CHAPTER 11: IMPLEMENTING FILE SYSTEMS (COMPACT) By I-Chen Lin Textbook: Operating System Concepts 9th Ed.

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1 CHAPTER 11: IMPLEMENTING FILE SYSTEMS (COMPACT) By I-Chen Lin Textbook: Operating System Concepts 9th Ed.

2 File-System Structure File structure Logical storage unit Collection of related information File system resides on secondary storage (disks) Provided user interface to storage, mapping logical to physical Provides efficient and convenient access to disk by allowing data to be stored, located retrieved easily Disk provides in-place rewrite and random access I/O transfers performed in blocks of sectors (usually 512 bytes) File control block storage structure consisting of information about a file Device driver controls the physical device File system organized into layers

3 Layered File System

4 Layered File System (cont.) I/O control Device drivers and interrupt handlers. Translate high-level commands to hardware-specific instructions. Given commands like read drive1, cylinder 72, track 2, sector 10, into memory location 1060 outputs low-level hardware specific commands to hardware controller. Basic file system given command like retrieve block 123, translates to device driver. Also manages memory buffers and caches (allocation, freeing, replacement) Buffers hold data in transit; caches hold frequently used data File-organization module Logical blocks -> physical blocks Free-space management

5 Layered File System (cont.) Logical file system Manage metadata. (all except the contents) Translates file name into file number, file handle Manage directories File-control block (FCB) Protection Layered structure Flexible Minimizes duplicate codes to support multiple file systems. Adds overhead and can decrease performance Logical layers can be implemented by any coding method according to OS designer E.g. CD-ROM: ISO 9660 Unix: UNIX file system (UFS) Windows: FAT, FAT32, NTFS (Windows NT File System) Linux: Extended file system (ext2, ext3 ) New ones ZFS, GoogleFS, Oracle ASM, FUSE

6 File-System Implementation (On-disk) We have system calls at the API level, but how do we implement their functions? On-disk and in-memory structures Boot control block contains info to boot OS from that volume UFS: boot block NTFS: partition boot sector (BIOS MBR boot sector of the active partition)

7 On-disk Structure (cont.) Volume control block contains volume details About volume ( logical drive ) details E.g. number of blocks, free block count, free block pointer, UFS: superblock NTFS: in master file table Directory structure organizes the files NTFS: in the master file table Per-file File Control Block (FCB) contains many details about the file UFS: inode NTFS: in the master file table

8 A Typical File Control Block

9 In-Memory File System Structures In-memory mounted table In-memory directory structure System-wide open-file table Copy of the FCB of each open file, etc. Per-process open-file table Per-process info. and a pointer to the system-wide open-file table Buffers hold data blocks from secondary storage Data from read eventually copied to specified user process memory address

10 In-Memory File System Structures (File Open and Read) File Open Check whether the file is already opened. If not, find the file and copy FCB into the system-wide open file table. Per-process open-file table points to the system-wide table. The process gets the file pointer.

11 Partitions and Mounting Partition can be a volume containing a file system ( cooked ) or raw just a sequence of blocks with no file system Boot block points to boot volume or boot loader set of blocks with codes to load the kernel from the file system Or a boot management program for multi-os booting Root partition contains the OS, other partitions can hold other Oses, other file systems, or be raw Mounted at boot time Other partitions can mount automatically or manually At mount time, file system consistency checked Is all metadata correct? If not, fix it, try again If yes, add to mount table, allow access

12 Virtual File Systems Virtual File Systems (VFS) provide an object-oriented way of implementing file systems. VFS concurrently supports multiple types of file systems. VFS allows the same system call interface (the API) to be used for different types of file systems. Separates file-system generic operations from implementation details Implements vnodes which hold inodes (for a single file system) or network file details The API is to the VFS interface, rather than any specific type of file system.

13 Schematic View of Virtual File System

14 Directory Implementation Linear list of file names with pointer to the data blocks. simple to program time-consuming to execute Hash Table linear list with hash data structure. decreases directory search time collisions situations where two file names hash to the same location fixed size

15 Allocation Methods An allocation method refers to how disk blocks are allocated for files: Contiguous allocation Linked allocation Indexed allocation

16 Contiguous Allocation Each file occupies a set of contiguous blocks on the disk Simple only starting location (block #) and length (number of blocks) are required Random access Best performance in most cases Wasteful of space (dynamic storage-allocation problem) External fragmentation: need for compaction off-line (downtime) or on-line Files cannot grow How to deal with these problems?

17 Contiguous Allocation of Disk Space

18 Extent-Based Systems Many newer file systems (I.e. Veritas File System) use a modified contiguous allocation scheme Extent-based file systems allocate disk blocks in extents An extent is a contiguous block of disks Extents are allocated for file allocation. A file consists of one or more extents. A link is assigned to the next extend. Internal fragmentation.

19 Linked Allocation Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk. block = pointer File ends at nil pointer Each block contains pointer to next block No compaction, external fragmentation Free space management system called when new block needed

20 Linked Allocation (Cont.) Simple need only starting address Free-space management system no waste of space No random access Locating a block can take many I/Os and disk seeks Improve efficiency by clustering blocks into groups but increases internal fragmentation Reliability can be a problem

21 Linked Allocation

22 File-Allocation Table File-allocation table (FAT) disk-space allocation used by MS-DOS and OS/2. Table located at beginning of the volume. Much like a linked list, but faster on disk and cacheable Unused blocks are set as 0: (for simpler new block allocation)

23 File-Allocation Table Improve random access. access both the table and content! (unless caching)

24 Indexed Allocation Brings all pointers together into the index block. Logical view. index table

25 Example of Indexed Allocation

26 Indexed Allocation (Cont.) Need index table Random access Dynamic access without external fragmentation, but have overhead of index block. How large should the index block be? Linked scheme Link blocks of index table (no limit on size). Multilevel index

27 Indexed Allocation Mapping (Cont.) outer-index index table file

28 Combined Scheme: UNIX inode (4K bytes per block)

29 Performance Best method depends on file access type Contiguous great for sequential and random Linked good for sequential, not random Declare access type at creation -> select either contiguous or linked Indexed more complex Single block access could require 2 index block reads then data block read Clustering can help improve throughput, reduce CPU overhead

30 Performance (Cont.) Adding instructions to the execution path to save one disk I/O is reasonable Intel Core i7 Extreme Edition 990x (2011) at 3.46Ghz = 159,000 MIPS Typical disk drive at 250 I/Os per second 159,000 MIPS / 250 = 630 million instructions during one disk I/O Fast SSD drives provide 60,000 IOPS 159,000 MIPS / 60,000 = 2.65 millions instructions during one disk I/O

31 Free-Space Management Bit vector (n blocks) n-1 bit[i] = 1 block[i] free 0 block[i] occupied Block number calculation (number of bits per word) * (number of 0-value words) + offset of first 1 bit

32 Free-Space Management (Cont.) Bit map requires extra space Example: block size = 2 12 bytes disk size = 2 40 bytes (1 terabyte) n = 2 40 /2 12 = 2 28 bits (or 256 MB) if clusters of 4 blocks -> 64MB of memory Easy to get contiguous files Linked list (free list) Cannot get contiguous space easily No waste of space No need to traverse the entire list (if # free blocks recorded) Grouping Modified free-list (n free block a group) Counting (with a variable free-contiguous-block length) Free space list then has entries containing addresses and counts

33 Free-Space Management (Cont.) Space Maps Used in ZFS Consider meta-data I/O on very large file systems Full data structures like bit maps couldn t fit in memory -> thousands of I/Os Divides device space into metaslab units and manages metaslabs Given volume can contain hundreds of metaslabs Each metaslab has associated space map Uses counting algorithm Metaslab activity -> load space map into memory in balanced-tree structure Combine contiguous free blocks into single entry

34 Linked Free Space List on Disk

35 Efficiency and Performance Efficiency dependent on: disk allocation and directory algorithms types of data kept in file s directory entry Pre-allocation or as-needed allocation of metadata structures Fixed-size or varying-size data structures Performance Keeping data and metadata close together Buffer cache separate section of main memory for frequently used blocks free-behind and read-ahead techniques to optimize sequential access improve PC performance by dedicating section of memory as virtual disk, or RAM disk

36 Page Cache A page cache caches pages rather than disk blocks using virtual memory techniques Memory-mapped I/O uses a page cache Routine I/O through the file system uses the buffer (disk) cache This leads to the following figure

37 I/O Without a Unified Buffer Cache

38 Unified Buffer Cache A unified buffer cache uses the same page cache to cache both memory-mapped pages and ordinary file system I/O

39 I/O Using a Unified Buffer Cache

40 Recovery Consistency checking compares data in directory structure with data blocks on disk, and tries to fix inconsistencies Use system programs to back up data from disk to another storage device (floppy disk, magnetic tape, other magnetic disk, optical) Recover lost file or disk by restoring data from backup

41 Log Structured File Systems Log structured (or journaling) file systems record each update to the file system as a transaction All transactions are written to a log A transaction is considered committed once it is written to the log However, the file system may not yet be updated The transactions in the log are asynchronously written to the file system When the file system is modified, the transaction is removed from the log If the file system crashes, all remaining transactions in the log must still be performed

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