Chapter 10: Storage in addition to File Structure Chapter 10: Storage in addition to File Structure Ov

Chapter 10: Storage in addition to File Structure Chapter 10: Storage in addition to File Structure Ov www.phwiki.com

Chapter 10: Storage in addition to File Structure Chapter 10: Storage in addition to File Structure Ov

Campbell, Ben, Host has reference to this Academic Journal, PHwiki organized this Journal Chapter 10: Storage in addition to File Structure Chapter 10: Storage in addition to File Structure Overview of Physical Storage Media Magnetic Disks RAID Tertiary Storage Storage Access File Organization Organization of Records in Files Data-Dictionary Storage Classification of Physical Storage Media 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 Can differentiate storage into: volatile storage: loses contents when power is switched off non-volatile storage: Contents persist even when power is switched off. Includes secondary in addition to tertiary storage, as well as batter- backed up main-memory.

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Physical Storage Media Cache – fastest in addition to most costly as long as m 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 capacities of up to a few Gigabytes widely used currently Capacities have gone up in addition to per-byte costs have decreased steadily in addition to 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. Physical Storage Media (Cont.) Flash memory Data survives power failure Data can be written at a location only once, but location can be erased in addition to 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 memory 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, in addition to USB keys Physical Storage Media (Cont.) Magnetic-disk Data is stored on spinning disk, in addition to read/written magnetically Primary medium as long as the long-term storage of data; typically stores entire database. Data must be moved from disk to main memory as long as access, in addition to written back as long as storage Much slower access than main memory (more on this later) 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 in addition to cost/byte than main memory/flash memory Growing constantly in addition to rapidly with technology improvements (factor of 2 to 3 every 2 years) Survives power failures in addition to system crashes disk failure can destroy data, but is rare

Physical Storage Media (Cont.) Optical storage non-volatile, data is read optically from a spinning disk using a laser CD-ROM (640 MB) in addition to DVD (4.7 to 17 GB) most popular as long as ms Blu-ray disks: 27 GB to 54 GB Write-one, read-many (WORM) optical disks used as long as archival storage (CD-R, DVD-R, DVD+R) Multiple write versions also available (CD-RW, DVD-RW, DVD+RW, in addition to DVD-RAM) Reads in addition to writes are slower than with magnetic disk Juke-box systems, with large numbers of removable disks, a few drives, in addition to a mechanism as long as automatic loading/unloading of disks available as long as storing large volumes of data Physical Storage Media (Cont.) Tape storage non-volatile, used primarily as long as backup (to recover from disk failure), in addition to as long as archival data sequential-access – much slower than disk very high capacity (40 to 300 GB tapes available) tape can be removed from drive storage costs much cheaper than disk, but drives are expensive Tape jukeboxes available as long as storing massive amounts of data hundreds of terabytes (1 terabyte = 109 bytes) to even multiple petabytes (1 petabyte = 1012 bytes) Storage Hierarchy

Storage Hierarchy (Cont.) 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 E.g. flash memory, magnetic disks tertiary storage: lowest level in hierarchy, non-volatile, slow access time also called off-line storage E.g. magnetic tape, optical storage Magnetic Hard Disk Mechanism NOTE: Diagram is schematic, in addition to simplifies the structure of actual disk drives Magnetic Disks Read-write head Positioned very close to the platter surface (almost touching it) Reads or writes magnetically encoded in as long as mation. Surface of platter divided into circular tracks Over 50K-100K tracks per platter on typical hard disks Each track is divided into sectors. A sector is the smallest unit of data that can be read or written. Sector size typically 512 bytes Typical sectors per track: 500 to 1000 (on inner tracks) to 1000 to 2000 (on outer tracks) To read/write a sector disk arm swings to position head on right track platter spins continually; data is read/written as sector passes under head Head-disk assemblies multiple disk platters on a single spindle (1 to 5 usually) one head per platter, mounted on a common arm. Cylinder i consists of ith track of all the platters

Magnetic Disks (Cont.) Earlier generation disks were susceptible to head-crashes Surface of earlier generation disks had metal-oxide coatings which would disintegrate on head crash in addition to damage all data on disk Current generation disks are less susceptible to such disastrous failures, although individual sectors may get corrupted Disk controller – interfaces between the computer system in addition to the disk drive hardware. accepts high-level comm in addition to s to read or write a sector initiates actions such as moving the disk arm to the right track in addition to actually reading or writing the data Computes in addition to attaches checksums to each sector to verify that data is read back correctly If data is corrupted, with very high probability stored checksum won’t match recomputed checksum Ensures successful writing by reading back sector after writing it Per as long as ms remapping of bad sectors Disk Subsystem 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 st in addition to ards families ATA (AT adaptor) range of st in addition to ards SATA (Serial ATA) SCSI (Small Computer System Interconnect) range of st in addition to ards SAS (Serial Attached SCSI) Several variants of each st in addition to ard (different speeds in addition to capabilities) Disk Subsystem 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

Per as long as mance Measures of Disks 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, in addition to we ignore the time to start in addition to stop arm movement 4 to 10 milliseconds on typical disks Rotational latency – time it takes as long as the sector to be accessed to appear under the head. Average latency is 1/2 of the worst case latency. 4 to 11 milliseconds on typical disks (5400 to 15000 r.p.m.) 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 as long as inner tracks Multiple disks may share a controller, so rate that controller can h in addition to le 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 Per as long as mance Measures (Cont.) 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 as long as a new disk E.g., an MTTF of 1,200,000 hours as long as a new disk means that given 1000 relatively new disks, on an average one will fail every 1200 hours MTTF decreases as disk ages Optimization of Disk-Block Access Block – a contiguous sequence of sectors from a single track data is transferred between disk in addition to main memory in blocks sizes range from 512 bytes to several kilobytes Smaller blocks: more transfers from disk Larger blocks: more space wasted due to partially filled blocks Typical block sizes today range from 4 to 16 kilobytes Disk-arm-scheduling algorithms order pending accesses to tracks so that disk arm movement is minimized elevator algorithm: R1 R5 R2 R4 R3 R6 Inner track Outer track

Optimization of Disk Block Access (Cont.) File organization – optimize block access time by organizing the blocks to correspond to how data will be accessed E.g. Store related in as long as mation on the same or nearby cylinders. Files may get fragmented over time E.g. if data is inserted to/deleted from the file Or free blocks on disk are scattered, in addition to newly created file has its blocks scattered over the disk Sequential access to a fragmented file results in increased disk arm movement Some systems have utilities to defragment the file system, in order to speed up file access Nonvolatile write buffers speed up disk writes by writing blocks to a non-volatile RAM buffer immediately Non-volatile RAM: battery backed up RAM or flash memory Even if power fails, the data is safe in addition to will be written to disk when power returns Controller then writes to disk whenever the disk has no other requests or request has been pending as long as some time Database operations that require data to be safely stored be as long as e continuing can continue without waiting as long as data to be written to disk Writes can be reordered to minimize disk arm movement Log disk – a disk devoted to writing a sequential log of block updates Used exactly like nonvolatile RAM Write to log disk is very fast since no seeks are required No need as long as special hardware (NV-RAM) File systems typically reorder writes to disk to improve per as long as mance Journaling file systems write data in safe order to NV-RAM or log disk Reordering without journaling: risk of corruption of file system data Optimization of Disk Block Access (Cont.) Flash Storage NOR flash vs NAND flash NAND flash used widely as long as storage, since it is much cheaper than NOR flash requires page-at-a-time read (page: 512 bytes to 4 KB) transfer rate around 20 MB/sec solid state disks: use multiple flash storage devices to provide higher transfer rate of 100 to 200 MB/sec erase is very slow (1 to 2 millisecs) erase block contains multiple pages remapping of logical page addresses to physical page addresses avoids waiting as long as erase translation table tracks mapping also stored in a label field of flash page remapping carried out by flash translation layer after 100,000 to 1,000,000 erases, erase block becomes unreliable in addition to cannot be used wear leveling

RAID 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 in addition to 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. 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 as long as 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 as long as “inexpensive’’ Today RAIDs are used as long as their higher reliability in addition to b in addition to width. The “I” is interpreted as independent Improvement of Reliability via Redundancy Redundancy – store extra in as long as mation that can be used to rebuild in as long as mation 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 Reads can take place from either disk If one disk in a pair fails, data still available in the other Data loss would occur only if a disk fails, in addition to its mirror disk also fails be as long as e the system is repaired Probability of combined event is very small Except as long as dependent failure modes such as fire or building collapse or electrical power surges Mean time to data loss depends on mean time to failure, in addition to 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 500106 hours (or 57,000 years) as long as a mirrored pair of disks (ignoring dependent failure modes) Improvement in Per as long as mance via Parallelism 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. Each access can read data at eight times the rate of a single disk. But seek/access time worse than as long as 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 as long as different blocks can run in parallel if the blocks reside on different disks A request as long as a long sequence of blocks can utilize all disks in parallel

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RAID Levels Schemes to provide redundancy at lower cost by using disk striping combined with parity bits Different RAID organizations, or RAID levels, have differing cost, per as long as mance in addition to reliability characteristics RAID Level 1: Mirrored disks with block striping Offers best write per as long as mance. Popular as long as applications such as storing log files in a database system. RAID Level 0: Block striping; non-redundant. Used in high-per as long as mance applications where data loss is not critical. RAID Levels (Cont.) RAID Level 2: Memory-Style Error-Correcting-Codes (ECC) with bit striping. RAID Level 3: Bit-Interleaved Parity a single parity bit is enough as long as error correction, not just detection, since we know which disk has failed When writing data, corresponding parity bits must also be computed in addition to written to a parity bit disk To recover data in a damaged disk, compute XOR of bits from other disks (including parity bit disk) RAID Levels (Cont.) 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, in addition to keeps a parity block on a separate disk as long as corresponding blocks from N other disks. When writing data block, corresponding block of parity bits must also be computed in addition to written to parity disk To find value of a damaged block, compute XOR of bits from corresponding blocks (including parity block) from other disks.

RAID Levels (Cont.) RAID Level 4 (Cont.) Provides higher I/O rates as long as 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 as long as reads of multiple blocks than no-striping Be as long as e writing a block, parity data must be computed Can be done by using old parity block, old value of current block in addition to new 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 as long as writing large amounts of data sequentially Parity block becomes a bottleneck as long as independent block writes since every block write also writes to parity disk RAID Levels (Cont.) RAID Level 5: Block-Interleaved Distributed Parity; partitions data in addition to parity among all N + 1 disks, rather than storing data in N disks in addition to parity in 1 disk. E.g., with 5 disks, parity block as long as nth set of blocks is stored on disk (n mod 5) + 1, with the data blocks stored on the other 4 disks. RAID Levels (Cont.) RAID Level 5 (Cont.) Higher I/O rates than Level 4. Block writes occur in parallel if the blocks in addition to their parity blocks are on different disks. Subsumes Level 4: provides same benefits, but avoids bottleneck of parity disk. RAID Level 6: P+Q Redundancy scheme; similar to Level 5, but stores extra redundant in as long as mation to guard against multiple disk failures. Better reliability than Level 5 at a higher cost; not used as widely.

Figure 10.18 Figure in-10.1

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