2 Systems Architecture, Fifth Edition Chapter Goals Describe the components and functions of a file management system Compare the logical and physical

2Systems Architecture, Fifth Edition

Chapter Goals

• Describe the components and functions of a file management system

• Compare the logical and physical organization of files and directories

• Explain how secondary storage locations are allocated to files and describe the data structures used to record those allocations


Chapter Goals (continued)

• Describe file manipulation operations, including open, close, read, delete, and undelete operations

• List access controls that can be applied to files and directories

• Describe security, backup, recovery, and fault tolerance methods and procedures

• Compare and contrast storage area networks and network-attached storage


File Management Systems (FMS)

• Collection of system software that manages all aspects of user and program access to secondary storage

• Usually part of the operating system• Translates operations into commands to physical

storage devices• Implemented in four layers (command layer, file

control, storage I/O control, and secondary storage devices)


Bridges between logical and physical views of secondary storage

Allocates secondary storage locations to individual files and directories

Includes software modules for device drivers for each storage device or device controller, interrupt handlers, buffers and cache managers


Logical and PhysicalStorage Views

• Logical view– Collection of files organized within directories and

storage volumes

• Physical view– Collection of physical storage locations organized

as a linear address space



The file is subdivided into multiple records and each record is composed of multiple fields.

Logical File Structure

• Is independent of the physical device on which it is stored

• Also includes physical data access considerations, such as sequential access, indexed access, or some other

• There are also data encoding issues including the data structures and coding methods used to represent individual fields

• And don’t forget encryption and compression9Systems Architecture, Fifth Edition


File Content and Type

• FMS supports limited number of file types:– Executable programs (.exe and .dll)

– Operating system commands (.bat)

– Textual or unformatted binary data (.txt)

• Modern FMSs can define new file types and install utility programs to manipulate them (file association)


File Types

• Normally declared when a file is created and:– Stored within a directory, or

– Declared through a filename convention

• Determine:– Physical organization of data items and data

structures within secondary storage

– Operations that may be performed upon the file

– Filename restrictions




Directory Content and Structure

• Contains information about files and other directories, typically name, file type, location, size, ownership, access controls, and time stamps

• Old MS-DOS used 8.3 naming convention• Newer NTFS allows 255 Unicode characters• Access controls include List, Read, Modify,

Change, Edit, Delete, etc.• Time stamps include file creation date, last

modified date, when file last read, …


Hierarchical Directory Structure

• Directories can contain other directories, creating a tree structure, but cannot be contained within more than one parent

• Ways that names of access paths can be specified:– Complete path (fully qualified reference)

– Relative path

• Each storage device has a root directory


Active (working) directory


Graph Directory Structure

• More flexible than hierarchical directory structure– Files and subdirectories can be contained within

multiple directories

– Directory links can form a cycle



Storage Allocation

• Secondary storage devices– The data structures and procedures used to manage

secondary storage devices is similar to memory management, except:

– Large number of storage locations; low frequency of allocation changes

• Secondary storage devices are divided into allocation units


Allocation Units

• Smallest number of secondary storage bytes that can be allocated to a file; cannot be smaller than unit of data transfer between storage device and controller (block); often smallest storage size is usually a multiple of block transfer size

• Assigned/reclaimed by FMS as files and directories are created or expanded/shrink or are deleted

• Size difficult to change once set


Allocation Unit Size

• Tradeoffs– Efficient use of secondary storage space for files

– Size of storage allocation data structures

– Efficiency of storage allocation procedures

• Benefit of smaller units:– More efficient use of storage space

• Benefit of larger units:– Allow smaller storage allocation data structures


Storage Allocation Tables

• Data structures that record which allocation units are free and which belong to files

• Format and content vary across FMSs• Can contain linked lists in simpler FMSs or

indices or other complex data structures in more complex FMSs

• Examples include bitmaps, tables, linked lists, B+ trees



Free allocation units are assigned to a hidden system file called SysFree.


All of a file allocation’s units are “chained” together in sequential order by a series of pointers. This is close to an example of a bitmap, such as in FAT.

B+ Tree (NTFS)


100

<100 >=100

30 125 155 181

3 5 10 30 38 101 109 120

etc.

128 133 140


Blocking

• Logical record grouping within physical records• Described by a numeric ratio of logical records to

physical records (blocking factor)


Blocking factor = 4:3

Blocking factor = 2:3

Usually the logical number is a wholemultiple of the physical number.


Buffering

• Temporary storage of data as it moves between programs and secondary storage devices– Physical records are stored in the buffer as they are

read from secondary storage

– FMS extracts logical records from buffers and copies them to data area of the application program

• Each buffer is the size of one allocation unit• Improves I/O performance if enough are used



Computing Record Location

• For a record r, we must compute where it is in the file.– Which allocation records (physical record) do we

need?

– What is offset within that physical record?

sizeunit Allocation

size Record 1) -number (Record offsetunit Allocation

Example

• We want to load File3 (from Table 12-1). Allocation units (block size) are 512 bytes; Record size = 55 bytes

• Table 12-1 tells us that the first allocation unit is unit 5

• System loads 512 bytes into buffer and moves first 55 bytes to application program


Example Continued

• Now we want the second record. Where is it?• (Pointer – 1) x RecordSize / BlockSize• = (2-1) x 55 / 512 = 0 remainder 55• So second record is still in this first block but

starting at byte 55 (which is in the buffer)• What if we want the 37th record?• (37-1) x 55 / 512 = 3 remainder 444 (typo in book)


Example Continued

• So since the answer is 3 remainder 444, you need to get the 4th block (because blocks are numbered 0, 1, 2, 3, …) and then start reading from the 444th byte

• Using Table 12-2, the 4th block is block number 19



File Manipulation

• Exact set of service layer functions varies among FMSs, but typically includes create, copy, move, delete, read, and write

• Application programs interact directly with FMS through OS service layer

• Users interact indirectly with FMS through command layer


File Open and Close Operations

• File open– Causes FMS to find the file, verify access

privileges, allocate buffers, and update internal table of open files

• File close– Causes FMS to flush buffer content to the storage

device, release buffers, update file time stamps, and update table of open files


Delete and Undelete Operations

• Delete– Does not immediately remove files; some content

remains on secondary storage unit all allocation units have been reassigned and overwritten

– File content can be visible to intruders

• Undelete– Can be used to reconstruct directory and storage

allocation table contents


Access Controls

• Granted by file owners and system administrators for reading, writing, and executing files

• Provide security at the expense of additional FMS overhead


File Migration, Backup, and Recovery

• Provided by most FMSs to protect files against damage or loss


File Migration(Version Control)

• Automatic storage and backup of old file versions • Balances storage cost of each file version with

anticipated user demand for that version


Original

Copy that has been updated to reflect new data


File Backup

• Protects against data loss (file content, directory content, and storage allocation tables)

• Store backup copies on a different storage device in a different physical location

• Manual or automatic• Full or incremental


Transaction Logging

• Automatically records all changes to file content and attributes in a separate storage area; also writes them to the file’s I/O buffer

• Provides high degree of protection against data loss due to program or hardware failure

• Imposes a performance penalty; used only when costs of data loss are high


File Recovery

• Automated and manual components• Can search backup logs for copies of lost or

damaged files• Can perform consistency checking and repair

procedures for crashed system or physically damaged storage device

Disk Scheduling

• A disk drive has multiple requests for read or write operations

• How does the disk scheduler schedule the read/write operations?

• Various techniques include: FCFS (First Come First Served), SSTF (Shortest Seek Time First), Scan, and Look


Disk Scheduling

• Assume the following disk sector requests:98, 183, 37, 122, 14, 124, 65, 57 Head is at 53

FCFS

SSTF

Scan

C-Scan

Look

C-Look



Fault Tolerance

• Methods of securing file content against hardware failure– File backup

– Recovery

– Transaction logging

– Mirroring

– RAID (Redundant Array of Inexpensive Disks)


Mirroring

• All disk write operations are made concurrently to two different storage devices

• Provides high degree of protection against data loss with no performance penalty if implemented in hardware

• Disadvantages– Cost of redundant disk drives

– Higher cost of disk controllers that implement mirroring


RAID

• Disk storage technique that improves performance and fault tolerance

• All levels except RAID 1 use data striping– Breaks a unit of data into smaller segments and

stores them on multiple disks

• Multiple levels can be layered to combine their best features (e.g. RAID 10)

• Can be implemented in hardware or software


Storage Consolidation

• Overcomes inefficiencies of direct-attached storage (DAS) in multiple-server environments

• Common approaches– Storage area network (SAN) - an architecture to

attach remote computer storage devices to servers such that the devices appear as locally connected

– Network-attached storage (NAS) – in contrast to SAN, uses file-based protocols such as NFS where it is clear the storage is remote


Storage ConsolidationStorage Area Network (SAN) Network-Attached Storage (NAS)

• High-speed interconnection among general-purpose servers and one or more storage servers

• Block-oriented access

• Common in multi-server environments with mainframes or supercomputers (large enterprises) and substantial overlap among server storage needs

• Expensive to purchase and administer, but avoid costs of duplicate storage and storage administration

• Dedicated to managing one or more file systems

• Accessed by other servers and clients over a local or wide area network

• File-oriented access (rather than block-oriented access)

• Common when geographically dispersed servers need access to a common file system

• Cheaper to acquire than SAN, but at the price of lower performance




Storage Consolidation

• Most systems use SCSI protocol to connect storage devices to servers

• To allow SAN extension over IP networks, many use iSCSI or Fibre Channel over IP

• SANs often use a Fibre Channel fabric technology


Fibre Channel

• Three major Fibre Channel architectures– Point to point

– Arbitrated loop (similar to Token Ring)

– Switched fabric (similar to Ethernet but with Fibre Channel switches instead of LAN switches)

• Can operate over twisted pair and fiber• Throughput speeds from 100 Mbps to 2 Gbps


Fibre Channel

• Five layers– FC0 – physical layer

– FC1 – data link (8B/10B)

– FC2 – network layer (core FC protocols)

– FC3 – common services layer (such as RAID)

– FC4 – protocol mapping layer (encapsulates SCSI)

Example File Systems

• FAT (File Allocation Table)• FAT16• FAT32 (up through Windows Me)• NTFS (New Technology File System, beginning

with Windows NT and continuing with Windows 2000, XP, 2003, 2008, and Vista)

• UNIX file system• HFS (Hierarchical File System) (MACs and

possible UNIX)57Systems Architecture, Fifth Edition

NTFS

• Supports– High-speed directory and file operations

– Ability to handle large disks, files, directories

– Secure file and disk content

– Reliability and fault tolerance

• NTFS organizes secondary storage as a set of volumes

• A volume is a partition on a disk


NTFS

• Volumes contain a collection of storage allocation units called clusters

• Cluster size can be 512, 1024, 2048, or 4096 bytes• Cluster size is set up during installation• Each cluster is identified by a 64-bit logical

cluster number (LCN)• A volume can be as large as 4096 x 264 bytes


NTFS

• A volume’s master directory is stored in a data structure called the master file table (MFT)

• MFT contains a sequential set of file records, one for each file on the volume

• All volume contents are stored as files, including user files, the MFT itself, and other volume management files such as the root directory, storage allocation table, bootstrap program, and bad cluster table


NTFS

• The first 16 MFT entries are reserved for the MFT• All subsequent entries numbered 16 and higher

store records about user files• A file is an object with a collection of attributes

such as name, restrictions, security descriptors, and the data itself (essentially a large attribute)

• Directory contents stored using B+ tree• See Burd pages 495-497 for further details



Summary

• File management systems• Directory content and structure• Storage allocation• File manipulation• Access controls• File migration, backup, and recovery• Storage consolidation

Documents

2 Systems Architecture, Fifth Edition Chapter Goals Describe the components and functions of a file management system Compare the logical and physical