Secondary Storage Management

1. Secondary Storage Management The Memory Hierarchy

2. The Memory Hierarchy • Computer systems have several different components in which data may be stored. • Data capacities & access speeds range over at least seven orders of magnitude • Devices with smallest capacity also offer the fastest access speed • The term memory hierarchy is used in computer architecture when discussing performance issues in computer architectural design, algorithm predictions, • The lower level programming constructs such as involving locality of reference. 

3. Description of Levels • Cache • Megabyte or more of Cache storage. • On-board cache : On same chip. • Level-2 cache : On another chip. • Cache data accessed in few nanoseconds. • Data moved from main memory to cache when needed by processor • Volatile

4. Description of Levels 2. Main Memory • 1 GB or more of main memory. • Instruction execution & Data Manipulation - involves information resident in main memory. • Time to move data from main memory to the processor or cache is in the 10-100 nanosecond range. • Volatile 3. Secondary Storage • Typically a magnetic disk. • Capacity upto 1 TB. • One machine can have several disk units. • Time to transfer a single byte between disk & main memory is around 10 milliseconds.

5. Description of Levels • Tertiary Storage • Holds data volumes measured in terabytes. • As capacious as a collection of disk units can be, there are databases much larger than what can be stored on the disk(s) of a single machine, or even several machines. • Significantly higher read/write times. • Tertiary storage is characterized by significantly higher read/write times than secondary storage • Smaller cost per bytes. • Retrieval takes seconds or minutes, but capacities in the petabyte range are possible.

6. Transfer of Data Between Levels • Data moves between adjacent levels of the hierarchy. • Each level is organized to transfer large amounts of data to or from the level below • Key technique for speeding up database operations is to arrange data so that when one piece of a disk block is needed • It is likely that other data on the same block will also be needed at about the same time.

7. Volatile & Non Volatile Storage • A volatile device “forgets” what is stored in it when the power goes off. • Example: Main Memory • A nonvolatile device, on the other hand, is expected to keep its contents intact even for long periods when the device is turned off or there is a power failure. • Example: Secondary & Tertiary Storage Note: No change to the database can be considered final until it has migrated to nonvolatile, secondary storage.

8. Virtual Memory • Managed by Operating System. • Typical software executes in virtual-memory, an address space that is typically 32 bits; • There are 232 bytes, or 4 gigabytes, in a virtual memory. • Some memory in main memory & rest on disk. • Transfer between the two is in units of disk blocks (pages). • Not a level of the memory hierarchy

9. Thank you!

11. Section 13.2 – Secondary storage management CS-257 Database System Principles Avinash Anantharamu (102) 008629907

12. Index • 13.2 Disks • 13.2.1 Mechanics of Disks • 13.2.2 The Disk Controller • 13.2.3 Disk Access Characteristics

13. Structure of a Disk

14. Mechanics of Disks • Two principal moving pieces of hard drive 1- Head Assembly 2- Disk Assembly • Disk Assembly has 1 or more circular platters that rotate around a central spindle. • Platters are covered with thin magnetic material • The upper and lower surfaces of the platters are covered with a thin layer of magnetic material,on which bits are stored. • 0’s and l ’s are represented by different patterns in the magnetic material. • A common diameter for disk platters is 3.5 inches, although disks with diameters from an inch to several feet have been built.

15. Top View of Disk Surface

16. Mechanics of Disks • Tracks are concentric circles on a platter. • The two principal moving pieces of a disk drive - disk assembly and a head assembly. • The disk is organized into tracks, • Tracks are organized into sectors which are segments of circular platter. • In 2008, a typical disk has about 100,000 tracks per inch but stores about a million bits per inch along the tracks. • Sectors are indivisible as far as errors are concerned. • Blocks are logical data transfer units.

17. Disk Controller • Control the actuator to move head assembly • Selecting the surface from which to read or write • Transfer bits from desired sector to main memory • buffering an entire track or more in local memory of the disk controller • additional accesses to the disk can be avoided.

18. Simple Single Processor Computer

19. Disk Access characteristics • Seek time • Rotational latency • Transfer time • Latency of the disk.

20. Thank you

21. 13.3 Accelerating Access to Secondary Storage San Jose State University Spring 2012

22. 13.3 Accelerating Access to Secondary StorageSection Overview • 13.3.1: The I/O Model of Computation • 13.3.2: Organizing Data by Cylinders • 13.3.3: Using Multiple Disks • 13.3.4: Mirroring Disks • 13.3.5: Disk Scheduling and the Elevator Algorithm • 13.3.6: Prefetching and Large-Scale Buffering

23. 13.3 Introduction • Average block access is ~10ms. • Disks may be busy. • Requests may outpace access delays, leading to infinite scheduling latency. • There are various strategies to increase disk throughput. • The “I/O Model” is the correct model to determine speed of database operations • the scheduling latency becomes infinite.

24. 13.3 Introduction (Contd.) • Actions that improve database access speed: • Place blocks closer, within the same cylinder • Increase the number of disks • Mirror disks • Use an improved disk-scheduling algorithm • Use prefetching • improve the throughput

25. 13.3.1 The I/O Model of Computation • If we have a computer running a DBMS that: • Is trying to serve a number of users • Has 1 processor, 1 disk controller, and 1 disk • Each user is accessing different parts of the DB • It can be assumed that: • Time required for disk access is much larger than access to main memory; and as a result: • The number of block accesses is a good approximation of time required by a DB algorithm

26. 13.3.2 Organizing Data by Cylinders • It is more efficient to store data that might be accessed together in the same or adjacent cylinder(s). • In a relational database, related data should be stored in the same cylinder. • we can approach the theoretical transfer rate for moving data on or off the disk.

27. 13.3.3 Using Multiple Disks • If the disk controller supports the addition of multiple disks and has efficient scheduling, using multiple disks can improve performance significantly • By striping a relation across multiple disks, each chunk of data can be retrieved in a parallel fashion, improving performance by up to a factor of n, where n is the total number of disks the data is striped over • The disk controller, bus, and main memorycan handle n times the data-transfer rate, • n disks will have approximately the performance of one disk that operates n times as fast.

28. 13.3.4 Mirroring Disks • A drawback of striping data across multiple disks is that you increase your chances of disk failure. • To mitigate this risk, some DBMS use a disk mirroring configuration • Disk mirroring makes each disk a copy of the other disks, so that if any disk fails, the data is not lost • Since all the data is in multiple places, access speedup can be increased by more than n since the disk with the head closest to the requested block can be chosen

29. 13.3.4 Mirroring Disks

30. 13.3.5 Disk Scheduling • One way to improve disk throughput is to improve disk scheduling, prioritizing requests such that they are more efficient • The elevator algorithm is a simple yet effective disk scheduling algorithm • The algorithm makes the heads of a disk oscillate back and forth similar to how an elevator goes up and down • The access requests closest to the heads current position are processed first

31. 13.3.5 Disk Scheduling • When sweeping outward, the direction of head movement changes only after the largest cylinder request has been processed • When sweeping inward, the direction of head movement changes only after the smallest cylinder request has been processed • Example:

32. 13.3.6 Prefetching and Large-Scale Buffering • In some cases we can anticipate what data will be needed • We can take advantage of this by prefetching data from the disk before the DBMS requests it • Since the data is already in memory, the DBMS receives it instantly

33. ? Questions ?

34. Disk Failures Presented by Timothy Chen Spring 2013

35. Index • 13.4 Disk Failures 13.4.1 Intermittent Failures 13.4.2 Organizing Data by Cylinders 13.4.3 Stable Storage 13.4.4 Error- Handling Capabilities of Stable Storage 13.4.5 Recovery from Disk Crashes 13.4.6 Mirroring as a Redundancy Technique 13.4.7 Parity Blocks 13.4.8 An Improving: RAID 5 13.4.9 Coping With Multiple Disk Crashers

36. Intermittent Failures • If we try to read the sector but the correct content of that sector is not delivered to the disk controller • with repeated tries we are able to read or write successfully. • Controller will check good and bad sector • If the write is correct: Read is performed • Good sector and bad sector is known by the read operation • The controller may attempt to write a sector, but the contents of the sector are not what was intended. • We assume the write was correct, and if the sector read is bad, then the write was apparently unsuccessful and must be repeated.

37. CheckSum • Read operation that determine the good or bad status • If, on reading, we find that the checksum is not proper for the data bits, then we know there is an error in reading. • If the checksum is proper, there is still a small chance that the block was not read correctly, but by using many checksum bits we can make the probability of missing a bad read arbitrarily small.

38. How CheckSum perform • Each sector has some additional bits • A simple form of checksum is based on the parity of all the bits in the sector. • Set depending on the values of the data bits stored in each sector • If the data bit in the not proper we know there is an error reading • Odd number of 1: bits have odd parity(01101000) • Even number of 1: bit have even parity (111011100) • Find Error is the it is one bit parity

39. Stable Storage • Deal with disk error • Sectors are paired, and each pair represents one sector-contents X . • Sectors are paired and each pair X showing left and right copies as Xl and Xr • It check the parity bit of left and right by substituting spare sector of Xl and Xr until the good value is returned • Assume that if the read function returns a good value w for either X l or X r , then w is the true value of X .

40. Error-Handling Capabilities of Stable Storage • Since it has XL and XR, one of them fail we can still read other one • Chance both of them fail are pretty small • The write Fail, it happened during power outage • Media Failure • Write Failure • The failure occurred as we were writing XL • The failure occurred after we wrote XL

41. Recover Disk Crash • The most serious mode of failure for disks is “head crash” where data permanently destroyed. • This situation represents a disaster for many DBMS applications, such as banking and other financial applications. • The way to recover from crash , we use RAID method • RAID- Redundant Arrays of Independent Disks.

42. Mirroring as a Redundancy Technique • it is call Raid 1 • Just mirror each disk • Mirroring, as a protection against data loss, is often referred to as RAID level 1. • Essentially, with mirroring and the other redundancy schemes we discuss, the only way data can be lost is if there is a second disk crash while the first crash is being repaired.

43. Raid 1 graph

44. Parity Block • It often call Raid 4 technical • read block from each of the other disks and modulo-2 sum of each column and get redundant disk disk 1: 11110000 disk 2: 10101010 disk 3: 00111000 get redundant disk 4(even 1= 0, odd 1 =1) disk 4: 01100010

45. Raid 4 graphic

46. Parity Block- Fail Recovery • It can only recover one disk fail • If it has more than one like two disk • Then it can’t be recover us modulo-2 sum • If the failed disk is one of the data disks, then we need to swap in a good disk and recompute its data from the other disks.

47. An Improvement Raid 5

48. Coping with multiple Disk Crash • For more one disk fail • Either raid 4 and raid 5 can’t be work • So we need raid 6 • It is need at least 2 redundant disk

49. Raid 6

50. Reference • http://www.definethecloud.net/wp-content/uploads/2010/12/325px-RAID_1.svg_.png • http://en.wikipedia.org/wiki/RAID