Download Lecture #19: Storage Management

Lecture 19
Ch 11: File System Implementation
Ch. 12: Mass Storage Structure
Modified from Silberschatz, Galvin and Gagne
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
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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
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Linked Free Space List on Disk
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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

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Efficiency and Performance
 Efficiency dependent on:

disk allocation and directory algorithms

types of data kept in file’s directory entry
 Performance

disk cache


free-behind and read-ahead


separate section of main memory for frequently used blocks
techniques to optimize sequential access
improve PC performance by dedicating section of memory as virtual
disk, or RAM disk
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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
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Unified Buffer Cache
 A unified buffer cache uses the same page cache to cache both memory-
mapped pages and ordinary file system I/O
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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
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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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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
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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
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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
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Three Independent File Systems
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Mounting in NFS
Mounts
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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
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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

NFS V4 is just coming available – stateful
 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
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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
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Schematic View of NFS Architecture
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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
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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
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Chapter 12: Mass-Storage Systems
 Overview of Mass Storage Structure
 Disk Structure
 Disk Attachment
 Disk Scheduling
 Disk Management
 Swap-Space Management
 RAID Structure
 Disk Attachment
 Stable-Storage Implementation
 Tertiary Storage Devices
 Operating System Issues
 Performance Issues
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Objectives
 Describe the physical structure of secondary and tertiary storage devices

and the resulting effects on the uses of the devices
 Explain the performance characteristics of mass-storage devices
 Discuss operating-system services provided for mass storage

including RAID and HSM
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Overview of Mass Storage Structure

Magnetic disks

provide bulk of secondary storage of modern computers

Drives rotate at 60 to 200 times per second

Transfer rate is rate at which data flow between drive and computer

Positioning time (random-access time) is time to move disk arm to desired
cylinder (seek time) and time for desired sector to rotate under the disk head
(rotational latency)

Head crash results from disk head making contact with the disk surface

That’s bad

Disks can be removable

Drive attached to computer via I/O bus

Busses vary, including EIDE, ATA, SATA, USB, Fibre Channel, SCSI

Host controller in computer uses bus to talk to disk controller built into drive or
storage array
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Moving-head Disk Mechanism
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Overview of Mass Storage Structure (Cont.)
 Magnetic tape

Was early secondary-storage medium

Relatively permanent and holds large quantities of data

Access time slow

Random access ~1000 times slower than disk

Mainly used for backup, storage of infrequently-used data, transfer
medium between systems

Kept in spool and wound or rewound past read-write head

Once data under head, transfer rates comparable to disk

20-200GB typical storage

Common technologies are 4mm, 8mm, 19mm, LTO-2 and SDLT
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Disk Structure
 Disk drives are addressed as large 1-dimensional arrays of logical blocks

the logical block is the smallest unit of transfer
 The 1-dimensional array of logical blocks is mapped into the sectors of the
disk sequentially

Sector 0 is the first sector of the first track on the outermost cylinder

Mapping proceeds in order through that track,

then the rest of the tracks in that cylinder, and

then through the rest of the cylinders from outermost to innermost.
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Disk Attachment
 Host-attached storage accessed through I/O ports talking to I/O busses
 SCSI itself is a bus, up to 16 devices on one cable,

SCSI initiator requests operation and SCSI targets perform tasks

Each target can have up to 8 logical units (disks attached to device
controller
 Fiber Channel is high-speed serial architecture

Can be switched fabric with 24-bit address space


the basis of storage area networks (SANs) in which many hosts
attach to many storage units
Can be arbitrated loop (FC-AL) of 126 devices
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Network-Attached Storage
 Network-attached storage (NAS) is storage made available over a network
rather than over a local connection (such as a bus)
 NFS and CIFS are common protocols
 Implemented via remote procedure calls (RPCs) between host and storage
 New iSCSI protocol uses IP network to carry the SCSI protocol
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Storage Area Network
 Common in large storage environments (and becoming more common)
 Multiple hosts attached to multiple storage arrays - flexible
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Disk Scheduling
 The operating system is responsible for using hardware efficiently

for the disk drives, this means having a fast access time and disk
bandwidth.
 Access time has two major components

Seek time is the time for the disk are to move the heads to the cylinder
containing the desired sector.

Rotational latency is the additional time waiting for the disk to rotate
the desired sector to the disk head.
 Minimize seek time
 Seek time  seek distance
 Disk bandwidth is the total number of bytes transferred, divided by the
total time between the first request for service and the completion of the
last transfer.
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Disk Scheduling (Cont.)
 Several algorithms exist to schedule the servicing of disk I/O requests.
 We illustrate them with a request queue (0-199).
98, 183, 37, 122, 14, 124, 65, 67
Head pointer 53
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FCFS
Illustration shows total head movement of 640 cylinders.
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SSTF
 Selects the request with the minimum seek time from the current head
position.
 SSTF scheduling is a form of SJF scheduling;

may cause starvation of some requests.
 Illustration shows total head movement of 236 cylinders.
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SCAN
 The disk arm starts at one end of the disk, and moves toward the other end,
servicing requests until it gets to the other end of the disk, where the head
movement is reversed and servicing continues.
 Sometimes called the elevator algorithm.
 Illustration shows total head movement of 208 cylinders.
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C-SCAN
 Provides a more uniform wait time than SCAN.
 The head moves from one end of the disk to the other. servicing requests
as it goes. When it reaches the other end, however, it immediately returns
to the beginning of the disk, without servicing any requests on the return
trip.
 Treats the cylinders as a circular list that wraps around from the last cylinder
to the first one.
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C-LOOK
 Version of C-SCAN
 Arm only goes as far as the last request in each direction, then reverses
direction immediately, without first going all the way to the end of the disk.
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Selecting a Disk-Scheduling Algorithm
 SSTF is common and has a natural appeal
 SCAN and C-SCAN perform better for systems that place a heavy load on
the disk.
 Performance depends on the number and types of requests.
 Requests for disk service can be influenced by the file-allocation method.
 The disk-scheduling algorithm should be written as a separate module of
the operating system, allowing it to be replaced with a different algorithm if
necessary.
 Either SSTF or LOOK is a reasonable choice for the default algorithm.
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