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COMP091 – Operating Systems 1

Linux Filesystems (EXTx) ISO9660 UDF. COMP091 – Operating Systems 1. Linux Filesystems. Filesystems. ReiserFS Hans Reiser led team at Namesys First journaling FS for linux Out of favor since Hans charged with murdering wife and Namesys folded JFS Journaling File System from IBM

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COMP091 – Operating Systems 1

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  1. Linux Filesystems (EXTx) ISO9660 UDF COMP091 – Operating Systems 1

  2. Linux Filesystems

  3. Filesystems • ReiserFS • Hans Reiser led team at Namesys • First journaling FS for linux • Out of favor since Hans charged with murdering wife and Namesys folded • JFS • Journaling File System from IBM • ext2, ext3, ext4 • Journaling introduced in version 3 • The “standard” FS for linux • FATxx, NTFS, XFS and others also supported

  4. Ext file systems • ext: Extended File System • 1992 • Created for the linux kernel • Based on Unix File System (UFS) • 2GB, 255 character file names • ext2 • 2TB • POSIX ACL's • Timestamps

  5. Sidetrack: POSIX • Portable Operating System Interface • Family of IEEE standards • Institute of Electrical and Electronics Engineers • IEEE Std 1003.1-1988 • ISO/IEC 9945 • Richard Stallman suggested the name • Designed to maintain compatibility between operating systems.

  6. ext3 • Journaling • Made JFS and ReiserFS unnecessary • Although ext3 not as fast • Backward compatible with ext2 • Just add journal file • Blocksize 1 – 8 KiB • Filesize 16 GiB – 2 TiB • FS size 2 TiB – 32 TiB

  7. Journal • After image log • Options: • Data and metadata, written before data • Metadata only, disk written before the journal is marked as committed • Metadata only, written in any order

  8. ext4 • Stable in linux 2.6.28 (2008) • Basically a batch of simultaneous updates to ext3 • Motivated partly by problems of large file systems • FS size 1 EiB(2**60) • File size 16 TiB

  9. ext4 • Extents replace block mapping for allocation • Extent = starting block and block count • Available pre-allocation • Delayed allocation • 64000 subdirectories (up from 32000) • Htree indexes • Nanosecond timestamps

  10. Blocks • Like clusters in FAT file system • In ext2/3: 1024, 2048 or 4096 • Specify when fs created • Block groups are large collections of contiguous blocks that partition the FS data structures • Usually size determined by FS but can be specified • Size limited by bitmaps so large FS may need multiple block groups

  11. Superblock • 2 sectors (1024 bytes) that describe the file system • Volume label • Block size • # blocks per group • # reserved blocks before the 1st block group • Count of free inodes & blocks (total all groups)

  12. Superblock • 1st superblock is1024 bytes past the beginning of the file system • Copies of the superblock are in the first block of each block group

  13. Group Descriptor Table • Stores • The group descriptors • One for each block group • Starting block addresses • block bitmap • inode bitmap • inode table • Count of free inodes & blocks for the group • Located in the block after the superblock • Backup copies are in the same block groups as the superblock backups

  14. Block Bitmap • One bit per block in the group • size = #blocks / 8 • Linux creates a block group to have as many blocks as there are bits in a block • Thus, a block bitmap is always 1 block in size • Tracks block allocation for the group

  15. Inode Bitmap • Tracks the allocation of inodes in the group • Size = #inodes per group / 8 • Size defined at file system creation • Typically fewer inodes than blocks in group

  16. inodes • Contained in inode table • Like MFT records for files • 256 bytes per inode • Number of inodes determines number of files • Specified when FS created • Contain file and directory metadata • Directory has file or directory name • And pointer to inode in the inode table • Inode points to the file content blocks

  17. inode contents • Type of the file • plain file, Directory, symbolic link, device file • Access permissions • Owner and group ID numbers • Size in bytes • Number of links (directory references) • Times of last access and last modification to the file • List of data blocks claimed by the file. • Address of the file's blocks on the disk

  18. Directories • Special files that associate names with the inode numbers used internally by the file system • Each entry associates one file name with one inode number • Consists of: • inode number, • length of the file name • actual text of the file name.

  19. Directory Structure • Hierarchical • Like FAT and NTFS • But all data stored in one tree • No C: N: etc • Includes network shares, all physical devices, removable devices and some virtual file systems

  20. / usr bin boot root mnt var Linux File Structure File system /mnt/floppy /mnt/cdrom /mnt/USB Points to a device /dev/fd0 /dev/hdc /dev/sda

  21. ISO9660

  22. ISO9660 • Filesystem spec for data on CDs (and DVDs) • 24 byte frames • 2352 byte sectors (98 frames) • 2048 user data, 288 bytes ECC and headers and sync data • A/V disks can use ECC area for data • Assumed contiguous allocation allows simpler FS structures

  23. ISO Limitations • For cross platform compatibility: • File names have upper case letters, digits, underscores and one “.” • No spaces • Eight level directories • 4GiB file size limit • Most OS ignore or circumvent these these limits • Path table (for efficiency) imposes limit 65,535 on number of directories (not linux)

  24. ISO Extensions • Sessions • ISO is read only, contiguous pre-allocating FS • Doesn't expect data to be appended • Sessions allow data to be added to a CD • Each new session contains an updated copy of the entire disk's directories and other data structures

  25. ISO Extensions • Joliet • Unicode names • Avoids file name restrictions • Rock Ridge supports POSIX acl's and longer names • El Torito allows cd's to be bootable • Apple's ISO 9660 extensions allow for Apple resource forks

  26. UDF x.x

  27. UDF • Universal Disk Format • Open specification designed for any media • Mostly used for DVD and BD instead of ISO9660 • Official file system for DVD-Video and Audio per DVD Forum • Design suitable for incremental updates • As opposed to creating ISO then burning • Specification maintained by Optical Storage Technology Association (OSTA) • Most of the world's optical product manufacturers and resellers

  28. UDF Revisions • 1.02: DVD video • 1.50: Introduced VAT for CD-R/DVD-R • 2.00: File types for DVD recording • 2.01: Fix bugs in 2.00 • 2.50: Adds metadata partition, Used on some Blu-ray disks • 2.60: Adds Pseudo OverWrite. Used by some other Blu-ray disks

  29. UDF “Builds” • Plain (the original one) • Must be built then burned to CD like ISO9660 • VAT • Like ISO9660 with lots of little sessions • Spared • For -RW media • Knows to move often changed metadata around to avoid wearing out sectors with re-writes

  30. Compatibility • Windows calls UDF “Live File System” • Because nothing should be referred to by its real name • Vista and later support all features • Linux support is evolving • Read for all versions • Write is “safe” up to 2.01 for plain build

  31. Terminology

  32. Terminology • Sector • Hard drives • At interface between disk and controller • Header, data and ECC • Header contains sync bytes, address identification, flaw flag and header parity bytes • Usually 512 bytes for data • Can be 2048 or 4096 (advanced format sectors) • Sometimes interleaved Data

  33. Terminology • Track • One ring of sectors around a disk • Can be read by one read head without moving the head • Invisible to FS and application • Cylinder • On multi-platter disk, one head per platter, all move in unison • Cylinder = the tracks read by the heads without moving the heads

  34. Terminology • Block • Sometimes called physical record • Consists of multiple logical records • On tape media where there is no sector • Or multiple sectors • On disk • Transfer unit from device to file system • Application can specify block size to fine tune performance • Block size = track size can make sense

  35. Terminology • Record • = logical record • Meaningful to applications • Internal structure of files • File is usually a collection of logical records • Not relevant to FS or OS • Except in files OS cares about, like directories, executables, MFT • Physical record usually means block • Sometimes sector is called physical record

  36. Terminology • Stream • The non-metadata part of a file • Actual data is a string of bytes • Byte 0 to byte filesize – 1 • This part of file sometimes called the data stream • NTFS allows alternate data streams • Some of the metadata are called streams • In MFT name value pairs, value is called stream

  37. Terminology • Cluster • Allocation unit • Sometimes called block • In ext FS allocation units are called blocks • Ext4 allocation unit called extent

  38. Terminology • Partition • A subdivision of the space on a disk • Contiguous, may need to start on physical boundaries • FSs are contained in partitions • Often called volumes • Partitions are the “mounting unit” • Note C: is where a FS in a partition or volume is mounted • But C:, D: etc are often called drives, or disks

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