CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst Storage Systems

CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst 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 / ( (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

CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst Magnetic Disk Physically consist of –Tracks –Sectors –Platters –Heads –Cylinders Performance characteristics are: –Rotational latency (delay) – time to rotate to the required sector (RPM) –Seek time – time to find the required track –Transfer rate – time to transfer the data

CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst Magnetic Disk Layout Platter Track Platters Sectors Tracks COPYRIGHT 1998 MORGAN KAUFMANN PUBLISHERS, INC. ALL RIGHTS RESERVED

CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst 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

CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst 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 bytes / (150 MB / sec) = (½ * 60) / / (150 * 2 20 ) = = sec = ms Suppose the rotation rate was 5400 rpm? Avg Access time = (½ * 60) / = = ms

CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst 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

CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst RAID Provide a set of physical disks that appear to be a single logical drive Distribute the data across the drives in the array Allow levels of redundancy to permit recovery of failed disks 7 (basic) levels of RAID (RAID0 – RAID6) RAID is NOT a backup system! –Its made to maintain uptime through failures

CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst RAID0 No redundancy (therefore not really RAID at all) Allocate sectors (or larger units) from a single logical drive in stripes across multiple physical drives PhysicalDrive Physical Drive Physical Drive Physical Drive LogicalDrive stripes

CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst 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

Physical Drive Physical Drive PhysicalDrive Physical Drive RAID1 Mirrored data Make a complete mirror (i.e. duplicate) of all data PhysicalDrive Physical Drive Physical Drive Physical Drive mirrors

CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst 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)

CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst RAID2 Redundancy through Hamming codes Store enough extra data in order to detect and correct errors, or in order to provide availability in the case of a failed drive Stripes are small  1 bit (!) per stripe originally  later bytes/words All disk heads are synchronized – does not permit parallel access as in RAID0 and RAID1 Requires  log 2 (#disks) extra disks to implement

CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst Hamming Codes A technique for enabling error detection & correction (and therefore redundancy as well) Ex: ECC Scheme: –store 1110, but suppose an error changes this to Even parity bit Wrong parity bit! Error in data Error detected

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

CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst 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

CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst 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

CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst RAID3 Summary Small stripes (byte or word) interleaved across disks Parity = XOR of all bits in the byte / word Advantage: –All disks participate in all data accesses  high transfer rate –If a single drive fails, lost bit (stripe) can be reconstructed easily Disadvantage: –Only 1 access per “time” –Can detect / correct only single failures

CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst Physical Drive 4 P(0-3) P(4-7) P(8-11) RAID4 Block level parity Same parity scheme as RAID3 but uses large blocks per stripe Physical Drive 0 Block0 Block4 Block8 Physical Drive 1 Block1 Block5 Block9 Physical Drive 2 Block2 Block6 Block10 Physical Drive 3 Block3 Block7 Block11 Stripes of large “blocks” Parity drive

CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst 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

CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst 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

CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst 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

CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst RAID6 Summary AKA “P + Q Redundancy” Same as RAID5 (interleaved blocks and interleaved parity) –But has a second set of parity bits Advantage: –Same as RAID5 –Plus the ability to tolerate a second failure (either disk or operator) Disadvantage: –Takes one more extra drive worth of space (more expensive) In case you want to be extra careful

CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst RAID Systems – Failure and Recovery RAID1 and RAID5 are the most common in practice –RAID1  read performance/redundancy –RAID5  cost/redundancy (performance not bad either) If a RAID system loses a disk, how do you fix it? –When a disk fails, we’ve entered a “danger zone” where one more error causes data loss Repairs need to be made quickly to minimize this time –Hot-swappable drives are doable, but expensive/difficult –Some systems use standby spares which are automatically used as a replacement when an error occurs Once a new disk is provided, the RAID system will reconstruct the data from redundancy or parity information –All done during uptime

CS 352 : Computer Organization and Design University of Wisconsin-Eau Claire Dan Ernst What RAID Can and Cannot Do RAID Can protect uptime –Downtime = $$$ RAID Can improve performance on certain applications –Anytime you’re accessing/moving around large files, striping gives you lots of access bandwidth RAID Cannot protect the data on the array! –One file system  single point of failure –Corruption, Virus, Fire, Flood, etc. – Not an alternative to a separate backup! RAID Cannot improve performance on all applications