Database Systems II Secondary Storage CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 29 The Memory Hierarchy Swapping, Main-memory DBMS s Tertiary Storage: Tape, Network Backup 3,200 MB/s (DDR-SDRAM @200MHz) 6,400 MB/s 12,800 MB/s (DDR2, dual channel, 800MHz) Virtual Memory Disk Disk-Cache (2 16MB) Main Memory CPU File System L1/L2-Cache (256KB 4MB) 300 MB/s (SATA-300) 16 GB/s (64bit@2GHz) CPU-to-Main-Memory: ~200 cycles latency CPU-to-L1-Cache: ~5 cycles initial latency, then burst mode CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 30
The Memory Hierarchy Cache Data and instructions in cache when needed by CPU. On-board (L1) cache on same chip as CPU, L2 cache on separate chip. Capacity ~ 1MB, access time a few nanoseconds. Main memory All active programs and data need to be in main memory. Capacity ~ 1 GB, access time 10-100 nanoseconds. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 31 The Memory Hierarchy Secondary storage Secondary storage is used for permanent storage of large amounts of data, typically a magnetic disk. Capacity up to 1 TB, access time ~ 10 milliseconds. Tertiary storage To store data collections that do not fit onto secondary storage, e.g. magnetic tapes or optical disks. Capacity ~ 1 PB, access time seconds / minutes. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 32
The Memory Hierarchy Trade-off The larger the capacity of a storage device, the slower the access (and vice versa). A volatile storage device forgets its contents when power is switched off, a non-volatile device remembers its content. Secondary storage and tertiary storage is nonvolatile, all others are volatile. DBS needs non-volatile (secondary) storage devices to store data permanently. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 33 The Memory Hierarchy RAM (main memory) for subset of database used by current transactions. Disk to store current version of entire database (secondary storage). Tapes for archiving older versions of the database (tertiary storage). CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 34
The Memory Hierarchy Typically programs are executed in virtual memory of size equal to the address space of the processor. Virtual memory is managed by the operating system, which keeps the most relevant part in the main memory and the rest on disk. A DBS manages the data itself and does not rely on the virtual memory. However, main memory DBS do manage their data through virtual memory. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 35 Moore s Law Gordon Moore in 1965 observed that the density of integrated circuits (i.e., number of transistors per unit) increased at an exponential rate, thus roughly doubles every 18 months. Parameters that follow Moore s law: - number of instructions per second that can be exceuted for unit cost, - number of main memory bits that can be bought for unit cost, - number of bytes on a disk that can be bought for unit cost. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 36
Moore s Law Number of transistors on an integrated circuit CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 37 Moore s Law Disk capacity CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 38
Moore s Law But some other important hardware parameters do not follow Moore s law and grow much slower. Theses are, in particular, - speed of main memory access, and - speed of disk access. For example, disk latencies (seek times) have almost stagnated for past 5 years. Thus, moving data from one level of the memory hierarchy to the next becomes progressively larger. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 39 Disks Secondary storage device of choice. Data is stored and retrieved in units called disk blocks or pages. Main advantage over tapes: random access vs. sequential access. Unlike RAM, time to retrieve a disk page varies depending upon location on disk. Therefore, relative placement of pages on disk has major impact on DBMS performance! CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 40
Disks Disk consists of two main, moving parts: disk assembly and head assembly. Disk assembly stores information, head assembly reads and writes information. Arm assembly Disk head Arm movement Spindle Tracks Platters CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 41 Disks The platters rotate around central spindle. Upper and lower platter surfaces are covered with magnetic material, which is used to store bits. The arm assembly is moved in or out to position a head on a desired track. All tracks under heads at the same time make a cylinder (imaginary!). Only one head reads/writes at any one time. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 42
Disks Sector Track Top view of a platter surface Gap CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 43 Disks Block size is a multiple of sector size (which is fixed). Time to access (read/write) a disk block (disk latency) consists of three components: - seek time: moving arms to position disk head on track, - rotational delay (waiting for block to rotate under head), and - transfer time (actually moving data to/from disk surface). Seek time and rotational delay dominate. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 44
Disks Time Seek time 3 or 5x x 1 N Cylinders Traveled CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 45 Disks Average seek time S = N N i=1 j=1 j i SEEKTIME (i j) N(N-1) Typical average seek time = 5 ms CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 46
Disks Average rotational delay Head Here Block I Want Average rotational delay R = 1/2 revolution Typical R = 5 ms CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 47 Disks Transfer time Typical transfer rate: 100 MB/sec Typical block size: 16KB Transfer time: block size transfer rate Typical transfer time = 0.16 ms CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 48
Disks Typical average disk latency is 10 ms, maximum latency 20 ms. In 10 ms, a modern microprocessor can execute millions of instructions. Thus, the time for a block access by far dominates the time typically needed for processing the data in memory. The number of disk I/Os (block accesses) is a good approximation for the cost of a database operation. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 49 Accelerating Disk Access Organize data by cylinders to minimize the seek time and rotational delay. Next block concept: - blocks on same track, followed by - blocks on same cylinder, followed by - blocks on adjacent cylinder. Blocks in a file are placed sequentially on disk (by next ). Disk latency can approach the transfer rate. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 50
Accelerating Disk Access Example Assuming 10 ms average seek time, no rotational delay, 40 MB/s transfer rate. Read a single 4 KB Block Random I/O: 10 ms Sequential I/O: 10 ms Read 4 MB in 4 KB Blocks (amortized) Random I/O: 10 s Sequential I/O: 0.1 s Speedup factor of 100 CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 51 Accelerating Disk Access Block size selection Bigger blocks amortize I/O cost. Bigger blocks read in more useless stuff and takes longer to read. Good trade-off block size from 4KB to 16 KB. With decreasing memory costs, blocks are becoming bigger! CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 52
Accelerating Disk Access Using multiple disks Replace one disk (with one independent head) by many disks (with many independent heads). Striping a relation R: divide its blocks over n disks in a round robin fashion. Assuming that disk controller, bus and main memory can handle n times the transfer rate, striping a relation across n disks can lead to a speedup factor of up to n. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 53 Accelerating Disk Access Disk scheduling For I/O requests from different processes, let the disk controller choose the processing order. According to the elevator algorithm, the disk controller keeps sweeping from the innermost to the outermost cylinder, stopping at a cylinder for which there is an I/O request. Can reverse sweep direction as soon as there is no I/O request ahead in the current direction. Optimizes the throughput and average response time. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 54
Accelerating Disk Access Double buffering In some scenarios, we can predict the order in which blocks will be requested from disk by some process. Prefetching (double buffering) is the method of fetching the necessary blocks into the buffer in advance. Requires enough buffer space. Speedup factor up to n, where n is the number of blocks requested by a process. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 55 Accelerating Disk Access (1) Read B1 Buffer (2) Process Data in Buffer Single buffering (3) Read B2 Buffer (4) Process Data in Buffer... Execution time = n(p+r) where P = time to process one block R = time to read in one block n = # blocks read. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 56
Accelerating Disk Access Double buffering (1) Read B1,..., Bn Buffer (2) Process B1 in Buffer (3) Process B2 in Buffer... Execution time = R + np as opposed to n(p+r). remember that R >> P CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 57 Disk Failures In an intermittent failure, a read or write operation is unsuccessful, but succeeds with repeated tries. parity checks to detect intermittent failures Media decay is a permanent corruption of one or more bits which make the corresponding sector impossible to read / write. stable storage to recover from media decay A disk crash makes the entire disk permanently unreadable. RAID to recover from disk crashes CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 58
Disk Failures Checksums Add n parity bits every m data bits. The number of 1 s among a collection of bits and their parity bit is always even. The parity bit is the modulo-2 sum of its data bits. m=8, n=1 Block A: 01101000:1 Block B: 11101110:0 If Block A instead contains Block A : 01100000:1 error detected (odd # of 1 s) (even # of 1 s) (has odd # of 1 s) CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 59 Disk Failures Checksums But what if multiple bits are corrupted? E.g., if Block A instead contains Block A : 01000000:1 (has even # of 1 s) error cannot be detected Probability that a single parity bit cannot detect a corrupt block is ½. This is assuming that the probability of disk failures involving an odd / even number of bits is identical. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 60
Disk Failures Checksums More parity bits decrease the probability of an undetected failure. With n m independent parity bits, this probability is only 1/2 n. E.g., we can have eight parity bits, one for the first bit of every byte, the second one for the second bit of every byte... The chance for not detecting a disk failure is then only 1/256. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 61 Disk Failures Stable storage Sectors are paired, and information X is written both on sectors X l and X r. Assume that both copies are written with a sufficient number of parity bits so that bad sectors can be detected. If sector is bad (according to checksum), write to alternative sector. Alternate reading X l and X r until a good value is returned. Probability of X l and X r both failing is very low. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 62
Disk Failures Disk arrays So far, we cannot recover from disk crashes. To address this problem, use Redundant Arrays of Independent Disks (RAID), arrangements of several disks that gives abstraction of a single, large disk. Goals: Increase reliability (and performance). Redundant information allows reconstruction of data if a disk fails. Data striping improves the disk performance. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 63 Disk Failures Failure Models for Disks What is the expected time until disk crash? We assume uniform distribution of failures over time. Mean time to failure: time period by which 50% of a population of disks have failed (crashed). Typical mean time to failure is 10 years. In this case, 5% of disks crash in the first year, 5% crash in the second year,..., 5% crash in the tenth year,..., 5% crash in the twentieth year. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 64
Disk Failures Failure Models for Disks Given the mean time to failure (mtf) in years, we can derive the probability p of a particular disk failing in a given year. p = 1 / (2 * mtf) Ex.: mtf = 10, p = 1/20 = 5% Mean time to data loss: time period by which 50% of a population of disks have had a crash that resulted in data loss. The mean time to disk failure is not necessarily the same as the mean time to data loss. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 65 Disk Failures Failure Models for Disks Failure rate: percentage of disks of a population that have failed until a certain point of time. Survival rate: percentage of disks of a population that have not failed until a certain point of time. While it simplifies the analysis, the assumption of uniform distribution of failures is unrealistic. Disks tend to fail early (manufacturing defects that have not been detected) or late (wear-andtear). CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 66
Disk Failures Failure Models for Disks Survival rate (realistic) time CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 67 Disk Failures Mirroring The data disk is copied unto a second disk, the mirror disk. When one of the disk crashes, we replace it by a new disk and copy the other disk to the new one. Data loss can only occur if the second disk crashes while the first one is being replaced. This probability is negligible. Mirroring is referred to as RAID level 1. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 68
Disk Failures Parity blocks Mirroring doubles the number of disks needed. The parity block approach needs only one redundant disk for n (arbitray) data disks. In the redundant disk, the ith block stores parity checks for the ith blocks of all the n data disks. A B C P Parity block approach is called RAID level 4. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 69 Disk Failures Parity blocks Reading blocks is the same as without parity blocks. When writing a block on a data disk, we also need to update the corresponding block of the redundant disk. This can be done using four (three additional) disk I/O: read old value of data disk block, read corresponding block of redundant (parity) disk, write new data block, recompute and write new redundant block. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 70
Disk Failures Parity blocks If one of the disks crashes, we bring in a new disk. The content of this disk can be computed, bit by bit, using the remaining n disks. No difference between data disks and parity disk. Computation based on the definition of parity, i.e. total number of ones is even. CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 71 Disk Failures n = 3 data disks Disk 1, block 1: 11110000 Disk 2, block 1: 10101010 Disk 3, block 1: 00111000 and one parity disk Disk 4, block 1: 01100010 Example Sum over each column is always an even number of 1 s Mod-2 sum can recover any missing single row CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 72
Disk Failures Suppose we have: Example Disk 1, block 1: 11110000 Disk 2, block 1:???????? Disk 3, block 1: 00111000 Disk 4, block 1: 01100010 (parity) Use mod-2 sums for block 1 over disks 1,3,4 to recover block 1 of failed disk 2: Disk 2, block 1: 10101010 CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 73 Disk Failures RAID level 5 In the RAID 4 scheme, the parity disk is the bottleneck. On average, n-times as many writes on the parity disk as on the data disks. However, the failure recovery method does not distinguish the types of the n + 1 disks. RAID level 5 does not use a fixed parity disk, but use block i of disk j as redundant if i MOD n+1 = j. A B C D CMPT 454, Simon Fraser University, Fall 2009, Martin Ester 74