CH 14

Chapter 14: File System
Implementation
Operating System Concepts – 10h Edition Silberschatz, Galvin and Gagne ©2018
Outline
 File-System Structure
 File-System Operations
 Directory Implementation
 Allocation Methods
 Free-Space Management
 Efficiency and Performance
 Recovery
 Example: WAFL File System
Operating System Concepts – 10th Edition 14.2 Silberschatz, Galvin and Gagne ©2018
Objectives
 Describe the details of implementing local file systems and directory
structures
 Discuss block allocation and free-block algorithms and trade-offs
 Explore file system efficiency and performance issues
 Look at recovery from file system failures
 Describe the WAFL file system as a concrete example
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 (FCB) – storage structure consisting of information
about a file
 Device driver controls the physical device
 File system organized into layers
Layered File System
File System Layers
 Device drivers manage I/O devices at the I/O control layer
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 understands files, logical address, and physical
blocks
 Translates logical block # to physical block #
 Manages free space, disk allocation
File System Layers (Cont.)
 Logical file system manages metadata information
• Translates file name into file number, file handle, location by
maintaining file control blocks (inodes in UNIX)
• Directory management
• Protection
 Layering useful for reducing complexity and redundancy, but
adds overhead and can decrease performance
 Logical layers can be implemented by any coding method
according to OS designer
File System Layers (Cont.)
 Many file systems, sometimes many within an operating system
• Each with its own format:
• CD-ROM is ISO 9660;
• Unix has UFS, FFS;
• Windows has FAT, FAT32, NTFS as well as floppy, CD, DVD Blu-ray,
• Linux has more than 130 types, with extended file system ext3 and
ext4 leading; plus distributed file systems, etc.)
• New ones still arriving – ZFS, GoogleFS, Oracle ASM, FUSE
File-System Operations
 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 needed by system to boot OS from
that volume
• Needed if volume contains OS, usually first block of volume
 Volume control block (superblock, master file table) contains volume
details
• Total # of blocks, # of free blocks, block size, free block pointers or
array
 Directory structure organizes the files
• Names and inode numbers, master file table
File Control Block (FCB)
 OS maintains FCB per file, which contains many details about
the file
• Typically, inode number, permissions, size, dates
• Example
In-Memory File System Structures
 Mount table storing file system mounts, mount points, file
system types
 System-wide open-file table contains a copy of the FCB of
each file and other info
 Per-process open-file table contains pointers to appropriate
entries in system-wide open-file table as well as other info
In-Memory File System Structures (Cont.)
• Figure 12-3(a) refers to opening a file

• Figure 12-3(b) refers to reading a file
Directory Implementation
 Linear list of file names with pointer to the data blocks
• Simple to program
• Time-consuming to execute
 Linear search time
 Could keep ordered alphabetically via linked list or use
B+ tree
 Hash Table – linear list with hash data structure
• Decreases directory search time
• Collisions – situations where two file names hash to the
same location
• Only good if entries are fixed size, or use chained-overflow
method
Allocation Method
 An allocation method refers to how disk blocks are allocated
for files:
• Contiguous
• Linked
• File Allocation Table (FAT)
Contiguous Allocation Method
 An allocation method refers to how disk blocks are allocated
for files:
 Each file occupies set of contiguous blocks
• Best performance in most cases
• Simple – only starting location (block #) and length
(number of blocks) are required
• Problems include:
 Finding space on the disk for a file,
 Knowing file size,
 External fragmentation, need for compaction off-line
(downtime) or on-line
Contiguous Allocation (Cont.)
 Mapping from logical to physical
(block size =512 bytes)
LA/512
R
 Block to be accessed = starting
address + Q
 Displacement into block = R
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
Linked Allocation
 Each file is a linked list of blocks
 File ends at nil pointer
 No external fragmentation
 Each block contains pointer to next block
 No compaction, external fragmentation
 Free space management system called when new block needed
 Improve efficiency by clustering blocks into groups but increases
internal fragmentation
 Reliability can be a problem
 Locating a block can take many I/Os and disk seeks
Linked Allocation Example
 Each file is a linked list of disk blocks: blocks may be scattered
anywhere on the disk
 Scheme
Linked Allocation (Cont.)
 Mapping
Q
LA/511
R
 Block to be accessed is the Qth block in the linked chain of

blocks representing the file.
 Displacement into block = R + 1
File-Allocation Table
Indexed Allocation Method
 Each file has its own index block(s) of pointers to its data blocks
 Logical view
index table
Example of Indexed Allocation
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
 For NVM, no disk head so different algorithms and optimizations needed
• Using old algorithm uses many CPU cycles trying to avoid non-
existent head movement
• Goal is to reduce CPU cycles and overall path needed for I/O
Free-Space Management
 File system maintains free-space list to track available blocks/clusters
• (Using term “block” for simplicity)
 Bit vector or bit map (n blocks)
01 2 n-1
…
1  block[i] free

bit[i] =
0  block[i] occupied
Block number calculation

(number of bits per word) *
(number of 0-value words) +
offset of first 1 bit
CPUs have instructions to return offset within word of first “1” bit
Free-Space Management
 File system maintains free-space list to track available blocks
 Bit vector or bit map (n blocks)
01 2 n-1
…
1  block[i] free

bit[i] =
0  block[i] occupied
 Bit map requires extra space
• Example:
block size = 4KB = 212 bytes
disk size = 240 bytes (1 terabyte)
n = 240/212 = 228 bits (or 32MB)
if clusters of 4 blocks -> 8MB of memory
 Easy to get contiguous files
Linked Free Space List on Disk
 Linked list (free list)
• Cannot get contiguous
space easily
• No waste. Linked Free
Space List on Disk of
space
• No need to traverse the
entire list (if # free blocks
recorded)
Free-Space Management (Cont.)
 Grouping
• Modify linked list to store address of next n-1 free blocks in
first free block, plus a pointer to next block that contains free-
block-pointers (like this one)
 Counting
• Because space is frequently contiguously used and freed,
with contiguous-allocation allocation, extents, or clustering
 Keep address of first free block and count of following free
blocks
 Free space list then has entries containing addresses and
counts
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 cannot 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
• But records to log file rather than file system

 Log of all block activity, in time order, in counting format
• Metaslab activity  load space map into memory in balanced-tree

structure, indexed by offset
 Replay log into that structure
 Combine contiguous free blocks into single entry
TRIMing Unused Blocks
 HDDS overwrite in place so need only free list

 Blocks not treated specially when freed
• Keeps its data but without any file pointers to it, until overwritten
 Storage devices not allowing overwrite (like NVM) suffer badly with same
algorithm
• Must be erased before written, erases made in large chunks (blocks,
composed of pages) and are slow
• TRIM is a newer mechanism for the file system to inform the NVM
storage device that a page is free
 Can be garbage collected or if block is free, now block can be
erased
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
Efficiency and Performance (Cont.)
 Performance
• Keeping data and metadata close together
• Buffer cache – separate section of main memory for frequently
used blocks
• Synchronous writes sometimes requested by apps or needed by
OS
 No buffering / caching – writes must hit disk before
acknowledgement
 Asynchronous writes more common, buffer-able, faster
• Free-behind and read-ahead – techniques to optimize sequential
access
• Reads frequently slower than writes
Page Cache
 A page cache caches pages rather than disk blocks using
virtual memory techniques and addresses
 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
I/O Without a Unified Buffer Cache
Unified Buffer Cache
 A unified buffer cache uses the same page cache to cache
both memory-mapped pages and ordinary file system I/O to
avoid double caching
 But which caches get priority, and what replacement
algorithms to use?
I/O Using a Unified Buffer Cache
Recovery
 Consistency checking – compares data in directory structure
with data blocks on disk, and tries to fix inconsistencies
• Can be slow and sometimes fails
 Use system programs to back up data from disk to another
storage device (magnetic tape, other magnetic disk, optical)
 Recover lost file or disk by restoring data from backup
Log Structured File Systems
 Log structured (or journaling) file systems record each metadata 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
(sequentially)
• Sometimes to a separate device or section of disk
• However, the file system may not yet be updated
 The transactions in the log are asynchronously written to the file system
structures
• When the file system structures are modified, the transaction is
removed from the log
 If the file system crashes, all remaining transactions in the log must still be
performed
 Faster recovery from crash, removes chance of inconsistency of metadata
Example: WAFL File System
 Used on Network Appliance “Filers” – distributed file system appliances
 “Write-anywhere file layout”
 Serves up NFS, CIFS, http, ftp
 Random I/O optimized, write optimized
• NVRAM for write caching
 Similar to Berkeley Fast File System, with extensive modifications
The WAFL File Layout
Snapshots in WAFL
The Apple File System
End of Chapter 14
Linked Allocation
Linked Allocation
 Each file is a linked list of disk blocks: blocks may be scattered anywhere
on the disk
block = pointer
 Mapping
Q
LA/511
R
Block to be accessed is the Qth block in the linked chain of blocks
representing the file.
Displacement into block = R + 1
End of Chapter 14
In-Memory File System Structures
 Mount table storing file system mounts, mount points, file system types
 System-wide open-file table contains a copy of the FCB of each file and
other info
 Per-process open-file table contains pointers to appropriate entries in
system-wide open-file table as well as other info
 The following figure illustrates the necessary file system structures
provided by the operating systems
 Figure 12-3(a) refers to opening a file
 Figure 12-3(b) refers to reading a file
 Plus buffers hold data blocks from secondary storage
 Open returns a file handle for subsequent use
 Data from read eventually copied to specified user process memory
address

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Chapter 14: File System

• Figure 12-3(a) refers to opening a file

 Block to be accessed is the Qth block in the linked chain of

Block number calculation

 Easy to get contiguous files

• Each metaslab has associated space map

• But records to log file rather than file system

• Metaslab activity  load space map into memory in balanced-tree

 HDDS overwrite in place so need only free list

Displacement into block = R + 1

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