Storage Architecture

1. Storage Architecture CE202 December 2, 2003 David Pease

2. Faster Smaller Higher Cache RAM Capacity Speed Cost Disk Optical Tape Slower Larger Lower Hierarchy of Storage

3. Storage System Components • Application • I/O Library • File System • Device Driver • Host Bus Adapter • Interconnect • Storage Controller • Devices I/O Context

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 spindle cylinder sector track platters arms with read/write heads rotation

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 1 2 3 4 5 6

16. RAID: 4x Small Write Penalty small data write xor 3 4 1 2 5

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. 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

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/