Mass-Storage Structure

CS 540 – Chapter 12

Kate DehbashiAnna Deghdzunyan

Fall 2010Dr. Behzad

Agenda

Review File System Parts of File System

Overview Magnetic Disks Magnetic Tapes

Disk Structure Disk Attachment

Host-attached Network-attached Storage-Area Networks

Disk Scheduling Scheduling Algorithms Selection of an algorithm

Agenda (Cont.)

Disk Management Disk Formatting Boot Block Bad-Block Recovery

Swap-Space Management How is it used Where is it located How is it managed

Review

File System Method of storing and organizing

computer files and their data Storage Organization Manipulation Retrieval

Maintain physical location

Review (Cont.)

Parts of File System Interface

User and programmer interface to the file system Implementation

Internal data structure and algorithms used to implement the interface

Storage Structure Physical structure Disk scheduling algorithms Disk formatting Disk reliability Stable-storage implementation

Overview

Magnetic Disks Magnetic Tape

Magnetic Disks

Structure Platter Track Sector Cylinder Disk arm Read-write head

Rotates 60-200 times/second Disk Speed

Transfer rate Positioning time

Seek time Rotational latency

Magnetic Disks (Cont.)

Head Crash Disk head making contact with the disk surface Permanent damage

Removable Magnetic Disks Floppy

Head sits directly on the surface Slow rotation and lower disk space

I/O bus Drive attached to computer via set of wires Busses vary, including EIDE, ATA, SATA, USB, Fiber

Channel, SCSI, Fire wire Disk Controller

Cache memory Host controller

First HDD – IBM RAMAC 1956

1.5 square meters (16 sq ft).

$3200.00

Magnetic Tape

Early secondary-storage medium First used in1951 as a computer storage

Holds large quantities of data LTO-5 (2010) 1.5 TB uncompressed data (book: 20-200GB)

Access time slow Random access ~1000 times slower than disk Once data under head, transfer rates comparable to disk

Modern Usage Backup, archive

For large amount of data tape can be substantially less expensive than disk

Common technologies 4mm, 8mm, 19mm LTO (Linear Tape-Open), SDLT (Digital Linear Tape)

LTO-2

SDLT ¼ , ½ inch

Disk Structure

Addressing One-dimensional array of blocks Logical Block

Smallest unit of transfer 512 bytes

Blocks maps to sectors sequentially Sector 0: first sector, first track, outmost cylinder Mapping order

Track Rest of the tracks in the same cylinder Rest of the cylinders from outermost to innermost

Disk Structure (Cont.)

Logical block number Cylinder#, track#, sector#

In practice it is difficult to perform Defective sectors Sectors/track is not constant

CLV (Constant Linear Velocity) Constant density of bits/track Variable rotational speed

CAV (Constant Angular Velocity) Constant rotational speed Variable density of bits per track

Disk Attachment

Host-Attached Storage (DAS) Accessed through local I/O ports

IDE, ATA, SATA SCSI, FC

Wide variety of storage devices HDD, RAID Arrays, CD/DVD Drives and Tape

Network-Attached Storage (NAS) NAS ISCSI

Storage-Area Network (SAN) SAN infiniBand

Host-Attached StorageSCSI

SCSI (Small Computer System Interface) Large variety of devices 16 devices per cable Controller card (SCSI Initiator) SCSI target 8 logical units per target

Host-Attached Storage FC

FC (Fiber Channel) High-speed serial architecture Optical cable, four-conductor copper cable Switched fabric (FC - SW)

All devices are connected to fiber channel switches 24-bit address space multiple hosts and storage

devices Dominate in future Basic of SANs

Arbitrated Loop (FC – AL) 126 devices All devices are in a loop or ring Historically lower cost but rarely used now

FC – Topologies

Network-Attached Storage

NAS Storage system Accessed remotely over a data network Clients access via remote-procedure-call

interface UNIX: NFS Windows: CIFS

RPCs carried via TCP/UDP Convenient way for all clients to share a pool of

storage NAS VS local-attached

Same ease of naming and access Less efficient and lower performance

Network-Attached Storage (Cont.)

ISCSI – Internet Small Computing System Interface Latest NAT protocol IP-based storage networking protocol Uses IP network to carry SCSI Protocol Clients are able to send SCSI commands

to remote targets TCP ports 860 and 3260

Storage-Area Networks

SAN Private network connecting servers and

storage devices Uses storage protocols instead of

networking protocols Multiple hosts and storage can attach to

the same SAN flexibility SAN Switch allows/prohibits client access

(exp.) FC is the most common SAN interconnect

Storage-Area Networks (Cont.)

InfiniBand Special-purpose bus architecture Supports high-speed interconnection

network Up to 2.5 gbps 64,000 addressable devices

Supports QoS and Failover

Disk Scheduling

Disk Drive Efficiency Access time

Seek time Rotational latency

Bandwidth Bytes transferred / Δt

Δt: Completion time of the last transfer – first request for service time

Improve? Scheduling the servicing of I/O requests in a good order

Disk Scheduling (Cont.)

I/O request procedure System Call Sent by the process to the OS System Call information

Input/output Disk address Memory address Number of sectors to be transferred

If disk available access

else Queue

Disk Scheduling (Cont.)

Algorithms FCFS SSTF SCAN C-SCAN LOOK/CLOOK

Disk Scheduling (Cont.)

FCFS (First come First Served)

640 cylinder

moves

Disk Scheduling (Cont.)

SSTF (Shortest Seek Time First) Service requests close to the current head

position Starvation

236 cylinder moves

Disk Scheduling (Cont.)

SCAN (Elevator Alg.) Head starts at one end and goes to the other

end Services each request on the current track

236 cylinder moves

Disk Scheduling (Cont.)

CSCAN Variant of SCAN More uniform wait time When head reaches the end, immediately moves to

the beginning without servicing any request

360 cylinder moves

Disk Scheduling (Cont.)

LOOK/CLOOK Head goes as far as the last request in each

direction

322 cylinder moves

Disk Scheduling (Cont.)

Selection of an algorithm Factors SSTF is common and better performance than

FCFS SCAN, CSCAN perform better for systems that

place a heavy load on the disk No starvation Scheduling alg. Performance (example1)

Number of requests Types of requests

Requests for disk service can be influenced by The file-allocation method (example2) Location of directories and indexed blocks

Caching directories and indexed blocks in the main memory reduces arm movement (example3)

Disk Scheduling (Cont.)

Selection of an algorithm Separate module of the OS can be replaced

if necessary Default: SSTF/LOOK

Rotational Delay Perspective Modern disks do not disclose the physical

location of logical blocks Disk controller takes over OS to choose the alg. Problem?

If only I/O OK But there are other constraints

Example: request for paging (example)

Disk Management

Disk Formatting Low-level Logical

Boot Block Bootstrap

Bad-Block Recovery Manually Sector Sparing (Forwarding) Sector Slipping

Disk Formatting Low-Level Formatting (Physical Formatting)

Header, Trailer Sector Number ECC

Error detection Soft error recovery

Data-Area 512 bytes

Logical Formatting Partition

One or more group of cylinders Each partition is treated as a separate disk (example)

Logical Formatting Storing of initial file system

Map of allocated and free space An initial empty Directory

Cluster Blocks are put together to increase efficiency Disk I/O done via blocks/File I/O done via clusters

Raw disk Some programs use the disk partition as a large sequential array of logic blocks bypassing the file system services

Boot Block

Bootstrap Program Initial program to start a computer system Initializes aspects of the system

CPU registers Device controllers Contents of the main memory

Starts the OS Finds the OS Kernel on disk and loads it into the memory Jumps to an initial address to begin the OS exec.

Stored in ROM No need for initialization No virus Problem? Hard to update solution: save the bootstrap loader in the ROM, full bootstrap on boot blocks (fixed location on HDD)

Booting from a Disk in Windows 2000

Bad-Block Recovery

Complete Disk Failure Replace the disk

Bad Sector Handling Manually

IDE: format, chkdsk Special entry into FAT

Sector Sparing (Forwarding) SCSI Controller maintains a bad sector list List is initialized during the low-level formatting Controller sets aside spare sectors to replace bad sectors logically (Example) Problem? Invalidate optimization done by disk scheduling alg. Solution? Spare sectors on each cylinder

Sector Slipping Move down every sector tom empty the next sector to the bad sector Example

Soft-error: repairable by disk controller through ECC Hard-error: lost data back up

Swap-Space Management

In modern Operating Systems, “Paging” and “Swapping” are used interchangeably

Virtual memory uses disk space as an extension of the main memory Performance decreases. why?

Swap-space management goal To get the best throughput for the

virtual memory system

Swap-Space Management (Cont.)

How is it used? Depends on memory management alg.

Swapping: Load entire process into disk Paging: Stores pages

Amount of swap space needed depends on Amount of physical memory Amount of virtual memory Way virtual memory is used Ranges from few MB to GB Better to overestimate why? No process is aborted Solaris: swap space = amount by which VM exceeds

pageable physical memory Linux: swap space = double the amount of physical memory Multiple swap spaces

Swap-Space Management (Cont.)

Where is it located on disk? In the normal file system

Large file within the file system File-system routines can be used Easy to implement but inefficient

Takes time to traverse the directory structure Separate disk partition

Raw partition Swap space storage manager Uses alg. Optimized for speed rather than storage efficiency why? Trade-off between speed and fragmentation acceptable (data life is short) Fixed amount of space is set aside during partitioning Adding more space requires re-partitioning

Linux: Supports both Who decides?

Swap-Space Management (Cont.)

How is it managed? Unix

Traditional: copy the entire processes Newer: combination of swapping & paging

Solaris1 File-system: text-segment pages containing

code Swap-space: pages of anonymous memory such as stack or heap Modern versions only allocate the swap space

if page is forced out of the main memory

Swapping on Linux System

References

Wikipedia.com PCTechGuide.com USRobotics.com allSAN.com Xenon.com.au