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Storage Architecture. CE202 December 2, 2003 David Pease. Faster. Smaller. Higher. Cache. RAM. Capacity. Speed. Cost. Disk. Optical. Tape. Slower. Larger. Lower. Hierarchy of Storage. Storage System Components. Application I/O Library File System Device Driver

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Storage Architecture

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Storage Architecture

CE202

December 2, 2003

David Pease


Faster

Smaller

Higher

Cache

RAM

Capacity

Speed

Cost

Disk

Optical

Tape

Slower

Larger

Lower

Hierarchy of Storage


Storage System Components

  • Application

  • I/O Library

  • File System

  • Device Driver

  • Host Bus Adapter

  • Interconnect

  • Storage Controller

  • Devices I/O Context


Disks


Disk Drives

  • “Workhorse” of modern storage systems

  • Capacity increasing, raw price dropping

    • can buy 1TB for only $1000!

    • bandwidth not keeping pace

    • reliability is actually decreasing

      • massive systems can mean even lower availability

  • Majority of cost of ownership in administration, not purchase price

    • backup, configuration, failure recovery


Disk Architecture

spindle

cylinder

sector

track

platters

arms with

read/write

heads

rotation


Disk Storage Density


Disk Capacity Growth


IBM Disk Storage Roadmap


Storage Costs


RAID

  • Redundant Arrays of Inexpensive Disks

  • Two orthogonal concepts:

    • data striping for performance

    • redundancy for reliability

  • Striped arrays can increase performance, but at the cost of reliability (next page)

    • redundancy can give arrays better reliability than an individual disk


Reliability of Striped Array


One-month Trace of Hardware Failures

Trace collected from the Internet Archive (March 2003)

(thanks Kelly Gottlib)

-- Over 100 terabytes of compressed data

-- 30 disk failures out of total 70 hardware problems


RAID Levels


RAID Levels

0

1

2

3

4

5

6


RAID: 4x Small Write Penalty

small data write

xor

3

4

1

2

5


Log-Structured File Systems

  • Based on assumption that disk traffic will become dominated by writes

  • Always writes disk data sequentially, into next available location on disk

    • no seeks on write

  • Eliminates problem of 4x write penalty

    • all writes are “new”, no need to read old data or parity

  • However, almost no examples in industry file systems


Tape


Tape Media

  • Inherently sequential

    • long time to first byte

    • no random I/O

  • Subject to mechanical stress

    • number of read-write cycles lower than disk

  • Problems as an archival medium:

    • readers go away after some years

      • most rapidly in recent years

    • tapes (with data) remain in a salt mine


Tape Media

  • Density will always trail that of disk

    • Tape stretches, more difficult to get higher density

  • Alignment also an issue

    • once it’s past the head, it’s gone

    • more conservative techniques required

  • Bottom line: mechanical engineering issues for tape are the difficult ones


Optical

  • CD, CD-R/RW, DVD, DVD-R/RW

    • Capacities:

      • CD: ~700MB (huge 20 years ago!)

      • DVD:

        • single sided, single layer: 5GB

        • single sided, double layer: 9GB

        • double sided, single layer: 10GB

        • double sided, double layer: 18GB

      • Size of cell limited by wavelength of light

        • current lasers are red

        • blue lasers are under development, then UV, ...


Optical

  • Magneto-optical (HAMR)

    • heat from laser makes changing direction of magnetization easier (so cell is smaller)


MEMS

  • MicroElectroMechanical Systems

    • 6-10 times faster than disk

    • cost and capacity issues


Magnetic RAM (MRAM)

  • Stores each bit in a magnetic cell rather than a capacitor or flip-flop

    • data is persistent

  • Can be read and written very quickly

    • Read and write times 0.5 – 10 µs or less

    • Individual bits are writeable (no block erase)

  • Density & cost comparable to DRAM

    • may require density/speed tradeoffs

    • denser MRAM may have to run slower because of heat dissipation on writes


Magnetic RAM (MRAM)

  • Several companies have announced partnerships to produce products ~2003

  • Ideas for use of MRAM in storage:

    • Persistent cache

      • Hot data in MRAM, cold data to disk

      • No need to flush write cache to avoid data loss

    • HeRMES

      • all metadata in MRAM

      • enough file data in MRAM to hide disk latency for first access to a file


Peripheral Buses

  • SCSI

  • IDE/ATA

  • HIPPI (High Performance Parallel Intf.)

  • IEEE 1394 (FireWire)

  • FibreChannel (FCP)

  • IP (e.g., iSCSI)

  • InfiniBand

  • Serial ATA


Peripheral Buses

  • Parallel

    • SCSI, most printers, IBM Channels

    • 1 or more bytes per clock

    • Skew problems at high speeds

  • Serial

    • FC, RS232, IEEE1394 (FireWire)

    • 1 bit per clock, self clocking

    • can be run at much higher speeds than parallel bus


Networked Storage

  • Storage attached by general-purpose or dedicated network (e.g., FibreChannel)

  • Motivations:

    • homogenous and heterogeneous file sharing

    • centralized administration

    • better resource utilization (shared storage resources, pooling)

  • Dedicated Networks:

    • Fibre-Channel: FCP (SCSI over FC)

    • iSCSI: SCSI over IP

    • InfiniBand


Networked Storage

  • Can mean many things:

    • NAS (Network-Attached Storage): file server appliances serving NFS and/or CIFS (for example, Network Appliance)

    • NASD (Network-Attached Secure Disk): intelligent, network-attached drives w/ security features (also, Network-Attached Storage Device)

    • SAN (Storage Area Network): network for attaching disks and computers, usually dedicated only to storage operations

      • OBSD (Object-Based Storage Device): similar to NASD


Solaris

Win2K

Linux

AIX

IFS w/cache

IFS w/cache

IFS w/cache

IFS w/cache

A SAN File System

NFS

CIFS

FTP

HTTP

Control Network (IP)

Meta-data

Server

Meta-data

Server

Meta-

data

SAN

Meta-data

Server

data

Security

assists

Storage

Management

Server

HSM &

Backup

Data

Data


Additional Reading

  • Hennessy & Patterson: Chapter 6

  • Chen, Lee, Gibson, Katz, & Patterson: RAID: high performance, reliable secondary storage. ACM Computing Surveys 26, June 1994, 145-185

  • Rosenblum & Ousterhout: The design and implementation of a log-structured file system. ACM Transactions on Computer Systems, Feb. 1992, 26-52

  • Gibson, Nagle, et al.: A cost-effective, high-bandwidth storage architecture. Proceedings of the Eight Conference on Architectural Support for Programming Languages and Operating Systems, 1998

  • http://www.almaden.ibm.com/cs/storagesystems/stortank/


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