Understanding Storage Systems: Magnetic Disks, RAID, and Performance, Slides of Computer Aided Design (CAD)

Download Understanding Storage Systems: Magnetic Disks, RAID, and Performance and more Slides Computer Aided Design (CAD) in PDF only on Docsity! Storage Systems Docsity.com Why Worry About Storage Systems • Response time = CPU time + I/O time • Suppose 95% of work is done in the CPU, 5% is I/O – If the CPU is improved by a factor of 100, what is the Speedup? – Sp = 1 / (1 - 0.95 + (0.95 / 100)) = 16.8 – 83.2% of the improvement is squandered due to poor I/O • Future performance gains must consider faster I/O • Amdahl’s Law  All parts of a system need to be improved somewhat equally Docsity.com Zone Bit Recording • Originally, all tracks had the same number of sectors – Each sector = 512 Bytes • Inefficient! Limited by the density of the smallest tracks • Outer tracks can hold more sectors (store more data) than inner tracks  called Zone Bit Recording (ZBR) • The sequence of information recorded on each sector is – sector number – gap – data including error correction code bits – gap Docsity.com Disk Performance • What is the average time to read / write a 512 byte sector given: – Avg seek time = 9 ms – Rotation = 7200 rpm – Transfer rate = 150 MB / sec (e.g. serial ATA) Avg access time = seek time + rotational delay + transfer time = 9 ms + (½ rev) / 7200 rpm + 512 bytes / (150 MB / sec) = .009 + (½ * 60) / 7200 + 512 / (150 * 220) = .009 + .004167 + .000003 = .01317 sec = 13.17 ms • Suppose the rotation rate was 5400 rpm? Avg Access time = .009 + (½ * 60) / 5400 + 0.000003 = 9 + 5.556 + 0.003 = 14.56 ms Docsity.com Disk Reliability vs Availability • In processing, our main concern is performance (and cost) • In I/O, our main concern is reliability (and cost) • Reliability  is anything broken? • Availability  is the system still available to the user, even if it is broken? • RAID technology is designed to provide increased availability for potentially unreliable devices • RAID – Redundant Array of Inexpensive Disks – Patterson / Katz / Gibson -- 1987 Docsity.com RAID0 Summary • Called striping the data – Each stripe is  1 sector • Advantage: – access to large sections of contiguous data can be done in parallel over all disks • Disadvantage: – no redundancy Docsity.com Physical Drive 6 2 6 10 Physical Drive 5 1 5 9 PhysicalDrive 4 0 4 8 Physical Drive 7 3 7 11 RAID1 • Mirrored data • Make a complete mirror (i.e. duplicate) of all data PhysicalDrive 0 0 4 8 Physical Drive 1 1 5 9 Physical Drive 2 2 6 10 Physical Drive 3 3 7 11 mirrors Docsity.com RAID1 Summary • Uses stripes (of sectors) across drives just like RAID0 • Replicates each disk with an exact copy • Advantage: – Allows parallel access (just like RAID0) – Failure of any number of drives does not impact availability • Disadvantage: – Writes must occur over both “arrays” – Very expensive (2 x disks) Docsity.com Physical Drive 5 0xxx xxxx Physical Drive 4 1xxx xxxx Physical Drive 6 0xxx xxxx RAID2 Disk Layout Physical Drive 0 1xxx xxxx Physical Drive 1 1xxx xxxx Physical Drive 2 1xxx xxxx Physical Drive 3 0xxx xxxx Parity bits Data Stripes of bytes or words Docsity.com RAID2 Summary • Error correction is done across disks • Advantage: – Useful in a high failure environment • Disadvantage: – Expensive – Modern disks do not exhibit high failure rate • Not used in practice Docsity.com Physical Drive 4 1xxx xxxx RAID3 • Bit interleaved parity • Like RAID2 but use only 1 parity drive Physical Drive 0 1xxx xxxx Physical Drive 1 1xxx xxxx Physical Drive 2 1xxx xxxx Physical Drive 3 0xxx xxxx Stripes of bytes or words Parity drive Docsity.com RAID4 Summary • Interleaved blocks across disks • Advantage: – Allows independent access due to large stripes  can support multiple independent reads – Error detection / correction supported up to single bit errors • Disadvantage: – On parallel writes  penalty incurred since all writes require access to the same parity disk to update the parity • Not used in practice Docsity.com Physical Drive 4 P(0-3) Block7 Block11 RAID5 • Block level distributed parity • Same scheme as RAID4 but parity is interleaved with the data Physical Drive 0 Block0 Block4 Block8 Physical Drive 1 Block1 Block5 Block9 Physical Drive 2 Block2 Block6 P(8-11) Physical Drive 3 Block3 P(4-7) Block10 Docsity.com RAID5 Summary • Interleaved blocks across disks and interleaved with data • Advantage: – Can support multiple independent reads as long as access is to independent disks – Can support multiple independent writes as long as write and parity disks are independent – Error detection / correction supported up to single bit errors – Reduces I/O bottleneck • Most common approach used in practice Docsity.com