Bài giảng Nguyên lý hệ điều hành - Nguyễn Hải Châu - Tuần 9: Cài đặt hệ thống tệp

hatz, Galvin and Gagne ©12.21Operating System Concepts
Indexed Allocation – Mapping (Cont.)
„ Mapping from logical to physical in a file of unbounded 
length (block size of 512 words).
„ Linked scheme – Link blocks of index table (no limit on 
size).
LA / (512 x 511)
Q1
R1
Q1 = block of index table
R1 is used as follows:
R1 / 512
Q2
R2
Q2 = displacement into block of index table
R2 displacement into block of file:
Silberschatz, Galvin and Gagne 12.22Operating System Concepts
Indexed Allocation – Mapping (Cont.)
„ Two-level index (maximum file size is 5123)
LA / (512 x 512)
Q1
R1
Q1 = displacement into outer-index
R1 is used as follows:
R1 / 512
Q2
R2
Q2 = displacement into block of index table
R2 displacement into block of file:
Silberschatz, Galvin and Gagne ©200212.23Operating System Concepts
Indexed Allocation – Mapping (Cont.)
#
outer-index
index table file
Silberschatz, Galvin and Gagne ©200212.24Operating System Concepts
Combined Scheme: UNIX (4K bytes per block)
55
Silberschatz, Galvin and Gagne ©200212.25Operating System Concepts
Free-Space Management
„ Bit vector (n blocks)
…
0 1 2 n-1
bit[i] =

 0 ⇒ block[i] free
1 ⇒ block[i] occupied
Block number calculation
(number of bits per word) *
(number of 0-value words) +
offset of first 1 bit
Silberschatz, Galvin and Gagne ©200212.26Operating System Concepts
Free-Space Management (Cont.)
„ Bit map requires extra space. Example:
block size = 212 bytes
disk size = 230 bytes (1 gigabyte)
n = 230/212 = 218 bits (or 32K bytes)
„ Easy to get contiguous files 
„ Linked list (free list)
) Cannot get contiguous space easily
) No waste of space
„ Grouping 
„ Counting
Silberschatz, Galvin and Gagne ©200212.27Operating System Concepts
Free-Space Management (Cont.)
„ Need to protect:
) Pointer to free list
) Bit map
Must be kept on disk
Copy in memory and disk may differ.
Cannot allow for block[i] to have a situation where bit[i] = 
1 in memory and bit[i] = 0 on disk.
) Solution:
 Set bit[i] = 1 in disk.
 Allocate block[i]
 Set bit[i] = 1 in memory
Silberschatz, Galvin and Gagne 12.28Operating System Concepts
Linked Free Space List on Disk
Silberschatz, Galvin and Gagne ©12.29Operating System Concepts
Efficiency and Performance
„ Efficiency dependent on:
) disk allocation and directory algorithms
) types of data kept in file’s directory entry
„ Performance
) disk 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.
Silberschatz, Galvin and Gagne 12.30Operating System Concepts
Various Disk-Caching Locations
66
Silberschatz, Galvin and Gagne ©200212.31Operating System Concepts
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.
Silberschatz, Galvin and Gagne ©12.32Operating System Concepts
I/O Without a Unified Buffer Cache
Silberschatz, Galvin and Gagne ©200212.33Operating System Concepts
Unified Buffer Cache
„ A unified buffer cache uses the same page cache to 
cache both memory-mapped pages and ordinary file 
system I/O.
Silberschatz, Galvin and Gagne 12.34Operating System Concepts
I/O Using a Unified Buffer Cache
Silberschatz, Galvin and Gagne ©12.35Operating System Concepts
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).
„ Recover lost file or disk by restoring data from backup.
Silberschatz, Galvin and Gagne ©200212.36Operating System Concepts
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.
77
Silberschatz, Galvin and Gagne ©12.37Operating System Concepts
The Sun Network File System (NFS)
„ An implementation and a specification of a software 
system for accessing remote files across LANs (or 
WANs).
„ The implementation is part of the Solaris and SunOS 
operating systems running on Sun workstations using an 
unreliable datagram protocol (UDP/IP protocol and 
Ethernet. 
Silberschatz, Galvin and Gagne 12.38Operating System Concepts
NFS (Cont.)
„ Interconnected workstations viewed as a set of 
independent machines with independent file systems, 
which allows sharing among these file systems in a 
transparent manner.
) A remote directory is mounted over a local file system 
directory. The mounted directory looks like an integral 
subtree of the local file system, replacing the subtree 
descending from the local directory.
) Specification of the remote directory for the mount operation 
is nontransparent; the host name of the remote directory 
has to be provided. Files in the remote directory can then 
be accessed in a transparent manner.
) Subject to access-rights accreditation, potentially any file 
system (or directory within a file system), can be mounted 
remotely on top of any local directory. 
Silberschatz, Galvin and Gagne ©200212.39Operating System Concepts
NFS (Cont.)
„ NFS is designed to operate in a heterogeneous 
environment of different machines, operating systems, 
and network architectures; the NFS specifications 
independent of these media. 
„ This independence is achieved through the use of RPC 
primitives built on top of an External Data Representation 
(XDR) protocol used between two implementation-
independent interfaces.
„ The NFS specification distinguishes between the services 
provided by a mount mechanism and the actual remote-
file-access services. 
Silberschatz, Galvin and Gagne 12.40Operating System Concepts
Three Independent File Systems
Silberschatz, Galvin and Gagne ©12.41Operating System Concepts
Mounting in NFS 
Mounts Cascading mounts
Silberschatz, Galvin and Gagne ©200212.42Operating System Concepts
NFS Mount Protocol
Establishes initial logical connection between server and 
client.
„ Mount operation includes name of remote directory to be 
mounted and name of server machine storing it. 
) Mount request is mapped to corresponding RPC and forwarded 
to mount server running on server machine. 
) Export list – specifies local file systems that server exports for 
mounting, along with names of machines that are permitted to 
mount them. 
„ Following a mount request that conforms to its export list, 
the server returns a file handle—a key for further accesses.
„ File handle – a file-system identifier, and an inode number to 
identify the mounted directory within the exported file 
system.
„ The mount operation changes only the user’s view and does 
not affect the server side. 
88
Silberschatz, Galvin and Gagne ©12.43Operating System Concepts
NFS Protocol
„ Provides a set of remote procedure calls for remote file 
operations. The procedures support the following operations:
) searching for a file within a directory 
) reading a set of directory entries 
) manipulating links and directories 
) accessing file attributes
) reading and writing files
„ NFS servers are stateless; each request has to provide a full set 
of arguments.
„ Modified data must be committed to the server’s disk before 
results are returned to the client (lose advantages of caching).
„ The NFS protocol does not provide concurrency-control 
mechanisms.
Silberschatz, Galvin and Gagne 12.44Operating System Concepts
Three Major Layers of NFS Architecture 
„ UNIX file-system interface (based on the open, read, 
write, and close calls, and file descriptors).
„ Virtual File System (VFS) layer – distinguishes local files 
from remote ones, and local files are further distinguished 
according to their file-system types.
) The VFS activates file-system-specific operations to handle 
local requests according to their file-system types. 
) Calls the NFS protocol procedures for remote requests.
„ NFS service layer – bottom layer of the architecture; 
implements the NFS protocol.
Silberschatz, Galvin and Gagne ©200212.45Operating System Concepts
Schematic View of NFS Architecture 
Silberschatz, Galvin and Gagne 12.46Operating System Concepts
NFS Path-Name Translation
„ Performed by breaking the path into component names 
and performing a separate NFS lookup call for every pair 
of component name and directory vnode.
„ To make lookup faster, a directory name lookup cache on 
the client’s side holds the vnodes for remote directory 
names. 
Silberschatz, Galvin and Gagne 12.47Operating System Concepts
NFS Remote Operations
„ Nearly one-to-one correspondence between regular UNIX 
system calls and the NFS protocol RPCs (except opening and 
closing files).
„ NFS adheres to the remote-service paradigm, but employs 
buffering and caching techniques for the sake of performance. 
„ File-blocks cache – when a file is opened, the kernel checks 
with the remote server whether to fetch or revalidate the cached
attributes. Cached file blocks are used only if the corresponding 
cached attributes are up to date.
„ File-attribute cache – the attribute cache is updated whenever 
new attributes arrive from the server.
„ Clients do not free delayed-write blocks until the server confirms 
that the data have been written to disk.