30. Disk Quota support • As a genuinely multi-user file system, NTFS support disk quotas • A quota can be set for a particular user or on a particular partition or the combination. • Allows for limits and warnings. The user is warned when he or she exceeds the warning amount. The user is blocked (from writing?) when he or she exceeds the limit amount. • Monitor and log events that cause a user to go over the "limit" or "warning" levels.

31. WinFS • Microsoft’s next version of the filesystem is WinFS. (supposedly?) • While NTFS brings many of the concepts of a database to the filesystem, WinFS is a database. • With the database ideas built in rather than overlaid, searching and querying should be enhanced. There will be metadata about the files. The dominating idea will become the properties and logical relationships of the file rather than its position in some hierarchical system of folders and files. • http://en.wikipedia.org/wiki/WinFS • http://msdn.microsoft.com/data/WinFS/default.aspx

32. DMA

33. Transfer Mode • The transfer mode describes the way in which the data moves from the hard disk through the interface (IDE/ATA) and to the memory. • For example, it tells how fast data is transferred or what device is in charge of the transfer. • There are two basic categories • PIO (Programmed I/O) Mode • The processor micro-manages data transfer • DMA (Direct Memory Access) Mode • The processor delegates data transfer

34. Programmed I/O (PIO) Modes • In the PIO category, the processor controls the data transfer. • There are various PIO modes which differ mainly by speed. • Through the early to mid-90s PIO was the standard way to transfer data to the hard disk. • The original ATA standards document defined the first three modes. • With ATA-2, two faster modes were introduced.

35. Standard PIO Modes 3.3 MB/s = 3.3  106 bytes / second = (2 bytes / 600  10-9 s) Two bytes are transferred every 600 nanoseconds.

36. External rates • The PIO rates on the previous slide are external rates meaning that they reflect the rate that data in the hard disk’s buffer/cache can be transferred. • Recall that access times to locate and read from a random sector are of the order of milliseconds. • Reading a sector (512 bytes) in 20 ms would correspond to a rate of 25 KB/s. • If one were not buffering and transferring consecutive data, the PIO mode rates would be sufficient. • But we do transfer buffered data and the PIO transfer rates are considered prohibitively slow by today’s standards.

37. PIO is too, too slow • PIO is slow in two ways: • One does not achieve the same data transfer rates as with Ultra DMA, which is the standard transfer mode used for IDE/ATA today. • Because the processor controls the details of the transfer in PIO, the processor is distracted from performing other tasks. • Despite its slowness, PIO is still around because: • PIO is simple (built into the BIOS so it does not require drivers). • Backward compatibility • Can be used as a backup when something goes wrong with DMA.

38. DMA • The alternative to PIO is DMA, Direct Memory Access. • In DMA, a device transfers information to or from the memory directly rather than in a processor-controlled fashion. • DMA has been around awhile but it was not always well supported early on. But speed requirements have made it preferred over PIO.

39. Various Modes • As with PIO, DMA has various modes differing mainly by speed. • DMA modes split into two categories: • Single-word modes which send one word (two bytes) at a time • Multiple-word modes which send several words in rapid succession (rather like the idea of bursting that accounts for improved memory speeds).

41. Assume it’s multiword • The single-word DMA modes are too slow, today it is understood that DMA is multiword DMA and the term is rarely mentioned and usually implied. • In fact, the single-word DMA modes were dropped from the standards with ATA-2. • Ultra DMA is multiword.

42. “The party of the first part shall be known in this contract as the party of the first part” First Party

43. First Party vs. Third Party • In Third-Party DMA there is a third device, the DMA controller mediating the transfer between the hard disk and memory. • Third party DMA is slow and old fashioned. • In First-Party DMA, a.k.a. bus mastering, the middle man is eliminated. The hard drive controls the transfer of data between itself and memory. • The device (hard drive in this case) takes control of (masters) the bus along which the information is sent.

44. Ultra DMA • DMA only became the norm with the introduction of Ultra DMA. • It just wasn’t well supported before. • DMA gained the advantage over PIO when Ultra DMA/33 doubled the interface transfer rate. • Support for it also improved. • Ultra DMA became an industry standard.

45. Ultra DMA uses DDR and CRC • One feature that made Ultra DMA “ultra” was that it transferred data on both the positive and negative edges of the clock. • Same idea as in DDR (Double Data Rate) memory • As Ultra DMA pushed the limit on transfer rate, it made the occurrence of errors somewhat more likely. Thus it introduced a CRC (Cyclic Redundancy Check) as part of the standard. • Recall CRC is error detection. If an error occurs in transmission the data is retransmitted.

46. Ultra DMA transfer rates DMA finally beats PIO Mode 4’s 16.7 MB/s with the added bonus of freeing up the processor. The modes are usually named after their maximum transfer rate and the interface they use: Ultra ATA/100 instead of Ultra DMA/Mode 5.

47. New cable needed • The faster speeds did require a change in the cable used to connect the drive. The 80-conductor ATA/EDE cable. • Color Code • Blue: connects to the host (motherboard or controller). • Gray: connects to the slave drive (if there is one) • Black: connects to the master drive

48. 80 wires and 40 pins?? (pre SATA) • Signals are varying currents. • Currents produce magnetic fields, varying currents produce varying magnetic fields. • Varying magnetic fields produce currents. • Oops, the current in wire 1 begins to change the current in wire 2. • There is interference (a.k.a. cross talk) • The extra wires shield the signal-carrying wires from each other.

49. Block Mode • Certain BIOSs allow for a Block Mode setting. Block Mode allows 16 or 32 sectors (512 bytes each) to be handled using a single interrupt (even if the processor is not running the show it needs to know something has happened).

50. References • PC Hardware in a Nutshell (Thompson and Thompson) • http://www.pcguide.com • All-in-One A+ Certification, Meyers and Jernigan • http://www.webopedia.com • http://www.serialata.org