Storage Architecture CE202 December 2, 2003 David Pease

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

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

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

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

Trace collected from the Internet Archive (March 2003) (thanks Kelly Gottlib) -- Over 100 terabytes of compressed data disk failures out of total 70 hardware problems One-month Trace of Hardware Failures

RAID Levels

RAID: 4x Small Write Penalty small data write xor

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 its past the head, its 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

Meta-data Server A SAN File System SAN IFS w/cache Win2K IFS w/cache AIX IFS w/cache Solaris Meta-data Server Meta-data Server Storage Management Server HSM & Backup Meta- data Control Network (IP) NFS CIFS FTP HTTP Data data Security assists IFS w/cache Linux

Additional Reading Hennessy & Patterson: Chapter 6 Chen, Lee, Gibson, Katz, & Patterson: RAID: high performance, reliable secondary storage. ACM Computing Surveys 26, June 1994, Rosenblum & Ousterhout: The design and implementation of a log-structured file system. ACM Transactions on Computer Systems, Feb. 1992, 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,