Chapter 12: Physical Storage Systems

Database System Concepts, 7th Ed.

©Silberschatz, Korth and Sudarshan
See www.db-book.com for conditions on re-use
 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 12.2 ©Silberschatz, Korth and Sudarshan
 Storage Hierarchy

Database System Concepts - 7th Edition 12.3 ©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
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 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

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 Magnetic Hard Disk Mechanism

Schematic diagram of magnetic disk drive Photo of magnetic disk drive

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 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 ith track of all the platters
Database System Concepts - 7th Edition 12.7 ©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

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 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

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 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

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 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

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 Flash Storage
 NOR flash vs NAND flash
 NAND flash
• used widely for storage, cheaper than NOR flash
• requires page-at-a-time read (page: 512 bytes to 4 KB)
 20 to 100 microseconds for a page read
 Not much difference between sequential and random
read
• Page can only be written once
 Must be erased to allow rewrite
 Solid state disks
• Use standard block-oriented disk interfaces, but store data on
multiple flash storage devices internally
• Transfer rate of up to 500 MB/sec using SATA, and
up to 3 GB/sec using NVMe PCIe

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 Flash Storage (Cont.)
 Erase happens in units of erase block
• Takes 2 to 5millisecs
• Erase block typically 256 KB to 1 MB (128 to 256 pages)
 Remapping of logical page addresses to physical page addresses
avoids waiting for erase
 Flash 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
and cannot be used
Page write
• wear leveling
Physical Page Address

Logical address and valid bit

Logical Page Address stored with each
physical page (extra bytes)

Flash Translation Table

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 SSD Performance Metrics
 Random reads/writes per second
• Typical 4 KB reads: 10,000 reads per second (10,000 IOPS)
• Typical 4KB writes: 40,000 IOPS
• SSDs support parallel reads
 Typical 4KB reads:
• 100,000 IOPS with 32 requests in parallel (QD-32) on
SATA
• 350,000 IOPS with QD-32 on NVMe PCIe
 Typical 4KB writes:
• 100,000 IOPS with QD-32, even higher on some models
 Data transfer rate for sequential reads/writes
• 400 MB/sec for SATA3, 2 to 3 GB/sec using NVMe PCIe
 Hybrid disks: combine small amount of flash cache with larger
magnetic disk

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 Storage Class Memory
 3D-XPoint memory technology pioneered by Intel
 Available as Intel Optane
• SSD interface shipped from 2017
 Allows lower latency than flash SSDs
• Non-volatile memory interface announced in 2018
 Supports direct access to words, at speeds comparable to
main-memory speeds

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 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

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 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)
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 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

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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,
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.

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 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

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 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.

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 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

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 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

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 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

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 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.

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 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

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 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
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 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

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 End of Chapter 12

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 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)
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