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COMP091 – Operating Systems 1

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1 COMP091 – Operating Systems 1
File Systems

2 File Allocation Various methods can be used by different OS to position files on disk Contiguous Allocation: Each file is stored on consecutive disk blocks Advantages: Simple to implement because we need to know only disk address of the first block of file and number of blocks The read performance is excellent because we may need only one disk operation to read the entire file.

3 Contiguous Allocation

4 Contiguous Allocation
The disadvantages of Contiguous allocation are: Disk fragmentation when files are removed. Compaction is difficult because all the blocks following the holes need to be copied. Need to know the final size of new file to be able to choose the correct hole in which to place it. Contiguous allocation is good for write once media such as CD-ROMS and DVDs

5 Linked List Allocation
A linked list of disk blocks is kept in this method First word is pointer Every disk block can be used Sequential read for the blocks of the file is easy Random access is hard because we have to read all the blocks of a file up to desired block Because of the pointer the amount of data stored in each block is not a whole block

6 Linked List Allocation

7 Linked List with Memory Table
Like linked list but keep a table of pointers to the blocks in memory. This File Allocation Table (FAT) was used in MSDOS and early windows OS Random access to blocks is easy because there is no disk reference involved. To reduce number of table entries allocation unit is a cluster of blocks Larger clusters means fewer FAT entries But more wasted space

8 File Allocation Table

9 Reliability Bad block management:
Most hard disks have bad blocks that can be avoided using bad block tables in the file system Bad block table points to spare blocks somewhere else on the disk that can be used instead of the bad block

10 Reliability Consistency checking
If the system crashes before writing all of the modified blocks, the file system becomes inconsistent. File system utilities can often resolve inconsistencies

11 The MS-DOS FAT File System (1)
Directory entry

12 The MS-DOS File System (2)
Partition (entire file system) and Cluster (Block) Size

13 The Windows 98 File System (1)
Extended MS-DOS directory entry

14 The Windows 98 File System (2)
Long name entry

15 The Windows 98 File System (3)
A long file name in storage

16 Reference Lots of detail


18 NTFS Windows NT file system More secure than FAT ACLs
Scales well to large disks Cluster size depends on disk size 64-bit file pointers Can address up to 16 exabytes of disk Multiple data streams Compression and encryption Journal

19 File System Structure Each NTFS volume (e.g., disk partition) contains files, directories, bitmaps, and other data structures Each volume is organized as a linear sequence of blocks (called clusters) usually 4 KB in size (can be 512 bytes to 64 KB) and pointed to by 64 bit pointers The main data structure in each volume is the MFT (Master File Table) which is a linear sequence of 1 KB records

20 NTFS Master File Table (1)
Each MFT record describes one file or directory and contains file attributes The MFT is a file itself and can be placed anywhere within the volume (eliminating the problem of defective sectors in the first track) The initial address of the MFT file is stored in boot sector at offset 30h bytes from its beginning. The first 16 MFT records are reserved for NTFS metadata files which contain volume related system data to describe the volume

21 NTFS Master File Table (2)

22 Attributes Used in MFT Records
Each record consists of a sequence of attribute header (= name & length) value pairs If attribute is small it is kept in the record, if it is long it is put in another block on disk and pointed to

23 MFT Record for A File

24 An MFT Record for A Small Directory

25 File Compression Transforms file to take less space on disk
Lempel-Ziv Compression Algorithm Transparent Applications access files using standard API calls System compresses and decompresses files Applications unaware if file compressed The compression algorithm considers 16 consecutive blocks If the compressed form takes less than 16 blocks then the compression is applied else not

26 File Encryption Protects files from illicit access
Encryption performed in compression units Keys Public key / private key encryption to encrypt copies of key Keys stored in X.509 certificates Recovery key given to system administrator In case user forgets password Encrypted versions of keys stored on disk Decrypted keys stored in non-paged pool

27 NTFS Log NTFS is a Journaling file system
Changes stored in reliable log first, then applied to disk After a crash, changes can be reconstructed from the log NTFS is a logical journal (not physical) Only contains changes to metadata USN (update sequence number) journal can be enabled to track all changes

28 More NTFS Features Volume Shadow Copy Transactional NTFS Quotas
Copy-on-write keeps before image journal Allows file to be reverted to previous state Transactional NTFS Users can define multi-operation transaction Technique similar to copy-on-write allows back- out of partially completed transactions Quotas Administrators can limit users use of disk space

29 Links Symbolic Links Volume Mount Points
Like an alias or bookmark to a file or directory Volume Mount Points Declare a directory to be a mount point Other volumes can be mounted there rather than at a drive letter

30 Links Directory junctions Single instance storage
Similar to mount points but mount a directory from same file system, rather than another volume Single instance storage Two identical files are linked, only stored once

31 ADS, Sparse Files Alternate Data Streams Rarely used
Introduced so Services for Macintosh could support Mac resource forks Filename:streamname refers to the alternate stream Sketchy support Used to store malware Sparse Files Files with lots of unused segments Unused aren't stored, FS returns zeros on read

32 Some Terminology Basic disks have four partition table entries in master boot record Each partition can hold a file system and is some times called a volume, or a drive (C:) One partition (an extended partition) can hold additional logical partitions, which are sometimes called logical drives, or logical volumes

33 References Old but official Comprehensive
us/library/cc758691%28v=WS.10%29.aspx us/library/cc781134%28v=ws.10%29.aspx Comprehensive

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