CS444/CS544 Operating Systems File Systems & Protection 4/20/2007 Prof. Searleman [email protected]
Transcript
Slide 1
CS444/CS544 Operating Systems File Systems & Protection
4/20/2007 Prof. Searleman [email protected]
Slide 2
Outline Secondary Storage Disk Scheduling SSTF, SCAN, C-SCAN,
LOOK, C-LOOK Disk Management formatting, partitioning, booting,
swap-space Storage Area Network Stable Storage Intro to Security
& Protection File Protection Read: Disk scheduling: Chapter 12,
sections 12.1 12.4 Chapters 14 & 15
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12.3 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts 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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12.4 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts SCAN (Cont.)
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12.5 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts 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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12.6 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts C-SCAN (Cont.)
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12.7 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts 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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12.8 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts C-LOOK (Cont.)
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12.9 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts 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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12.10 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts Disk Management Low-level formatting, or physical
formatting Dividing a disk into sectors that the disk controller
can read and write. To use a disk to hold files, the operating
system still needs to record its own data structures on the disk.
Partition the disk into one or more groups of cylinders. Logical
formatting or making a file system. Boot block initializes system.
The bootstrap is stored in ROM. Bootstrap loader program. Methods
such as sector sparing used to handle bad blocks.
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12.11 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts Swap-Space Management Swap-space Virtual memory uses disk
space as an extension of main memory. Swap-space can be carved out
of the normal file system,or, more commonly, it can be in a
separate disk partition. Swap-space management 4.3BSD allocates
swap space when process starts; holds text segment (the program)
and data segment. Kernel uses swap maps to track swap-space use.
Solaris 2 allocates swap space only when a page is forced out of
physical memory, not when the virtual memory page is first
created.
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12.12 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts 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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12.13 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts Storage Area Network Common in large storage environments
(and becoming more common) Multiple hosts attached to multiple
storage arrays - flexible
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12.14 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts RAID Structure RAID multiple disk drives provides
reliability via redundancy. RAID is arranged into six different
levels. Several improvements in disk-use techniques involve the use
of multiple disks working cooperatively. Disk striping uses a group
of disks as one storage unit. RAID schemes improve performance and
improve the reliability of the storage system by storing redundant
data. Mirroring or shadowing keeps duplicate of each disk. Block
interleaved parity uses much less redundancy.
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12.15 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts RAID Levels RAID (0 + 1) and (1 + 0)
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12.16 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts Stable-Storage Implementation Write-ahead log scheme
requires stable storage. To implement stable storage: Replicate
information on more than one nonvolatile storage media with
independent failure modes. Update information in a controlled
manner to ensure that we can recover the stable data after any
failure during data transfer or recovery.
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12.17 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts Tertiary Storage Devices Low cost is the defining
characteristic of tertiary storage. Generally, tertiary storage is
built using removable media Common examples of removable media are
floppy disks and CD-ROMs; other types are available.
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12.18 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts WORM Disks The data on read-write disks can be modified
over and over. WORM (Write Once, Read Many Times) disks can be
written only once. Thin aluminum film sandwiched between two glass
or plastic platters. To write a bit, the drive uses a laser light
to burn a small hole through the aluminum; information can be
destroyed by not altered. Very durable and reliable. Read Only
disks, such ad CD-ROM and DVD, come from the factory with the data
pre-recorded.
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12.19 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts Tapes Compared to a disk, a tape is less expensive and
holds more data, but random access is much slower. Tape is an
economical medium for purposes that do not require fast random
access, e.g., backup copies of disk data, holding huge volumes of
data. Large tape installations typically use robotic tape changers
that move tapes between tape drives and storage slots in a tape
library. stacker library that holds a few tapes silo library that
holds thousands of tapes A disk-resident file can be archived to
tape for low cost storage; the computer can stage it back into disk
storage for active use.
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12.20 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts Operating System Issues Major OS jobs are to manage
physical devices and to present a virtual machine abstraction to
applications For hard disks, the OS provides two abstraction: Raw
device an array of data blocks. File system the OS queues and
schedules the interleaved requests from several applications.
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12.21 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts Application Interface Most OSs handle removable disks
almost exactly like fixed disks a new cartridge is formatted and an
empty file system is generated on the disk. Tapes are presented as
a raw storage medium, i.e., and application does not not open a
file on the tape, it opens the whole tape drive as a raw device.
Usually the tape drive is reserved for the exclusive use of that
application. Since the OS does not provide file system services,
the application must decide how to use the array of blocks. Since
every application makes up its own rules for how to organize a
tape, a tape full of data can generally only be used by the program
that created it.
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12.22 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts File Naming The issue of naming files on removable media
is especially difficult when we want to write data on a removable
cartridge on one computer, and then use the cartridge in another
computer. Contemporary OSs generally leave the name space problem
unsolved for removable media, and depend on applications and users
to figure out how to access and interpret the data. Some kinds of
removable media (e.g., CDs) are so well standardized that all
computers use them the same way.
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12.23 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts Hierarchical Storage Management (HSM) A hierarchical
storage system extends the storage hierarchy beyond primary memory
and secondary storage to incorporate tertiary storage usually
implemented as a jukebox of tapes or removable disks. Usually
incorporate tertiary storage by extending the file system. Small
and frequently used files remain on disk. Large, old, inactive
files are archived to the jukebox. HSM is usually found in
supercomputing centers and other large installations that have
enormous volumes of data.
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12.24 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts Reliability A fixed disk drive is likely to be more
reliable than a removable disk or tape drive. An optical cartridge
is likely to be more reliable than a magnetic disk or tape. A head
crash in a fixed hard disk generally destroys the data, whereas the
failure of a tape drive or optical disk drive often leaves the data
cartridge unharmed.
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12.25 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts Cost Main memory is much more expensive than disk storage
The cost per megabyte of hard disk storage is competitive with
magnetic tape if only one tape is used per drive. The cheapest tape
drives and the cheapest disk drives have had about the same storage
capacity over the years. Tertiary storage gives a cost savings only
when the number of cartridges is considerably larger than the
number of drives.
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12.26 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts Price per Megabyte of DRAM, From 1981 to 2004
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12.27 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts Price per Megabyte of Magnetic Hard Disk, From 1981 to
2004
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12.28 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts Price per Megabyte of a Tape Drive, From 1984-2000
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Disk Performance Model /Trends Capacity + 100%/year (2X / 1.0
yrs) Transfer rate (BW) + 40%/year (2X / 2.0 yrs) Rotation + Seek
time 8%/year (1/2 in 10 yrs) MB/$ > 100%/year (2X /
Fallacy: Use Data Sheet Average Seek Time Manufacturers needed
standard for fair comparison (benchmark) Calculate all seeks from
all tracks, divide by number of seeks => average Real average
would be based on how data laid out on disk, where seek in real
applications, then measure performance Usually, tend to seek to
tracks nearby, not to random track Rule of Thumb: observed average
seek time is typically about 1/4 to 1/3 of quoted seek time (i.e.,
3X-4X faster) UltraStar 73 avg. seek: 4.9 ms 1.6 ms
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Fallacy: Use Data Sheet Transfer Rate Manufacturers quote the
speed of the data rate off the surface of the disk Sectors contain
an error detection and correction field (can be 20% of sector size)
plus sector number as well as data There are gaps between sectors
on track Rule of Thumb: disks deliver about 3/4 of internal media
rate (1.3X slower) for data For example, UlstraStar 73 quotes 49 to
87 MB/s internal media rate Expect 37 to 65 MB/s user data
rate
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Disk Performance Example One More Time Calculate time to read 1
sector for UltraStar 72 again, this time using 1/3 quoted seek
time, 3/4 of internal outer track bandwidth; (8.0 ms before) Disk
latency = average seek time + average rotational delay + transfer
time + controller overhead = (0.33 * 4.9 ms) + 0.5 * 1/(10000 RPM)
+ 0.5 KB / (0.75 * 87 MB/s) + 0.1 ms = 1.62 ms + 0.5 /(10000
RPM/(60000ms/M)) + 0.5 KB / (65 KB/ms) + 0.1 ms = 1.62 + 3.0 +
0.0077 + 0.1 ms = 4.73 ms
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Security & Protection
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Security overall problem of ensuring that resources (files,
cpus, memory, etc.) are accessed only by authorized users e.g. PC
in lecture room was stolen Protection OS mechanisms to provide
security for the enforcement of policies governing use e.g. What is
the policy regarding e-mail?
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Protection Protecting processes/users from each other is one of
the core OS responsibilities Control access of processes or users
to resources of the computer system (both HW and SW) Ensure
resources are operated on by only those processes that have gained
proper authorization Enforcing resource limits
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Cross-cutting issue CPU Scheduling Protection by timer
interrupts and OS scheduling policy Process Management Protection
by access control and enforcement of resource limits (most OS?)
Virtual Memory Protection by inability to name other processes
memory space File System User defined access controls per
file/directory Note: Synchronization more voluntary protection by
observing rules within a set of processes/threads that share data
(Monitors maybe protection?)
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How to do protection? From that brief survey of OS topics it is
clear that protection can be accomplished in many ways Protection
can be based on the design of the system which makes access
impossible (cant even name things you shouldnt access) e.g. VM
Protection can be controllable by an OS wide policy (OS controls
resource allocation) e.g. timer interrupts Protection can be
controlled by user definable access controls e.g. User can set FS
access controls Implies ability to deny authorized access! Ability
to enforce the policy!
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Principles Generally the more restrictive the system the more
protection Need to know principle says only grant those rights
absolutely necessary to accomplish a task Start out granting none
and see where it breaks, add the smallest new privileges as
possible Ex. If a process only needs to read/write one specific
file then dont give it access to all the users files Ex. Dont give
full root privileges just because need to open a port <
1024
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Policy vs Mechanism Mechanism says what types of access are
possible and defines the means for identifying authorized vs
unauthorized access Policy says which processes/users should have
which kinds of access When building system best to make mechanism
match the problem domain rather than a particular desired policy
More flexible if separate mechanism from policy! Example: if your
mechanism does not distinguish between read and execute rights then
impossible to hand out one without the other; if mechanism does
distinguish then policy may never choose to hand out one without
the other but it could
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Types of access The possible types of access depend on the
resource CPUs can be executed upon File can be
read/written/executed Directories can be read/inserted into/deleted
from/traversed without displaying all Tape drives can be
read/written/rewound Begin by thinking about all the possible
actions you might want to allow/disallow on an object
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Example: File Sharing & Protection File Sharing Sharing of
files on multi-user systems is desirable Sharing may be done
through a protection scheme On distributed systems, files may be
shared across a network Network File System (NFS) is a common
distributed file-sharing method
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10.44 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts File Sharing Remote File Systems Uses networking to allow
file system access between systems Manually via programs like FTP
Automatically, seamlessly using distributed file systems Semi
automatically via the world wide web Client-server model allows
clients to mount remote file systems from servers Server can serve
multiple clients Client and user-on-client identification is
insecure or complicated NFS is standard UNIX client-server file
sharing protocol CIFS is standard Windows protocol Standard
operating system file calls are translated into remote calls
Distributed Information Systems (distributed naming services) such
as LDAP, DNS, NIS, Active Directory implement unified access to
information needed for remote computing
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10.45 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts File Sharing Consistency Semantics Consistency semantics
specify how multiple users are to access a shared file
simultaneously Similar to Ch 7 process synchronization algorithms
Tend to be less complex due to disk I/O and network latency (for
remote file systems Andrew File System (AFS) implemented complex
remote file sharing semantics Unix file system (UFS) implements:
Writes to an open file visible immediately to other users of the
same open file Sharing file pointer to allow multiple users to read
and write concurrently AFS has session semantics Writes only
visible to sessions starting after the file is closed
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10.46 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts File Protection File owner/creator should be able to
control: what can be done by whom Types of access Read Write
Execute Append Delete List
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10.47 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts Access Lists and Groups Mode of access: read, write,
execute Three classes of users RWX a) owner access 7 1 1 1 RWX b)
group access 6 1 1 0 RWX c) public access1 0 0 1 Ask manager to
create a group (unique name), say G, and add some users to the
group. For a particular file (say game) or subdirectory, define an
appropriate access. owner grouppublic chmod 761game Attach a group
to a file chgrp G game
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10.48 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts Windows XP Access-control List Management
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10.49 Silberschatz, Galvin and Gagne 2005 Operating System
Concepts A Sample UNIX Directory Listing
