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  • 1. Storage Architecture CE202 December 2, 2003 David Pease
  • 2. Hierarchy of Storage Smaller Larger Capacity Higher Lower Cost RAM Disk Optical Tape Cache Faster Slower Speed
  • 3.
    • Application
    • I/O Library
    • File System
    • Device Driver
    • Host Bus Adapter
    • Interconnect
    • Storage Controller
    • Devices I/O Context
    Storage System Components
  • 4. Disks
  • 5. 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
  • 6. Disk Architecture track platters spindle sector arms with read/write heads rotation cylinder
  • 7. Disk Storage Density
  • 8. Disk Capacity Growth
  • 9. IBM Disk Storage Roadmap
  • 10. Storage Costs
  • 11. 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
  • 12. Reliability of Striped Array
  • 13. 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
  • 14. RAID Levels
  • 15. RAID Levels 0 2 3 4 5 6 1
  • 16. RAID: 4x Small Write Penalty small data write 1 2 4 5 3 xor
  • 17. 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
  • 18. Tape
  • 19. 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
  • 20. 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
  • 21. 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, ...
  • 22. Optical
    • Magneto-optical (HAMR)
      • heat from laser makes changing direction of magnetization easier (so cell is smaller)
  • 23. MEMS
    • MicroElectroMechanical Systems
      • 6-10 times faster than disk
      • cost and capacity issues
  • 24. 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
  • 25. 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
  • 26. Peripheral Buses
    • SCSI
    • IDE/ATA
    • HIPPI (High Performance Parallel Intf.)
    • IEEE 1394 (FireWire)
    • FibreChannel (FCP)
    • IP (e.g., iSCSI)
    • InfiniBand
    • Serial ATA
  • 27. 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
  • 28. 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
  • 29. 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
  • 30. Meta-data Server A SAN File System SAN Meta-data Server Meta-data Server Storage Management Server HSM & Backup Meta- data Control Network (IP) NFS CIFS FTP HTTP Data Data data Security assists IFS w/cache Win2K IFS w/cache AIX IFS w/cache Solaris IFS w/cache Linux
  • 31. 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/