File Systems Examples

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MS-DOS File System

• Naming: 8+3 in upper case• Directories: Hierarchical directory structure

– No soft or hard links– 32 byte directory entry

– Max file size: 232 = 4 GB (not possible due to other reasons)

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FAT

• MS-DOS uses a File Allocation Table (FAT)– FAT-12, FAT-16, FAT-32 based on # bits in disk address

• Disk block is some multiple of 512 bytes– Block size also known as cluster size in Microsoft terminology– 1st MS-DOS version used FAT-12 with 512-byte blocks

• Partition size: 212*512, actually 4086*512 ~ 2 MB

• Memory space: 4096 entries of 2 bytes each

– Later versions had variable disk block sizes 16 MB partitions

• MS-DOS supported 4 disk partitions 64 MB disks• MS-DOS uses FAT to track free disk blocks

– Mark free blocks with special code– Does not need free-list of bitmap

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FAT-16 and FAT-32

• FAT-16: 16-bit disk pointers, with varying block sizes– Block sizes from 512 bytes to 32 KB were supported– Memory requirement: 216*2 bytes = 128 KB– Largest disk partition: 216*32 KB ~ 2 GB

• Total of 8 GB disk space

• FAT-32: 28-bit disk pointers– Introduced with 2nd version of Windows 95– Theoretical partition size: 228*32 KB

• Internal representation using 32-bits and 512-byte sectors 2 TB

– Pros over FAT-16: other than larger disks• 8 GB can be single partition

• Smaller block size can be used for same disk partition

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FAT Comparison

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Windows 98 FS

• Uses FAT-32 and long file names– Also used in Windows Me

• New directory structure:

• How to store long file names?– Challenge: compatibility with earlier DOS versions– Solution: 2 names: DOS compliant (8 + 3), and original name

• SampleFile is also known as SAMPLE~1

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Storing Long File Names

• Use long file name fields to store the longer file name– Attributes field ensures that MS-DOS ignores these entries

Checksum

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NTFS• Partition also called a volume

– Cluster size from 512 bytes to 64 KB, usually 4 KB used

• Addressing: uses logical cluster numbers• File Structure: object with certain attributes

– Opposed to stream of bytes in MS-DOS and UNIX– User data in data attributes

• NTFS disk structure:– 12% allocated for MFT area

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Master File Table (MFT)

• Every system component is a file, and MFT is most imp.– Has information of every other file on the disk

• MFT divided into records of fixed size (1 to 4 KB)– Each record corresponds to a file– Each file described by one or more records

• Small (resident) attributes are stored in MFT record– For small files, even data might be stored in MFT record

• Large (nonresident) attributes stored on disk– Pointer stored in MFT

• For files with large # of attributes, or high fragmentation– Base file record has info of other records with file info

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MFT Record Attributes

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MFT Record for 3-run, 9-block file

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Storing Large Files

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Other Details

• Each file in NTFS has unique ID called File Reference– 64 bit in length with 48-bit file number, 16-bit sequence number– File number is array slot in MFT for that file– Sequence number incremented on every MFT reuse

• Used for internal consistency checks

• Directory Structure– As a B+ Tree: no tree reorganization, height of all leaves same– Index root of dir contains top level of B+ Tree

• Might point to disk extents (seq. of contiguous blocks) for large dirs

– Each dir entry has name, file ref., copy of update timestamp, size

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NTFS vs FAT directory structure

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NTFS Metadata

• First 16 NTFS files are system files, called metafiles• Disk structure:

– The first NTFS file is the MFT file– Second file contains copy of 1st 16 entries of MFT– Next few files are also special

• $LogFile: metadata updates to FS

• $Volume: housekeeping info

• $Attrdef: list of attributes on volume

• $. : root directory

• $Bitmap: volume free space bitmap

• $Boot: boot sector

• $Quota: users rights on disk space (from NT5)

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NTFS Journaling

• All FS data structure updates done within transactions– Before altering a data structure write redo and undo information– Write commit record to log after a successful update

• Restore FS data structure after a crash by processing log– Redo committed transactions, and undo unsuccessful ones

• Periodically write a checkpoint record to log– Log records before checkpoint not required

• Note: Journaling does not ensure file data consistency

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WinFS

• Bridge worlds of file systems, relation DBs, objects, XML– Eg. Store your data as object relations, which is also useful to

other apps

– Just too much work to be done!• Rewrite OS (apps), compatibility with NTFS, …

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Unix V7 FS

• Structured as a tree, starting at the root– File names are 14 ASCII chars, other than / and NUL

• Directories: one entry for each file– Each entry has 2 fields: file name (14 bytes) & i-node # (2 bytes)

• Number of files is 216 = 64 K

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Unix I-nodes

• I-node attributes: size, times (creation, modification, last access), owner, group, protection, num. of link pointers

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Steps for /usr/ast/mbox

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Linux ext2 File System

• Very common on Linux systems• Extends on Unix FS

– Generic code in /usr/src/linux/fs– ext2 specific code in /usr/src/linux/fs/ext2

• Disk partitioned into “block groups”– Ext2fs block structure:

• Similar to FFS cylinder groups

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The Linux Ext2fs File System

• File Allocation: first select a block group– For data blocks, tries to allocate block to group with file’s i-node

– For i-node allocations, selects group of file’s parent’s dir

– Directory files are dispersed across all block groups

• Within block groups, tries to allocate contiguous blocks– Uses a bitmap for free blocks in the group

– For first blocks for a file, searches free block from the beginning

– For extending file, search for a free byte in bitmap from last alloc

– If no free byte, then choose any free bit

– After choosing block, back track to not leave holes!

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Ext2fs Block-Allocation Policies

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Linux Virtual File System

• Operations differ on various OSes – Linux provides abstraction called virtual file system (VFS)

• The Linux VFS is designed around OO principles– is composed of two components:– Set of definitions defining what a file object is allowed to look like

• inode-object & file-object structures represent individual files• the file system object represents an entire file system

– A layer of software to manipulate those objects