explain in simple terms  following text 
Chapter 12: Physical Storage Systems

Database System Concepts - 7th Edition 12.2 ©Silberschatz, Korth and Sudarshan

Classification of Physical Storage Media

§ 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 and tertiary storage, as well as batter-backed
up main-memory.

§ Factors affecting choice of storage media include
• Speed with which data can be accessed
• Cost per unit of data
• Reliability

Database System Concepts - 7th Edition 12.3 ©Silberschatz, Korth and Sudarshan

Storage Hierarchy

Database System Concepts - 7th Edition 12.4 ©Silberschatz, Korth and Sudarshan

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 and used for archival storage
• e.g., magnetic tape, optical storage
• Magnetic tape
§ Sequential access, 1 to 12 TB capacity
§ A few drives with many tapes
§ Juke boxes with petabytes (1000’s of TB) of storage

Database System Concepts - 7th Edition 12.5 ©Silberschatz, Korth and Sudarshan

Storage Interfaces

§ Disk interface standards families
• SATA (Serial ATA)
§ SATA 3 supports data transfer speeds of up to 6 gigabits/sec
• SAS (Serial Attached SCSI)
§ SAS Version 3 supports 12 gigabits/sec
• NVMe (Non-Volatile Memory Express) interface
§ Works with PCIe connectors to support lower latency and higher
transfer rates
§ Supports data transfer rates of up to 24 gigabits/sec

§ 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

Database System Concepts - 7th Edition 12.6 ©Silberschatz, Korth and Sudarshan

Magnetic Hard Disk Mechanism

Schematic diagram of magnetic disk drive Photo of magnetic disk drive

Database System Concepts - 7th Edition 12.7 ©Silberschatz, Korth and Sudarshan

Magnetic Disks

§ Read-write head
§ 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 i

th track of all the platters

Database System Concepts - 7th Edition 12.8 ©Silberschatz, Korth and Sudarshan

Magnetic Disks (Cont.)

§ Disk controller – interfaces between the computer system and the disk
drive hardware.
• accepts high-level commands to read or write a sector
• initiates actions such as moving the disk arm to the right track and
actually reading or writing the data
• Computes and 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
• Performs remapping of bad sectors

Database System Concepts - 7th Edition 12.9 ©Silberschatz, Korth and Sudarshan

Performance 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, 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 under the head.
§ 4 to 11 milliseconds on typical disks (5400 to 15000 r.p.m.)
§ Average latency is 1/2 of the above latency.
• Overall latency is 5 to 20 msec depending on disk model
§ Data-transfer rate – the rate at which data can be retrieved from or stored
to the disk.
• 25 to 200 MB per second max rate, lower for inner tracks

Database System Concepts - 7th Edition 12.10 ©Silberschatz, Korth and Sudarshan

Performance Measures (Cont.)

§ Disk block is a logical unit for storage allocation and retrieval
• 4 to 16 kilobytes typically
§ Smaller blocks: more transfers from disk
§ Larger blocks: more space wasted due to partially filled blocks

§ Sequential access pattern
• Successive requests are for successive disk blocks
• Disk seek required only for first block
§ Random access pattern
• Successive requests are for blocks that can be anywhere on disk
• Each access requires a seek
• Transfer rates are low since a lot of time is wasted in seeks
§ I/O operations per second (IOPS)
• Number of random block reads that a disk can support per second
• 50 to 200 IOPS on current generation magnetic disks

Database System Concepts - 7th Edition 12.11 ©Silberschatz, Korth and Sudarshan

Performance 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 for a new disk
§ E.g., an MTTF of 1,200,000 hours for a new disk means that given
1000 relatively new disks, on an average one will fail every 1200
hours

• MTTF decreases as disk ages

Database System Concepts - 7th Edition 12.16 ©Silberschatz, Korth and Sudarshan

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 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.
• 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

Database System Concepts - 7th Edition 12.17 ©Silberschatz, Korth and Sudarshan
Improvement of Reliability via Redundancy
§ 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
§ 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, and its mirror disk also
fails before the system is repaired
• Probability of combined event is very small
§ Except for dependent failure modes such as fire or building
collapse or electrical power surges

§ 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)

Database System Concepts - 7th Edition 12.18 ©Silberschatz, Korth and Sudarshan
Improvement in Performance 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 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

Database System Concepts - 7th Edition 12.19 ©Silberschatz, Korth and Sudarshan

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,
performance and reliability characteristics
§ RAID Level 0: Block striping; non-redundant.
• Used in high-performance applications where data loss is not critical.
§ RAID Level 1: Mirrored disks with block striping
• Offers best write performance.
• Popular for applications such as storing log files in a database system.

Database System Concepts - 7th Edition 12.20 ©Silberschatz, Korth and Sudarshan

RAID Levels (Cont.)

§ Parity blocks: Parity block j stores XOR of bits from block j of each disk
• When writing data to a block j, parity block j must also be computed
and written to disk
§ Can be done by using old parity block, old value of current block
and 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 for writing large amounts of data sequentially
• To recover data for a block, compute XOR of bits from all other
blocks in the set including the parity block

Database System Concepts - 7th Edition 12.21 ©Silberschatz, Korth and Sudarshan

RAID Levels (Cont.)

§ RAID Level 5: Block-Interleaved Distributed Parity; partitions data and
parity among all N + 1 disks, rather than storing data in N disks and parity
in 1 disk.
• E.g., with 5 disks, parity block for nth set of blocks is stored on disk
(n mod 5) + 1, with the data blocks stored on the other 4 disks.

Database System Concepts - 7th Edition 12.22 ©Silberschatz, Korth and Sudarshan

RAID Levels (Cont.)

§ RAID Level 5 (Cont.)
• Block writes occur in parallel if the blocks and their parity blocks are
on different disks.

§ RAID Level 6: P+Q Redundancy scheme; similar to Level 5, but stores
two error correction blocks (P, Q) instead of single parity block to guard
against multiple disk failures.
• Better reliability than Level 5 at a higher cost
§ Becoming more important as storage sizes increase

Database System Concepts - 7th Edition 12.23 ©Silberschatz, Korth and Sudarshan

RAID Levels (Cont.)

§ Other levels (not used in practice):
• RAID Level 2: Memory-Style Error-Correcting-Codes (ECC) with bit
striping.
• RAID Level 3: Bit-Interleaved Parity
• RAID Level 4: Block-Interleaved Parity; uses block-level striping,
and keeps a parity block on a separate parity disk for corresponding
blocks from N other disks.
§ RAID 5 is better than RAID 4, since with RAID 4 with random
writes, parity disk gets much higher write load than other disks
and becomes a bottleneck

Database System Concepts - 7th Edition 12.24 ©Silberschatz, Korth and Sudarshan

Database System Concepts - 7th Edition 12.25 ©Silberschatz, Korth and Sudarshan

Choice of RAID Level

§ 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

§ RAID 0 is used only when data safety is
not important
• E.g., data can be recovered quickly
from other sources

Database System Concepts - 7th Edition 12.26 ©Silberschatz, Korth and Sudarshan

Choice of RAID Level (Cont.)

§ 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 had higher storage cost than level 5
§ Level 5 is preferred for applications where writes are sequential and large
(many blocks), and need large amounts of data storage
§ RAID 1 is preferred for applications with many random/small updates
§ Level 6 gives better data protection than RAID 5 since it can tolerate two
disk (or disk block) failures
• Increasing in importance since latent block failures on one disk,
coupled with a failure of another disk can result in data loss with RAID
1 and RAID 5.

Database System Concepts - 7th Edition 12.27 ©Silberschatz, Korth and Sudarshan

Hardware Issues

§ 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 mirrored system
§ 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

Database System Concepts - 7th Edition 12.28 ©Silberschatz, Korth and Sudarshan

Hardware Issues (Cont.)

§ 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
§ Many systems maintain spare disks which are kept online, and used as
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

Database System Concepts - 7th Edition 12.29 ©Silberschatz, Korth and Sudarshan

Optimization of Disk-Block Access

§ Buffering: in-memory buffer to cache disk blocks
§ Read-ahead: Read extra blocks from a track in anticipation that they will
be requested soon
§ Disk-arm-scheduling algorithms re-order block requests so that disk arm
movement is minimized
• elevator algorithm

Database System Concepts - 7th Edition 12.30 ©Silberschatz, Korth and Sudarshan

Optimization of Disk-Block Access

§ Buffering: in-memory buffer to cache disk blocks
§ Read-ahead: Read extra blocks from a track in anticipation that they will
be requested soon
§ Disk-arm-scheduling algorithms re-order block requests so that disk arm
movement is minimized
• elevator algorithm

R6 R3 R1 R5 R2 R4

Inner track Outer track

Database System Concepts - 7th Edition 12.32 ©Silberschatz, Korth and Sudarshan

End of Chapter 12

Database System Concepts - 7th Edition 12.33 ©Silberschatz, Korth and Sudarshan

Magnetic Tapes

§ Hold large volumes of data and provide high transfer rates
• Few GB for DAT (Digital Audio Tape) format, 10-40 GB with DLT
(Digital Linear Tape) format, 100 GB+ with Ultrium format, and 330 GB
with Ampex helical scan format
• Transfer rates from few to 10s of MB/s
§ Tapes are cheap, but cost of drives is very high
§ Very slow access time in comparison to magnetic and optical disks
• limited to sequential access.
• Some formats (Accelis) provide faster seek (10s of seconds) at cost of
lower capacity

§ Used mainly for backup, for storage of infrequently used information, and
as an off-line medium for transferring information from one system to
another.
§ Tape jukeboxes used for very large capacity storage
• Multiple petabyes (1015 bytes)