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Computer Architecture Lecture 24 Lecture 24 Announcement CSC3501 - S.J. Park Announcement Computer Architecture (CSC-3501) Lecture 24 (22 April 2008) Seung-Jong Park (Jay) http://www.csc.lsu.edu/~sjpark 1 2 CSC3501 - S.J. Park CSC3501 - S.J. Park 7.7 Optical Disks 7.7 Optical Disks Optical disks provide large storage capacities very CD-ROMs were designed by the music industry in inexpensively. the 1980s, and later adapted to data. They come in a number of varieties including CD- This history is reflected by the fact that data is ROM, DVD, and WORM. recorded in a single spiral track, starting from the Many large computer installations produce center of the disk and spanning outward . document output on optical disk rather than on Binary ones and zeros are delineated by bumps in paper. This idea is called COLD-- Computer Output the polycarbonate disk substrate. The transitions Laser Disk. between pits and lands define binary ones. It is estimated that optical disks can endure for a If you could unravel a full CD-ROM track, it would be hundred years. Other media are good for only a nearly five miles long! decade-- at best. 3 4 CSC3501 - S.J. Park CSC3501 - S.J. Park 7.7 Optical Disks 7.7 Optical Disks The logical data format for a CD-ROM is much DVDs can be thought of as quad-density CDs. more complex than that of a magnetic disk. (See Varieties include single sided, single layer, single the text for details.) sided double layer, double sided double layer, and double sided double layer. Different formats are provided for data and music. Where a CD-ROM can hold at most 650MB of data, Two leve ls o f error correc tion are provid e d for the DVDs can hold as much as 17GB. data format. One of the reasons for this is that DVD employs a Because of this, a CD holds at most 650MB of laser that has a shorter wavelength than the CD’s data, but can contain as much as 742MB of music. laser. This allows pits and land to be closer together and the spiral track to be wound tighter. 5 6 1 CSC3501 - S.J. Park CSC3501 - S.J. Park 7.7 Optical Disks 7.7 Optical Disks A shorter wavelength light can read and write bytes Blu-Ray was developed by a consortium of nine in greater densities than can be done by a longer companies that includes Sony, Samsung, and wavelength laser. Pioneer. Maximum capacity of a single layer Blu-Ray disk is This is one reason that DVD’s density is greater 25GB. than that of CD. The manufacture of blue-violet lasers can now be HD-DVD was developed under the auspices of the DVD Forum with NEC and Toshiba leading the effort. done economically, bringing about the next generation of laser disks. Maximum capacity of a single layer HD-DVD is 15GB. The big difference between the two is that HD-DVD is Two incompatible formats, HD-CD and Blu-Ray, are backward compatible with red laser DVDs, and Blu- competing for market dominance. Ray is not. 7 8 CSC3501 - S.J. Park CSC3501 - S.J. Park 7.7 Optical Disks 7.9 RAID Blue-violet laser disks have also been designed for RAID, an acronym for Redundant Array of use in the data center. Independent Disks was invented to address The intention is to provide a means for long term problems of disk reliability, cost, and performance. data storage and retrieval. In RAID, data is stored across many disks, with Two types are now dominant: extra disks added to the array to provide error Sony’s Professional Disk for Data (PDD) that can correction (redundancy). store 23GB on one disk and The inventors of RAID, David Patterson, Garth Plasmon’s Ultra Density Optical (UDO) that can hold Gibson, and Randy Katz, provided a RAID up to 30GB. taxonomy that has persisted for a quarter of a It is too soon to tell which of these technologies will century, despite many efforts to redefine it. emerge as the winner. 9 10 CSC3501 - S.J. Park CSC3501 - S.J. Park 7.9 RAID 7.9 RAID RAID Level 0, also known as drive spanning, RAID Level 1, also known as disk mirroring, provides improved performance, but no redundancy. provides 100% redundancy, and good performance. Data is written in blocks across the entire array Two matched sets of disks contain the same data. The disadvantage of RAID 0 is in its low reliability. The disadvantage of RAID 1 is cost. 11 12 2 CSC3501 - S.J. Park CSC3501 - S.J. Park 7.9 RAID 7.9 RAID A RAID Level 2 configuration consists of a set of data RAID Level 3 stripes bits across a set of data drives drives, and a set of Hamming code drives. and provides a separate disk for parity. Hamming code drives provide error correction for the data Parity is the XOR of the data bits. drives. RAID 3 is not suitable for commercial applications, but is good for personal systems. RAID 2 performance is poor and the cost is relatively high. 13 14 CSC3501 - S.J. Park CSC3501 - S.J. Park 7.9 RAID 7.9 RAID RAID Level 4 is like adding parity disks to RAID 0. RAID Level 5 is RAID 4 with distributed parity. Data is written in blocks across the data disks, and a parity With distributed parity, some accesses can be serviced block is written to the redundant drive. concurrently, giving good performance and high reliability. RAID 4 would be feasible if all record blocks were the RAID 5 is used in many commercial systems. same size. 15 16 CSC3501 - S.J. Park CSC3501 - S.J. Park 7.9 RAID 7.9 RAID RAID Level 6 carries two levels of error protection Double parity RAID (RAID DP) employs pairs of over- over striped data: Reed-Soloman and parity. lapping parity blocks that provide linearly independent parity functions. It can tolerate the loss of two disks. RAID 6 is write-intensive, but highly fault-tolerant. 17 18 3 CSC3501 - S.J. Park CSC3501 - S.J. Park 7.9 RAID 7.9 RAID Like RAID 6, RAID DP can tolerate the loss of two Large systems consisting of many drive arrays disks. may employ various RAID levels, depending on The use of simple parity functions provides RAID the criticality of the data on the drives. DP with better performance than RAID 6. A disk array that provides program workspace (say for file sorting) does not req uire hig h fault tolerance. Of course, b ecause t wo parit y func tions are involved, RAID DP’s performance is somewhat Critical, high-throughput files can benefit from degraded from that of RAID 5. combining RAID 0 with RAID 1, called RAID 10. Keep in mind that a higher RAID level does not RAID DP is also known as EVENODD, diagonal parity RAID, RAID 5DP, advanced data guarding necessarily mean a “better” RAID level. It all RAID (RAID ADG) and-- erroneously-- RAID 6. depends upon the needs of the applications that use the disks. 19 20 CSC3501 - S.J. Park CSC3501 - S.J. Park 7.10 The Future of Data Storage 7.10 The Future of Data Storage Advances in technology have defied all efforts to As data densities increase, bit cells consist of define the ultimate upper limit for magnetic disk proportionately fewer magnetic grains. storage. There is a point at which there are too few grains to In the 1970s, the upper limit was thought to be hold a value, and a 1 might spontaneously change around 2Mb/in2. to a 0, or vice versa. Today’s disks commonly support 20Gb/in2. This point is called the superparamagnetic limit. In 2006, the superparamagnetic limit is thought to lie Improvements have occurred in several different between 150Gb/in2 and 200Gb/in2 . technologies including: Even if this limit is wrong by a few orders of Materials science magnitude, the greatest gains in magnetic storage Magneto-optical recording heads. have probably already been realized. Error correcting codes. 21 22 CSC3501 - S.J. Park CSC3501 - S.J. Park 7.10 The Future of Data Storage 7.10 The Future of Data Storage Future exponential gains in data storage most Present day biological data storage systems likely will occur through the use of totally new combine organic compounds such as proteins or oils technologies. with inorganic (magentizable) substances. Early prototypes have encouraged the expectation Research into finding suitable replacements for that densities of 1Tb/in2 are attainable. magnetic disks is taking place on several fronts. Of course, the ultimate biological data storage Some of the more interesting technologies include: medium is DNA. Biological materials Trillions of messages can be stored in a tiny strand of DNA. Holographic systems and Micro-electro-mechanical devices. Practical DNA-based data storage is most likely decades away. 23 24 4 CSC3501 - S.J. Park CSC3501 - S.J. Park 7.10 The Future of Data Storage 7.10 The Future of Data Storage Micro-electro-mechanical storage (MEMS) Millipede consists of thousands of cantilevers that record devices offer another promising approach to a binary 1 by pressing a heated tip into a polymer mass storage. substrate. IBM’s Millipede is one such device. • The tip reads a bin Pro to types have ac hieve d dens ities o f 100Gb/in2 ary 1 when it dips i with 1Tb/in2 expected as the technology is nto the imprint in th refined. e polymer A photomicrograph of Millipede is shown on the next sli de.
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