CS计算机代考程序代写 data structure algorithm assembly arm scheme database flex file system Chapter 10: Mass-Storage Systems

Chapter 10: Mass-Storage Systems

Chapter 10: Mass-Storage Systems
Overview of Mass Storage Structure
Disk Structure
Disk Attachment
Disk Scheduling
Disk Management
Swap-Space Management
RAID Structure
Stable-Storage Implementation

Objectives
To describe the physical structure of secondary storage devices and its effects on the uses of the devices
To explain the performance characteristics of mass-storage devices
To evaluate disk scheduling algorithms
To discuss operating-system services provided for mass storage

Moving-head Disk Mechanism

Tracks get bigger as you move away from the spindle…
Stay tuned…

Overview of Mass Storage Structure
Magnetic disks provide bulk of secondary storage of modern computers
Drives rotate at 60 to 250 times per second
Transfer rate is rate at which data flow between drive and computer
Positioning 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)
Seek time is given in track-to-track and full-stroke measures
Head crash results from disk head making contact with the disk surface — (That’s bad, by the way!)
Disks can be removable
Drive attached to computer via I/O bus
Busses vary, including EIDE, ATA, SATA, USB, Fibre Channel, SCSI, SAS, Firewire
Host controller in computer uses bus to talk to disk controller built into drive or storage array

Hard Disks
Platters range from .85” to 14” (historically)
Commonly 3.5”, 2.5”, and 1.8”
Range from 30GB to 3TB per drive
Performance
Transfer Rate – theoretical – 6 Gb/sec
Effective Transfer Rate – real – 1Gb/sec
Seek time from 3ms to 12ms – 9ms common for desktop drives

(From Wikipedia)

Hard Disk Performance
Access Latency = Average access time = average seek time + average latency
For fastest disk 3ms + 2ms = 5ms
For slow disk 9ms + 5.56ms = 14.56ms
Average I/O time = average access time + (amount to transfer / transfer rate) + controller overhead
For example to transfer a 4KB block on a 7200 RPM disk with a 5ms average seek time, 1Gb/sec transfer rate with a .1ms controller overhead =
5ms + 4.17ms + 0.1ms + transfer time =
Transfer time = 4KB / 1Gb/s * 8Gb / GB * 1GB / 10242KB = 32 / (10242) = 0.031 ms
Average I/O time for 4KB block = 9.27ms + .031ms = 9.301ms

The First Commercial Disk Drive

1956
IBM RAMDAC computer included the IBM Model 350 disk storage system

5M (7 bit) characters
50 x 24” platters
Access time = < 1 second In 1958, the system was enhanced to permit an optional additional 350 Disk Storage Unit, thereby doubling storage capacity; and an additional access arm for each 350. Remember that we were taking about latency in milliseconds for current disk storage systems. HARD DISK DRIVE IN ACTION https://www.youtube.com/watch?v=NtPc0jI21i0 7:22 Solid-State Disks Nonvolatile memory used like a hard drive Many technology variations Can be more reliable than HDDs More expensive per MB May have shorter life span* Less capacity But much faster Standard Bus interfaces can be too slow -> connect directly to PCI (Peripheral Component Interconnect) for example
No moving parts, so no seek time or rotational latency

*this myth is overrated. Most PC SSDs will outlast other PC components.

Watch the video at 1.5 speed.
18

Magnetic Tape
Was early secondary-storage medium
Evolved from open spools to cartridges
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
140MB/sec and greater
200GB to 1.5TB typical storage
Common technologies are LTO-{3,4,5} and T10000

3:42

Disk Structure
Disk drives are addressed as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer
Low-level formatting creates logical blocks on physical media
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

The outer tracks are ‘longer’ than the inner tracks.
Do they hold more information than do the inner tracks/cylinders?
Do they contain more sectors?

Track Length Variance Compensation
Constant Angular Velocity (i.e. RPM):
Non-constant # of sectors per track
decreasing bit density from in to out (to maintain constant data rate)
consistent rotation speed
Constant Linear Velocity:
Non-constant # of sectors per track
consistent bit density
increasing speed going from in to out (to maintain constant data rate)

2:34

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)
FC 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
I/O directed to bus ID, device ID, logical unit (LUN)

Storage Array
Can just attach disks, or arrays of disks
Storage Array has controller(s), provides features to attached host(s)
Ports to connect hosts to array
Memory, controlling software (sometimes NVRAM, etc)
A few to thousands of disks
RAID, hot spares, hot swap (discussed later)
Shared storage -> more efficiency
Features found in some file systems
Snaphots, clones, thin provisioning, replication, deduplication, etc

Storage Area Network
Common in large storage environments
Multiple hosts attached to multiple storage arrays – flexible

Storage Area Network (Cont.)
SAN is one or more storage arrays
Connected to one or more Fibre Channel switches
Hosts also attach to the switches
Storage made available via LUN Masking from specific arrays to specific servers
Easy to add or remove storage, add new host and allocate it storage
Over low-latency Fibre Channel fabric
Why have separate storage networks and communications networks?
Consider iSCSI, FCOE

Network-Attached Storage
Network-attached storage (NAS) is storage made available over a network rather than over a local connection (such as a bus)
Remotely attaching to file systems
NFS and CIFS are common protocols
Implemented via remote procedure calls (RPCs) between host and storage over typically TCP or UDP on IP network
iSCSI protocol uses IP network to carry the SCSI protocol
Remotely attaching to devices (blocks)

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
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

Disk Scheduling (Cont.)
There are many sources of disk I/O requests
OS
System processes
Users processes
I/O request includes:
input or output mode
disk address
memory address
number of sectors to transfer
OS maintains queue of requests, per disk or device
Idle disk can immediately work on I/O request, busy disk means work must queue

Optimization algorithms only make sense when a queue exists

Disk Scheduling (Cont.)
Drive controllers have small buffers and can manage a queue of I/O requests (of varying “depth”)
Several algorithms exist to schedule the servicing of disk I/O requests
The analysis is true for one or many platters
We illustrate scheduling algorithms with a request queue (0-199)

98, 183, 37, 122, 14, 124, 65, 67
Head pointer 53

FCFS
Illustration shows total head movement of 640 cylinders

SSTF
Shortest Seek Time First 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 (vs 640 for FCFS)

SCAN
This approach works like an elevator does. It scans ‘down’ towards the nearest end and then, when it hits the bottom, it scans ‘up’ servicing the requests that it didn’t get going down.
SCAN algorithm Sometimes called the elevator algorithm

SCAN (Cont.)

total head movement of 208 cylinders

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

C-SCAN (Cont.)

C-SCAN WILL TRAVEL UNTIL THE END OF THE DISK EVEN IF THERE ARE NO OTHER REQUESTS IN THAT DIRECTION

look
LOOK Disk Scheduling Algorithm:
LOOK is the advanced version of SCAN
The LOOK algorithm services request similar to SCAN
It also “looks” ahead as if there are more tracks that are needed to be serviced in the same direction.
If there are no pending requests in the moving direction the head reverses the direction and start servicing requests in the opposite direction.
LOOK provides better performance than SCAN because in SCAN, the head is not allowed to switch directions until it reaches the end of the disk.

LOOK

LOOK SATISFIES REQUESTS IN BOTH DIRECTIONS.

C-LOOK
C-LOOK is the modified version of both LOOK and SCAN algorithms.
The head starts from first request in one direction and moves towards the last request at other end, serving all request in between.
After reaching last request in one end, the head jumps in the other direction and move towards the remaining requests and then satisfies them in same direction as before.
Unlike LOOK, it satisfies requests only in one direction.

C-LOOK

C-LOOK SATISFIES REQUESTS ONLY IN ONE DIRECTION.
C-LOOK WILL TRAVEL ONLY AS FAR AS THERE ARE REQUESTS. C-SCAN GOES ALL THE WAY TO THE END.

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
Less starvation
Performance depends on the number and types of requests
Requests for disk service can be influenced by the file-allocation method
And metadata layout
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
What about rotational latency?
Difficult for OS to calculate

Disk Management
Low-level formatting, or physical formatting — Dividing a disk into sectors that the disk controller can read and write
Each sector can hold header information, plus data, plus error correction code (ECC)
Usually 512 bytes of data but can be selectable
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, each treated as a logical disk
Logical formatting or “making a file system”
To increase efficiency most file systems group blocks into clusters
Disk I/O done in blocks
File I/O done in clusters

Disk Management (Cont.)

Raw disk access for apps that want to do their own block management, keep OS out of the way (databases for example)
Boot block initializes system
The bootstrap is stored in ROM
Bootstrap loader program stored in boot blocks of boot partition
Methods such as sector sparing used to handle bad blocks

Booting from a Disk in Windows

Swap-Space Management
Swap-space — Virtual memory uses disk space as an extension of main memory
Less common now due to memory capacity increases
Swap-space can be carved out of the normal file system, or, more commonly, it can be in a separate disk partition (raw)
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 dirty page is forced out of physical memory, not when the virtual memory page is first created
File data written to swap space until write to file system requested
Other dirty pages go to swap space due to no other home
Text segment pages thrown out and reread from the file system as needed
What if a system runs out of swap space?
Some systems allow multiple swap spaces

Data Structures for Swapping on Linux Systems

RAID Structure
RAID – redundant array of inexpensive disks
multiple disk drives provides reliability via redundancy
Increases the mean time to failure
Mean time to repair – exposure time when another failure could cause data loss
Mean time to data loss based on above factors
If mirrored disks fail independently, consider disk with 1300,000 mean time to failure and 10 hour mean time to repair
Mean time to data loss is 100, 0002 / (2 ∗ 10) = 500 ∗ 106 hours, or 57,000 years!
Frequently combined with NVRAM to improve write performance
Several improvements in disk-use techniques involve the use of multiple disks working cooperatively

RAID (Cont.)

Disk striping uses a group of disks as one storage unit
RAID is arranged into six different levels
RAID schemes improve performance and improve the reliability of the storage system by storing redundant data
Mirroring or shadowing (RAID 1) keeps duplicate of each disk
Striped mirrors (RAID 1+0) or mirrored stripes (RAID 0+1) provides high performance and high reliability
Block interleaved parity (RAID 4, 5, 6) uses much less redundancy
RAID within a storage array can still fail if the array fails, so automatic replication of the data between arrays is common
Frequently, a small number of hot-spare disks are left unallocated, automatically replacing a failed disk and having data rebuilt onto them

RAID Levels

RAID (0 + 1) and (1 + 0)

Other Features
Regardless of where RAID implemented, other useful features can be added
Snapshot is a view of file system before a set of changes take place (i.e. at a point in time)
More in Ch 12
Replication is automatic duplication of writes between separate sites
For redundancy and disaster recovery
Can be synchronous or asynchronous
Hot spare disk is unused, automatically used by RAID production if a disk fails to replace the failed disk and rebuild the RAID set if possible
Decreases mean time to repair

Extensions
RAID alone does not prevent or detect data corruption or other errors, just disk failures
Solaris ZFS adds checksums of all data and metadata
Checksums kept with pointer to object, to detect if object is the right one and whether it changed
Can detect and correct data and metadata corruption
ZFS also removes volumes, partitions
Disks allocated in pools
Filesystems with a pool share that pool, use and release space like malloc() and free() memory allocate / release calls

ZFS Checksums All Metadata and Data

Traditional and Pooled Storage

Stable-Storage Implementation
Write-ahead log scheme requires stable storage
Stable storage means data is never lost (due to failure, etc)
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
Disk write has 1 of 3 outcomes
Successful completion – The data were written correctly on disk
Partial failure – A failure occurred in the midst of transfer, so only some of the sectors were written with the new data, and the sector being written during the failure may have been corrupted
Total failure – The failure occurred before the disk write started, so the previous data values on the disk remain intact

Stable-Storage Implementation (Cont.)
If failure occurs during block write, recovery procedure restores block to consistent state
System maintains 2 physical blocks per logical block and does the following:
Write to 1st physical
When successful, write to 2nd physical
Declare complete only after second write completes successfully
Systems frequently use NVRAM as one physical to accelerate

Write a program that implements the following disk-scheduling algorithms: (Pick three)
a. FCFS
b. SSTF
c. SCAN
d. C-SCAN
e, LOOK
f. C-LOOK
Your program will service a disk with 5,000 cylinders numbered 0 to 4,999. The program will generate a random series of 50 requests and service them according to each of the algorithms you chose.
The program will be passed the initial position of the disk head as a parameter on the command line and report the total amount of head movement required for each algorithm. (Programming Assignment 10.24 in the text)

Send email to CISC3320.bc@gmail.com
Within the next 90 seconds

End of Chapter 10

2.05.eps

track t

sector s

spindle

cylinder c

platter

arm

read-write
head

arm assembly

rotation

n.12.eps

LAN/WAN

storage
array

data-processing
center

web content
provider

server
client

client

client
server

tape
library

SAN

12.750.eps

swap area
page
slot

swap partition
or swap file

swap map 1 0 3 0 1

n.09.eps

(a) RAID 0: non-redundant striping.

(b) RAID 1: mirrored disks.

C C C C

(d) RAID 3: bit-interleaved parity.

(e) RAID 4: block-interleaved parity.

(f) RAID 5: block-interleaved distributed parity.

P P

P P P

(g) RAID 6: P � Q redundancy.

PP P

P
PPP

P
P

n.10.eps

mirror

a) RAID 0 � 1 with a single disk failure.

stripe

mirror

b) RAID 1 � 0 with a single disk failure.

stripe
mirror mirror mirror

12.13.eps

metadata block 1

address 1

checksum MB2 checksum

address 2

metadata block 2

address

checksum D1 checksum D2

data 1 data 2

address

12.14.eps

volume

ZFS ZFS

storage pool

ZFS

volume volume

FS FS

(a) Traditional volumes and file systems.

(b) ZFS and pooled storage.

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