DBMS 4 PDF

Storage Structure Reference: Database System Concepts – 7th Edition,  Speed with which data can be accessed Cost per unit of data Reliability data loss on power failure or system crash physi...

Storage Structure Reference: Database System Concepts – 7th Edition,  Speed with which data can be accessed Cost per unit of data Reliability data loss on power failure or system crash physical failure of the storage device Classification Can differentiate storage into: of Physical volatile storage: loses contents when power is switched off Storage Media non-volatile storage:  Contents persist even when power is switched off.  Includes secondary and tertiary storage, as well as batter- backed up main-memory. Reference:  Cache – fastest and most costly form of storage; volatile; managed by the computer system hardware. Main memory: fast access (10s to 100s of nanoseconds; 1 nanosecond = 10–9 seconds) generally too small (or too expensive) to store the entire database Physical capacities of up to a few Gigabytes widely used currently Storage Media Capacities have gone up and per-byte costs have decreased steadily and rapidly (roughly factor of 2 every 2 to 3 years) Volatile — contents of main memory are usually lost if a power failure or system crash occurs. Reference:  Flash memory Data survives power failure Data can be written at a location only once, but location can be erased and written to again  Can support only a limited number (10K – 1M) of write/erase cycles.  Erasing of memory has to be done to an entire bank of Physical memory Storage Media Reads are roughly as fast as main memory But writes are slow (few microseconds), erase is slower Widely used in embedded devices such as digital cameras, phones, and USB keys Reference:  Magnetic-disk Data is stored on spinning disk, and read/written magnetically Primary medium for the long-term storage of data; typically stores entire database. Data must be moved from disk to main memory for access, and written back for storage Much slower access than main memory (more on this later) Physical  Storage Media direct-access – possible to read data on disk in any order, unlike magnetic tape Capacities range up to roughly 1.5 TB as of 2009  Much larger capacity and cost/byte than main memory/flash memory  Growing constantly and rapidly with technology improvements (factor of 2 to 3 every 2 years) Survives power failures and system crashes  disk failure can destroy data, but is rare Reference:  Tape storage non-volatile, used primarily for backup (to recover from disk failure), and for archival data sequential-access – much slower than disk very high capacity (40 to 300 GB tapes available) Physical tape can be removed from drive  storage costs much cheaper than Storage Media disk, but drives are expensive Tape jukeboxes available for storing massive amounts of data  hundreds of terabytes (1 terabyte = 109 bytes) to even multiple petabytes (1 petabyte = 1012 bytes) Reference:  Storage Hierarchy Reference:  primary storage: Fastest media but volatile (cache, main memory). secondary storage: next level in hierarchy, non-volatile, moderately fast access time also called on-line storage Storage E.g. flash memory, magnetic disks Hierarchy tertiary storage: lowest level in hierarchy, non-volatile, slow access time also called off-line storage E.g. magnetic tape, optical storage Reference: Magnetic Hard Disk Mechanism Reference:  Multiple disks connected to a computer system through a controller Controllers functionality (checksum, bad sector remapping) often carried out by individual disks; reduces load on controller Disk interface standards families ATA (AT adaptor) range of standards Disk Sub-system SATA (Serial ATA) SCSI (Small Computer System Interconnect) range of standards SAS (Serial Attached SCSI) Several variants of each standard (different speeds and capabilities) Disks usually connected directly to computer system In Storage Area Networks (SAN), a large number of disks are connected by a high- speed network to a number of servers In Network Attached Storage (NAS) networked storage provides a file system interface using networked file system protocol, instead of providing a disk system interface Reference:  Access time – the time it takes from when a read or write request is issued to when data transfer begins. Consists of: Seek time – time it takes to reposition the arm over the correct track.  Average seek time is 1/2 the worst case seek time. – Would be 1/3 if all tracks had the same number of sectors, and we ignore the time to start and stop arm movement  4 to 10 milliseconds on typical disks Rotational latency – time it takes for the sector to be accessed to appear Disk under the head.  Average latency is 1/2 of the worst case latency. Performance  4 to 11 milliseconds on typical disks (5400 to 15000 r.p.m.) Measurement Data-transfer rate – the rate at which data can be retrieved from or stored to the disk. 25 to 100 MB per second max rate, lower for inner tracks Multiple disks may share a controller, so rate that controller can handle is also important  E.g. SATA: 150 MB/sec, SATA-II 3Gb (300 MB/sec)  Ultra 320 SCSI: 320 MB/s, SAS (3 to 6 Gb/sec)  Fiber Channel (FC2Gb or 4Gb): 256 to 512 MB/s Reference:  Mean time to failure (MTTF) – the average time the disk is expected to run continuously without any failure. Typically 3 to 5 years Probability of failure of new disks is quite low, corresponding to a “theoretical MTTF” of 500,000 to 1,200,000 hours for a new disk  E.g.,an MTTF of 1,200,000 hours for a new disk means that Disk given 1000 relatively new disks, on an average one will fail every Performance 1200 hours Measurement MTTF decreases as disk ages Reference:  RAID: Redundant Arrays of Independent Disks disk organization techniques that manage a large numbers of disks, providing a view of a single disk of high capacity and high speed by using multiple disks in parallel, high reliability by storing data redundantly, so that data can be recovered even if a disk fails The chance that some disk out of a set of N disks will fail is much higher than the chance that a specific single disk will fail. RAID E.g., a system with 100 disks, each with MTTF of 100,000 hours (approx. 11 years), will have a system MTTF of 1000 hours (approx. 41 days) Techniques for using redundancy to avoid data loss are critical with large numbers of disks Originally a cost-effective alternative to large, expensive disks I in RAID originally stood for ``inexpensive’’ Today RAIDs are used for their higher reliability and bandwidth. The “I” is interpreted as independent Reference:  Redundancy – store extra information that can be used to rebuild information lost in a disk failure E.g., Mirroring (or shadowing) Duplicate every disk. Logical disk consists of two physical disks. Every write is carried out on both disks Improvement  Reads can take place from either disk of Reliability vs If one disk in a pair fails, data still available in the other Redundancy Mean time to data loss depends on mean time to failure, and mean time to repair E.g. MTTF of 100,000 hours, mean time to repair of 10 hours gives mean time to data loss of 500*106 hours (or 57,000 years) for a mirrored pair of disks (ignoring dependent failure modes) Reference:  Two main goals of parallelism in a disk system: 1. Load balance multiple small accesses to increase throughput 2. Parallelize large accesses to reduce response time. Improve transfer rate by striping data across multiple disks. Bit-level striping – split the bits of each byte across multiple disks In an array of eight disks, write bit i of each byte to disk i. Improvement in Performance Each access can read data at eight times the rate of a single disk. vs Parallelism But seek/access time worse than for a single disk  Bit level striping is not used much any more Block-level striping – with n disks, block i of a file goes to disk (i mod n) + 1 Requests for different blocks can run in parallel if the blocks reside on different disks A request for a long sequence of blocks can utilize all disks in parallel Reference:  Schemes to provide redundancy at lower cost by using disk striping combined with parity bits Different RAID organizations, or RAID levels, have differing cost, performance and reliability characteristics RAID Level 0: Block striping; non-redundant. Used in high-performance applications where data loss is not critical. RAID Levels RAID Level 1: Mirrored disks with block striping Offers best write performance. Popular for applications such as storing log files in a database system. Reference:  RAID Level 2: Memory-Style Error-Correcting-Codes (ECC) with bit striping. RAID Level 3: Bit-Interleaved Parity a single parity bit is enough for error correction, not just detection, since we know which disk has failed  When writing data, corresponding parity bits must also be computed and written to a parity bit disk RAID Levels  To recover data in a damaged disk, compute XOR of bits from other disks (including parity bit disk) Reference:  RAID Level 3 (Cont.) Faster data transfer than with a single disk, but fewer I/Os per second since every disk has to participate in every I/O. Subsumes Level 2 (provides all its benefits, at lower cost). RAID Level 4: Block-Interleaved Parity; uses block-level striping, and keeps a parity block on a separate disk for corresponding blocks from N other disks. RAID Levels When writing data block, corresponding block of parity bits must also be computed and written to parity disk To find value of a damaged block, compute XOR of bits from corresponding blocks (including parity block) from other disks. Reference:  RAID Level 4 (Cont.) Provides higher I/O rates for independent block reads than Level 3  block read goes to a single disk, so blocks stored on different disks can be read in parallel Provides high transfer rates for reads of multiple blocks than no-striping Before writing a block, parity data must be computed  Can be done by using old parity block, old value of current block and new RAID Levels value of current block (2 block reads + 2 block writes)  Or by recomputing the parity value using the new values of blocks corresponding to the parity block – More efficient for writing large amounts of data sequentially Parity block becomes a bottleneck for independent block writes since every block write also writes to parity disk Reference:  Factors in choosing RAID level Monetary cost Performance: Number of I/O operations per second, and bandwidth during normal operation Performance during failure Performance during rebuild of failed disk  Including time taken to rebuild failed disk Choice of RAID Levels RAID 0 is used only when data safety is not important E.g. data can be recovered quickly from other sources Level 2 and 4 never used since they are subsumed by 3 and 5 Level 3 is not used anymore since bit-striping forces single block reads to access all disks, wasting disk arm movement, which block striping (level 5) avoids Level 6 is rarely used since levels 1 and 5 offer adequate safety for most applications Reference:  Level 1 provides much better write performance than level 5 Level 5 requires at least 2 block reads and 2 block writes to write a single block, whereas Level 1 only requires 2 block writes Level 1 preferred for high update environments such as log disks Level 1 had higher storage cost than level 5 disk drive capacities increasing rapidly (50%/year) whereas disk access times have decreased much less (x 3 in 10 years) Choice of RAID I/O requirements have increased greatly, e.g. for Web servers Levels When enough disks have been bought to satisfy required rate of I/O, they often have spare storage capacity  so there is often no extra monetary cost for Level 1! Level 5 is preferred for applications with low update rate, and large amounts of data Level 1 is preferred for all other applications Reference:  Software RAID: RAID implementations done entirely in software, with no special hardware support Hardware RAID: RAID implementations with special hardware Use non-volatile RAM to record writes that are being executed Beware: power failure during write can result in corrupted disk  E.g.failure after writing one block but before writing the second in a Hardware mirrored system Issues  Such corrupted data must be detected when power is restored – Recovery from corruption is similar to recovery from failed disk – NV-RAM helps to efficiently detected potentially corrupted blocks » Otherwise all blocks of disk must be read and compared with mirror/parity block Reference:  Latent failures: data successfully written earlier gets damaged can result in data loss even if only one disk fails Data scrubbing: continually scan for latent failures, and recover from copy/parity Hot swapping: replacement of disk while system is running, without power down Supported by some hardware RAID systems, reduces time to recovery, and improves availability greatly Hardware Many systems maintain spare disks which are kept online, and used as Issues replacements for failed disks immediately on detection of failure Reduces time to recovery greatly Many hardware RAID systems ensure that a single point of failure will not stop the functioning of the system by using Redundant power supplies with battery backup Multiple controllers and multiple interconnections to guard against controller/interconnection failures Reference: