1. COT 4600 Operating Systems Spring 2011
Dan C. Marinescu
Office: HEC 304
Office hours: Tu-Th 5:00 – 6:00 PM

2. Lecture 14 – Tuesday, March 15, 2011
Last time:
Midterm
Today
Solution of the midterm problems and midterm discussion
Next time
Virtualization
Locks
Lecture 14

3. Problem 1 Naming
Conceptual/abstract model: a description which retains the most important characteristics of the process/object in a given context.
The model of an airplane wing
The abstraction of an interpreter
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4. Example 1- abstract model of an interpreter – Figure 2.5
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5. Naming MODEL – (see Lecture 8)
Naming allows objects
to refer to each other
to locate another objects
to determine the properties of other objects
Naming model  a general scheme to resolve a name which can be applied for a spectrum of objects and implementations; examples
Searching for an address in phone book
Compiling a program which uses a library
Identifying the registers used during execution of a compiled program
Four components:
Name space (the universe of names)
Mapping function
The universe of values
Context
A function which has as input: a name and a context and produces a value
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6. Example 2: Abstract model of naming: Figure 2.10
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7. The role of the context for modular sharing
Modular sharing: allows one module to use another module developed independently, without the danger of name collision (page 117 of the text book)
All names encountered during the name resolution for module A are resolved using the context of A; in other words trhe context of module A points to all modules used by A (including A);
The context allows modular sharing; if both modules A and B use a function called W, but the two functions are different (call them WA and WB for convenience) then the context of A will point to the function WA and the context of B will point to WB
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8. Example: context used for modular sharing, Figure 3.5
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9. Lecture 14

10. Problem 2 – UNIX file system
Lecture 8
Basic concepts related to file handling
How UNIX users access a file – API
Internal implementation of file operations for a generic system
How to map logical to physical organization
UNIX
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11. B. The software layer: the file abstraction
File: memory abstraction used by the application and OS layers
linear array of bits/bytes
properties:
durable  information will not be changed in time
has a name 
allows access to individual bits/bytes  has a cursor which defines the current position in the file.
The OS provides an API (Application Programming Interface) supporting a range of file manipulation operations.
A user must first OPEN a file before accessing it and CLOSE it after it has finished with it. This strategy:
allows different access rights (READ, WRITE, READ-WRITE)
coordinate concurrent access to the file
Some file systems
use OPEN and CLOSE to enforce before-or-after atomicity
support all-or-nothing atomicity  e.g., ensure that if the system crashes before a CLOSE either all or none of WRITEs are carried out
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12. API for the Unix File System
OPEN(name, flags, model)  connect to a file
Open an existing file called name, or
Create a new file with permissions set to mode if flags is set.
Set the file pointer (cursor) to 0.
Return the file descriptor (fd).
CLOSE(fd)  disconnect from a file
Delete file descriptor fd.
READ(fd, buf,n)  read from file
Read n bytes from file fd into buf; start at the current cursor position and update the file cursor (cursor = cursor + n).
WRITE(fd, buf,n)  write to file
Write n bytes to the file fd from buf; start at the current cursor position and update the file cursor (cursor = cursor + n).
SEEK(fd, offset,whence)  move cursor of file
Set the cursor position of file fd to offset from the position specified by whence (beginning, end, current position)
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13. API for the Unix File System (cont’d)
FSYNC(fd)  make all changes to file fd durable.
STAT(name) read metadata
CHMOD, CHOWN  change access mode/ownership
RENAME(from_name,to_name)  change file name
LINK(name, link_name) create a hard link
UNLINK(name) remove name from directory
SYMLINK(name, link_name) create a symbolic link
MKDIR(name) create directory name
RMDIR(name) delete directory name
CHDIR(name)  change current directory to name
CHROOT  Change the default root directory name
MOUNT(name,device)  mount the file system name onto device
UNMOUNT(name)  unmount file system name
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14. Figure 2.20 and 2.22
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15. Generic OPEN and READ operations (Lecture 8)

16. Logical and physical organization
Logical/physical records:
a file consists of a set of logical records
a persistent storage media (e.g., disk) is organized as a sequence of blocks/physical records.
Hierarchical logical organization
Records
File
Directory
File system
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17. Lecture 14

18. How to map logical to physical organization
Define a control structure which holds information about a file a directory, or a file system. For a file this information includes:
Creation date
Owner
Size
Access rights (read, write, execute)
The physical location of the file
Store this meta information on the persistent storage (disk).
In UNIX
call such a control structure – inode
an inode describes a file, a directory, or a file system
The inode table stores the block where the inodes for directories are located
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19. Lecture 14

20. How to access the inode for a file in UNIX
Given a path name the following steps are taken
Go to Block0 to locate the physical bloc where the inode table is stored
Go to the inode table to locate the inode for the root file system
Search the inode of the root file system for the inode of the next directory in the path for the file name.
Reccursively search until you locate the inode of the directory containing the file.
Identify the inode for the file.
Note: this process is conducted when an OPEN file name is issued; then the control information from the inode is transferred to a control block in main memory.
A READ operation does not need to access the inode table. It
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21. Unix OPEN and READ
If the file exists then the OPEN consists of the following steps
Locate the inode of the file using the procedure described earlier
Check if the operation is allowed
If the operation is allowed then transfer the information from the inode to the open file table for the process
If the file does not exists then the OPEN creates the inode and adds the file to the table of open files.
The READ only accesses the table of open files in main memory
Uses the file descriptor to locate the file entry in the table of open files
Determines the position of the cursor
Maps the cursor to a disk block
Transfers the data a from the block where the cursor is pointing at
Continues transferring blocks from the disk to the memory buffer until the entire length of the data is covered.
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22. Lecture 14

23. Problem 3  NFS (see Lecture 13)
Developed at Sun Microsystems in early to early 1980s.
Application of the client-server paradigm.
Objectives:
Design a shared file system to support collaborative work
Simplify the management of a set of workstations
Facilitate the backups
Uniform, administrative policies
Main design goals
Compatibility with existing applications  NFS should provide the same semantics as a local UNIX file system
Ease of deployment  NFS implementation should be easily ported to existing systems
Broad scope  NSF clients should be able to run under a variety of operating systems
Efficiency  the users of the systems should not notice a substantial performance degradation when accessing a remote file system relative to access to a local file system
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24. NFS clients and servers
Should provide transparent access to remote file systems.
It mounts a remote file system in the local name space  it perform a function analogous to the MOUNT UNIX call.
The remote file system is specified as Host/Path
Host  the host name of the host where the remote file system is located
Path  local path name on the remote host.
The NFS client sends to the NFS server an RPC with the file Path information and gets back from the server a file handle
A 32 bit name that uniquely identifies the remote object.
The server encodes in the file handle:
A file system identifier
An inode number
A generation number
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25. Lecture 14

26. Implementation
Vnode – a structure in volatile memory which abstracts if a file or directory is local or remote. A file system call (Open, Read, Write, Close, etc.) is done through the vnode-layer. Example:
To Open a file a client calls PATHNAME_TO_VNODE
The file name is parsed and a LOOKUP is generated
if the directory is local and the file is found the local file system creates a vnode for the file
else
the LOOKUP procedure implemented by the NFS client is invoked . The file handle of the directory and the path name are passed as arguments
The NFS client invokes the LOOKUP remote procedure on the sever via an RPC
The NFS server extracts the file system id and the inode number and then calls a LOOKUP in the vnode layer.
The vnode layer on the server side does LOOKUP on the local file system passing the path name as an argument.
If the local file system on the server locates the file it creates a vnode for it and returns the vnode to the NFS server.
The NFS server sends a reply to the RPC containing the file handle of the vnode and some metadata
The NFS client creates a vnode containing the file handle
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27. Lecture 14

28. Why file handles and not path names
Example
Program 1 on client Program 2 on client 2
CHDIR (‘dir1’)
fd  OPEN(“f”, READONLY)
RENAME(‘dir1’,’dir2)
RENAME(‘dir3’,’dir1’)
READ(fd,buf,n)
To follow the UNIX specification if both clients would be on the same system client1 would read from dir2.f. If the inode number allows the client 1 to follw the same semantics rather than read from dir1/f
Example
fd  OPEN(“file1”, READONLY)
UNLINK(“f”)
fd  OPEN(“f”,CREATE)
If the NFS server reuses the inode of the old file then the RPC from client 2 will read from the new file created by client 1. The generation number allows the NSF server to distinguish between the old file opened by client 2 and the new one created by client 1.
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29. Read/Write coherence
Enforcing Read/Write coherence is non-trivial even for a local operations.
For performance reasons a device driver may delay a Write operation issued by Client1; caching could cause problems when Client2 tries to Read the file.
Possible solutions
Close-to-Open consistency:
Client 1 executes the sequence : Open Write  Close before Client 2 executes the sequence : Open  Read  Close  Read/Write coherence is provided
Client 1 executes the sequence : Open Write before Client 2 executes the sequence : Open  Read . Read/Write coherence may or may not be provided
Consistency for every operation: no caching 
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30. Lecture 14

31. NFS Close-to-Open semantics
A client stores with each block in its cache the timestamp of the block’s vnode at the time the client got the block from the NFS server.
When a user program opens a file the client sends a GETATTR request to get the timestamp of the latest modification for the file.
The client gets a new copy only if the file has been modified since it has last accessed it; else it uses the local copy.
To implement a Write a client updates only its copy in cache without a an RPC Write.
At Close time the client sends the cached copy to the server
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