ISO/TR 10255:2009
(Main)Document management applications — Optical disk storage technology, management and standards
Document management applications — Optical disk storage technology, management and standards
ISO/TR 10255:2009 gives recommendations and provides guidance for maintaining archival optical disk collections. It describes the various services that would be necessary for the management of an optical media‑based system to ensure a successful implementation of this technology. ISO/TR 10255:2009 also provides guidance in the maintenance of data residing on on-line, off-line, and near-line digital optical storage devices; establishes a plan to ensure the migration path of digital information from early and current technology and optical media to future technologies and media; provides guidance for the short- and long-term effect of the finite life of digital optical storage devices. ISO/TR 10255:2009 also describes all forms of optical disk media including write-once-read-many (WORM), magneto-optical (MO), compact disk (CD), digital versatile disk (DVD) and newer technologies.
Applications de la gestion des documents — Technologie de stockage sur disque optique, gestion et normes
General Information
Standards Content (Sample)
TECHNICAL ISO/TR
REPORT 10255
First edition
2009-11-15
Document management applications —
Optical disk storage technology,
management and standards
Applications de la gestion des documents — Technologie de stockage
sur disque optique, gestion et normes
Reference number
©
ISO 2009
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ii © ISO 2009 – All rights reserved
Contents Page
Foreword .iv
Introduction.v
1 Scope.1
2 Abbreviated terms.1
3 Optical storage concepts .5
3.1 General .5
3.2 On-line versus off-line storage .5
3.3 Data layout formats.6
3.4 Rotational models .6
3.5 Physically writing to the disk surface .6
4 Optical media .8
4.1 General .8
4.2 DVD technologies.8
4.3 Blue-indigo laser technologies .8
4.4 Blu-ray Disc.9
4.5 Ultra Density Optical (UDO).9
5 Optical device characteristics.9
5.1 General .9
5.2 Readers / writers.9
5.3 Multi-function drives .10
5.4 Jukeboxes/libraries.10
5.5 Software support for optical libraries .11
6 Implementation strategies.12
6.1 General .12
6.2 Relationships between applications and optical storage management.12
6.3 Optical media interchange across environments .12
6.4 Rewritable and WORM support.13
6.5 Operating system independence.13
6.6 Vendor independence.14
6.7 Massive volume support .14
6.8 Document/records management concerns .14
7 Information management .15
7.1 Retention .15
7.2 Archival support .15
7.3 Deterioration .15
7.4 Migration .16
7.5 Disposal.16
7.6 Legal admissibility concerns .16
7.7 Evolving technology and vendor support .17
8 Technical issues.17
8.1 Optical storage device file structures .17
8.2 Periodic testing.18
9 Optical disk standards.18
Annex A (informative) Related International Standards .19
Bibliography.24
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
In exceptional circumstances, when a technical committee has collected data of a different kind from that
which is normally published as an International Standard (“state of the art”, for example), it may decide by a
simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely
informative in nature and does not have to be reviewed until the data it provides are considered to be no
longer valid or useful.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO/TR 10255 was prepared by Technical Committee ISO/TC 171, Document management applications,
Subcommittee SC 2, Application issues.
iv © ISO 2009 – All rights reserved
Introduction
This Technical Report specifies the recommendations and provides guidance for maintaining archival optical
disk collections. The problem identified is one of systems becoming obsolete prior to the expiration of the
useful life of the information. Additionally, technology is evolving so rapidly that the systems might be obsolete
prior to the storage media reaching its life expectancy. These issues require a considerable amount of
planning to occur in the initial stages of the development and implementation of imaging systems to provide a
plan for migrating the information from a system utilizing obsolete technology to a system employing
advanced technology. This planning is invaluable to the overall success of the system as the information itself
might have a lifespan greater than the media and technology combined, resulting in inaccessibility.
The purpose of this Technical Report is to recommend methodologies by which optical disk users can
understand various optical disk issues, such as implementation, retention, obsolescence, and basic data
management. In addition, this report provides information describing the differences between various optical
components as well as some basic concepts that should be used when determining which optical solution
best fits the users' needs.
A list of related standards is given in Annex A.
TECHNICAL REPORT ISO/TR 10255:2009(E)
Document management applications — Optical disk storage
technology, management and standards
1 Scope
This Technical Report gives recommendations and provides guidance for maintaining archival optical disk
collections. It describes the various services that would be necessary for the management of an optical
media-based system to ensure a successful implementation of this technology.
This Technical Report also
⎯ provides guidance in the maintenance of data residing on on-line, off-line, and near-line digital optical
storage devices;
⎯ establishes a plan to ensure the migration path of digital information from early and current technology
and optical media to future technologies and media;
⎯ provides guidance for the short- and long-term effect of the finite life of digital optical storage devices.
This Technical Report also describes all forms of optical disk media including write-once-read-many (WORM),
magneto-optical (MO), compact disk (CD), digital versatile disk (DVD) and newer technologies.
2 Abbreviated terms
2.1
BD
Blu-ray Disc
2.2
CAV
constant angular velocity
2.3
CCS
continuous composite servo
2.4
CCW
continuous composite write-once
2.5
CD-DA
compact disk-digital audio
2.6
CD-R
compact disk-recordable
2.7
CD-ROM
compact disk-read only memory
2.8
CD-RW
compact disk-rewriteable
2.9
CD-I
compact disk-interactive
2.10
CLV
constant linear velocity
2.11
DBF
discrete block format
2.12
DIF
document interchange format
2.13
DVD
digital versatile disk
2.14
DVD-Audio
digital versatile disk-audio read only
2.15
DVD-R
digital versatile disk-recordable
NOTE One of three competing recordable DVD standards; the others are DVD+R(W) and DVD-RAM.
2.16
DVD+R
digital versatile disk+recordable
NOTE One of three competing recordable DVD standards; the others are DVD-R(W) and DVD-RAM.
2.17
DVD-RAM
digital versatile disk-random access memory
NOTE One of three competing recordable DVD standards; the others are DVD-R(W) and DVD+R(W).
2.18
DVD-RW
digital versatile disk-rewriteable
NOTE One of three competing recordable DVD standards; the others are DVD+R(W) and DVD-RAM.
2.19
DVD+RW
digital versatile disk+rewriteable
NOTE One of three competing recordable DVD standards; the others are DVD-R(W) and DVD-RAM.
2 © ISO 2009 – All rights reserved
2.20
DVD-ROM
digital versatile disk-recorded optical media or read only memory
2.21
DVD-Video
digital versatile disk-video
2.22
ECC
error correcting coding
2.23
FAT
file allocation table
NOTE Originally developed for the MS-DOS operating system.
2.24
GIF
graphics interchange format
2.25
HD-DVD
high definition-digital versatile disk
2.26
HFS
hierarchical file system
NOTE Developed for the Apple Macintosh operating system.
2.27
HPFS
high-performance file system
NOTE Developed for the OS/2 operating system.
2.28
INCITS
InterNational Committee for Information Technology Standards
2.29
ISO
International Organization for Standardization
2.30
IEC
International Electrotechnical Commission
NOTE Standards developed jointly between the IEC and the International Organization for Standardization are given
the designation ISO/IEC.
2.31
JPEG
Joint Photographic Experts Group
NOTE Used to refer to both the International Standards Committee (ISO/IEC JTC 1/SC 29/WG 1) and the
standard(s) they developed for coding and compression of still images.
2.32
JTC 1
Joint Technical Committee 1 on Information Technology
NOTE This is an International Standards development committee jointly operated by ISO and IEC.
2.33
LIMDOW
light intensity modulated direct overwrite
2.34
MPEG
Moving Picture Experts Group
NOTE Used to refer to both the International Standards Committee (ISO/IEC JTC 1/SC 29/WG 11) and the
standard(s) they developed for video and audio encoding.
2.35
MO
magneto-optical
2.36
NFS
Network File System
2.37
NIST
National Institute of Standards and Technology
2.38
NSR
non-sequential recording for information interchange
2.39
ODC
optical disk cartridges
2.40
OSTA
Optical Storage Technology Association
2.41
PCX
PiCture eXchange
NOTE A graphics file format.
2.42
PDD
Professional Disk for DATA
2.43
RTF
rich text format
2.44
TC
Technical Committee
NOTE A committee designated by ISO to develop International Standards in a particular area.
4 © ISO 2009 – All rights reserved
2.45
TIFF
tagged image file format
2.46
UDO
ultra density optical
2.47
UDF
universal disc format
2.48
UNIX
trademark used for a computer operating system
2.49
VTP
variable track pitch
2.50
WAV
waveform audio format
2.51
WORM
write-once-read-many
2.52
WORM/MO
write-once-read-many/magneto-optical
3 Optical storage concepts
3.1 General
Optical storage has been used for data storage for over 20 years. Data is recorded on reflective media using a
laser-powered head. The preciseness of the laser and the properties of the media combine to allow data to be
stored at very high densities. For example, the current generation of optical storage technology can store up
to 8,5 GB of data on a 120 mm disk and up to 50 GB of data on a 130 mm disk. The steady increase of
storage capacity on removable optical media enables organizations to consider long-term storage of
information for archival use taking into account reliability and technology trustworthiness.
3.2 On-line versus off-line storage
The storage components within any computer system directly affect the overall system operation. There are
several different types of storage components that can be attached to any of these systems. Before
discussing each of these components, let us consider the various storage groupings, including on-line storage,
near-line storage, and off-line storage.
⎯ On-line storage is considered to be any storage device that is always available to a system user. An
example of this type of storage is a fixed hard disk either attached directly to a computer or available
across a local area network. Removable storage media, including removable hard disks and optical
media, are considered to be on-line storage devices when they are mounted, or in other words, can be
accessed by a user without any system intervention other than reading or writing the requested data.
⎯ Off-line storage defines any storage media that is removed from the system and typically stored in a
separate area for archival purposes. Removable optical devices and magnetic tapes that are not mounted
fall into this category.
⎯ Near-line storage devices are stored in a mechanical library, which can be defined as a hardware
component consisting of media drives, such as optical or tape, and numerous storage slots or bays to
store the media. These libraries are often referred to as jukeboxes for their mechanical similarities to
musical jukeboxes. These systems typically contain a robotics arm, which is used to store and retrieve
the optical media. In addition, most optical libraries also provide a mailbox slot which is used to insert
and/or remove optical media from the system for offsite storage or simple removal from the system.
The most important aspect of the optical library is its ability to store numerous platters or other types of
media as well as multiple drives in a single storage cabinet. Libraries typically only support one form of
media and often only one particular class, such as CD or DVD.
3.3 Data layout formats
There are three different data formats used in the manufacturing process of 130 mm optical disks. These
formats are not compatible, and media can only be read by optical disk drives supporting that particular format.
Two of these formats, Format A and Format B, are described in ISO/IEC 9171.
In the list of currently available industry International Standards (see Annex A), some of these International
Standards refer to continuous composite write-once capability (CCW) while others refer to WORM. The CCW
media uses MO disks to emulate a WORM-like function. The recording technology used for this emulation is
the same as that used for rewritable media. The references to WORM refer to ablative or permanent change
write-once media (see section 3.5.2).
See Annex A for a detailed list of relevant International Standards.
3.4 Rotational models
There are two basic rotational modes used by optical disk drives. The first mode is the Constant Angular
Velocity (CAV). Within this mode, the media is spun at a constant rate so the angular velocity of the optical
media does not change. This simple implementation means that the outer edge of the optical disk rotates
faster than the inner edge, storing data further apart toward the outer edge. Since the amount of data stored in
each track is constant, the data density is greater on the innermost tracks. With this approach, the amount of
data stored is limited by the data rate achieved on the inner tracks.
The second mode is the Constant Linear Velocity (CLV) mode. This mode requires that the disk speed
change as the laser head moves from the innermost portion of the optical media to the outermost. The most
significant aspect of this mode, in contrast to the CAV, is that the data density does not change throughout the
disk. The result of the greater data density towards the outermost edge of the optical device is greater storage
capabilities. In addition, since the rotation speeds are slower as the laser head is positioned over the
outermost tracks, the data transfer rates are higher than they would be with CAV. Seek times are slightly
longer than for CAV because the angular velocity is required to change at the same time as the head moves.
3.5 Physically writing to the disk surface
3.5.1 General
Optical media may be written using several different mechanisms. Some of these mechanisms are write once;
that is, the changes made to the surface of the media are irreversible and controlled by the media recording
layer, which actually identifies the media type to the optical drive.
3.5.2 Ablative
Originally, data was written onto optical media by physically etching pits into the surface. These techniques
have largely been replaced by dye-layer and phase-change recording.
6 © ISO 2009 – All rights reserved
3.5.3 Dye-laser recording
This technique is primarily used for CD-R recording. The media contains a thin layer of light-sensitive dye.
During the recording process, a laser hits the dye and causes it to change colour to denote bits. Once the
recording is complete, a lower-powered laser is used to read the resulting differences between lighter and
darker bits.
3.5.4 Phase change
Phase change technologies are used in many rewritable operations. The media contains a crystalline
recording layer that is highly reflective. A higher-intensity laser strikes the media, causing the recording layer
to change from the crystalline state to an amorphous state which does not reflect as well. A lower-intensity
laser is then used to read the bits. When the information is to be overwritten, a moderate-intensity pulse is
used to restore the crystalline state.
3.5.5 Magneto-optical
This technology uses a combination of magnetic and optical techniques to store large quantities of information.
A laser is used to heat the surface of the media to a specific temperature which changes the magnetic
properties of the media to allow polarity changes. The magnetic head is activated and changes the polarity of
the bit. When the spot cools, the information is stored and accessed using a lower-powered laser. There is
both a WORM and Rewritable format as discussed below.
3.5.6 WORM
WORM (Write-Once Read Many) allows a user to write data one time but read the data as many times as
necessary. This technology does not allow users to easily delete or modify previously stored data. Data is
always read and written to optical devices in blocks of information, or sectors. Sectors are marked when they
are written on. When a request to write data is made, the drive first checks to ensure a sector is not already
used. If it has been, the optical drive will not allow that sector to be rewritten, regardless of how full or empty it
is; and the new data, regardless of whether it is 10 or 511 bytes in size, will be written to the next unmarked
sector. The important point to remember is that data cannot be written to the previously written sector even
though the sector is not full.
WORM systems are typically not inter-compatible because they use different methods to write data to the
media. Originally, WORM employed ablative methods to permanently write grooves into the recording
substrate. Today, most WORM drives rely on continuous composite write, or CCW – a firmware-based
technique to prevent recorded areas from being rewritten or erased – or as phase change technology to
permanently change the reflectance of the recording layer.
3.5.7 Rewritable
Rewritable optical data storage allows users to write over previously used sectors multiple times. This
technology has been gathering popularity throughout the industry and consists of three basic approaches. The
more common of these is the Magneto-optical (MO) technology, which combines features from the magnetic
and optical technologies. The MO technology uses a laser to heat the surface of the media to the point at
which its magnetic properties change allowing polarity changes. The magnetic head then writes the data onto
the heated surface. The magnetic head reads the data without heating.
Erasure is performed by heating and re-magnetising the surface. This process requires two or three steps,
depending on the recording head, to process a block of data on a sector previously written. The first pass is
needed to erase existing data, the second pass to write the new data, and the third pass to verify the accuracy
of the data. The second approach to rewritable media is the phase change process, which requires only one
overwrite pass as compared to the MO process. The existing data is erased and the new data is written in the
same pass. A third technology incorporates the new light intensity modulation-direct overwrite (LIMDOW)
technology. LIMDOW technology allows magneto/optic (MO) disks to be rerecorded without using a two- or
three-pass technique to change the content of the disk. At a minimum, this technology doubles the data
recording performance of optical drives.
Rewritable storage technologies enable organizations to erase data from the optical device whenever required
and allowed by the overall storage solution security features and/or configuration(s). For applications that
require the ability to delete or modify data, rewritable optical systems offer the best solution, even though they
reduce data security. Some multifunction optical drives can be modified to not allow updates to data
previously written. This feature is similar to a write-protect function. It is a fairly simple matter for a user to
insert the optical media in a non-modified drive (the majority of the market) thereby bypassing the so-called
non-updateable feature. Caution should be exercised when using this type of WORM. Before selecting this
type of optical storage, users should determine whether they truly need the level of data security provided with
the WORM technology.
4 Optical media
4.1 General
Optical media are available in several form factors ranging from a diameter of 63,5 mm to 355,6 mm, and
capacities of 140 MB to 60 GB. A library is required to include drives and robotics to handle the appropriate
form factor. The earliest optical libraries in the late 1980's tended to incorporate large form factor cartridges.
As densities have increased, the smaller form factors have become more prevalent— currently the most
common disk formats are 120 mm, used by CD, DVD, and BD; and 130 mm, used by WORM/MO and UDO.
4.2 DVD technologies
DVDs originated in the consumer market but have become the preferred optical media for many applications
due to the increased storage capacity per disk and the compatibility between systems.
DVDs are based on a 120 mm form factor which can support writing to one or two layers, and theoretically on
both sides, with a top storage capacity of 17 GB. DVDs can be written in session-at-once, track-at-once, and
packet writing modes. Both DVD+R and DVD-R support multi-session disks and can write them in track-at-
once mode.
DVDs come in write-once and rewritable versions. However, there are competing standards for DVD and not
all older readers/writers support all formats. The DVD Forum has released the DVD Multi-specification, which
provides that compliant devices are required to be able to read and write DVD-R, DVD-RW, and DVD-RAM
disks, as well as the read-only DVD-Audio, DVD-Video, and DVD-ROM formats. The DVD+RW Alliance
maintains the DVD+RW and DVD+R specifications. Most DVD readers/writers available today will read both
the DVD- and DVD+ formats. DVD-RAM is a unique technology from the DVD Forum that uses a dual-sided,
single-layer disk and protective cartridge form factor. This provides up to 9,4 GB storage capacity and the
ability to rewrite the disk more than 100 000 times.
DVD drives originally provided a data transfer rate of 1,25 MB/s, referred to today as 1×. Subsequent
generations have increased the data transfer rate from 2,4×, or 3 MB/s, to 16×, or 20 MB/s.
4.3 Blue-indigo laser technologies
Technologies that use blue-indigo lasers to increase data density and throughput are commercially available.
The current implementations include 130 mm media that use protective cartridges, and 120 mm DVD without
protective cartridges
Current examples of blue laser-based technologies offer dramatically increased data density through several
mechanisms. The blue laser operates at a much shorter wavelength than red laser. In addition, the lens used
to focus the beam is more precise, resulting in storage capacities today between 20 and 60 GB per disk. The
leading vendors' roadmaps describe data storage in excess of 100 GB in the next two years through
improvement in optics technologies and multi-substrate disks.
8 © ISO 2009 – All rights reserved
4.4 Blu-ray Disc
Blu-ray disc (BD) is an optical disc storage media format. Its main uses are high-definition video and data
storage. The disc has the same dimensions as a standard DVD or CD.
BD uses a “blue” (technically violet) laser operating at a wavelength of 405 nm to read and write data.
Conventional DVDs and CDs use red and near infrared lasers at 650 nm and 780 nm respectively.
The blue-violet laser's shorter wavelength makes it possible to store more information on a 120 mm CD/DVD
sized disc. Currently, BD media comes in 25 GB for single layer recording and 50 GB for dual layer recording.
BD features improvements in data encoding that further increase capacity.
4.5 Ultra Density Optical (UDO)
An Ultra Density Optical disc or UDO is a 133,35 mm ISO/IEC 17345 cartridge optical disc which can store up
to 60 GB of data. Utilizing a design based on a Magneto-optical disc, but using phase change technology
combined with a blue violet laser, a UDO disc can store substantially more data than a magneto-optical disc or
MO, because of the shorter wavelength (405 nm) of the blue-violet laser employed. MOs use a
650 nm-wavelength red laser. Because its beam width is shorter when burning to a disc than a red-laser for
MO, a blue-violet laser allows more information to be stored digitally in the same amount of space.
Current generations of UDO store up to 60 GB (30 GB per side), and a 120 GB version of UDO is in
development.
It should be noted that these technologies use different lens optics and mechanisms that result in different
densities of data and different read/write times. Some blue laser technologies use 120 mm media, while
others use 130 mm. These differences result in all current blue laser-based products being incompatible.
Some initiatives have been proposed to develop an industry standard for recording with blue laser
technologies, but little progress has been made to date in this area.
Note also that the technology is sufficiently advanced to render it incompatible with prior generations of optical
media, e.g. CD and DVD. In other words, blue-laser media readers are unable to read red-laser media and
vice versa. Manufacturers have recently announced plans to make devices capable of reading both red- and
blue-laser media, but these have not been put into production as yet. Moreover, there are still questions about
which versions of red- and blue-laser-based technologies will be supported by particular manufacturers.
5 Optical device characteristics
5.1 General
The fundamental component in optical storage is the optical drive. This device contains the laser, actuator,
and loading mechanism to allow data to be stored on the reflective optical media. A drive can be attached
directly to a computer, and if the appropriate device drivers are in place, the computer can access the device
for data storage and retrieval.
5.2 Readers / writers
As optical technologies have been introduced, the pattern has typically been to introduce a reader followed by
a writer. These devices typically hold one media at a time and require intervention to physically unload and
reload other media.
Historically these have been specific to particular optical technologies and media. For media that required
cartridges, this distinction is even more pronounced.
Later generations of technology often offer increased compatibility with earlier formats.
5.3 Multi-function drives
There are three basic approaches to the development and use of multi-function drives. The first approach is
the combination of WORM and phase change media. This approach provides the combined functionality of
write-once capability and rewritable functionality using the phase change technology. The second approach is
similar to the first but uses MO technology in place of phase change technology. The third approach is to use
a single-function drive and through modification to the media provide both write-once and rewritable
capabilities.
Multi-function drives provide the ability to read both WORM disks and rewritable disks using a mode which
can be changed through the software drivers accessing the optical drive. This functionality was an important
step forward in the optical drive industry. Most system integrators recognize the importance of using rewritable
disks for those customers who need the ability to change the data on the disk. Before rewritable systems
became available, users were sometimes told that even though WORM disks were write-once devices, the
software provided the ability to update the data by redirecting pointers away from the old data and toward new
data. In this case, the old data remains, but security features would prevent unauthorized access to any but
the appropriate version. There are two significant problems with this approach. First, as the data is updated,
more and more space is used. Perhaps more importantly, the data is never truly overwritten. The advantage
to the approach of never erasing the older data is that the software system can be designed to store several
pointers, which supports an audit trail of document changes.
Some vendors have developed a different approach to providing multi-function drives to their customers. This
approach utilizes special encoding of the media when manufactured.
5.4 Jukeboxes/libraries
5.4.1 General
Commonly, optical drives are incorporated in a library configuration with an autochanger and a number of
storage slots for media. This configuration is often referred to as a jukebox, since it resembles the jukeboxes
of the fifties. Attaching a jukebox to a computer or, network of computers gives potential access to a very large
amount of data.
Jukeboxes can be very slow in production environments where requests are made to different media
constantly, because of the requirement for the robotics to unload and reload media frequently. There are
strategies available to mitigate this issue, including prefetching, disk optimization, and copying the data to
attached magnetic disk.
The following should be noted.
⎯ Information stored in jukeboxes is typically used for archival purposes and access is required by the user
community and usually configured along with a transient or temporary magnetic cache that is used for
frequent and/or “in-process” work activities. This enables users to utilize optical storage technology for
archival purposes while having rapid access to the information as required by the organization.
⎯ Jukeboxes commonly have a standard capacity from 750 GB to 35 TB and can incorporate many different
types of storage media in a secure environment.
⎯ Access time when the media is not in a drive can average 5 s (this access time is important to understand
as related to the concept of prefetching, disk optimization, copy to magnetic, etc.). In the case of data
caching to hard disk, there is no latency for reading and writing of data if the information is in cache. If it is
not, then the hardware latency applies, which can be on average 5 s before the media is ready for access.
5.4.2 Data cache
Data caching is the process of using a hard disk cache to manage the directory structure of the data and the
reads and writes to and from the library, while in the background the cached data is archived to the optical
media. This process is transparent to the user and provides significant performance benefits to the archive
10 © ISO 2009 – All rights reserved
system. Another benefit is system longevity. Multiple requests for data may be retrieved from the cache rather
than having to go and fetch the media for each request. This mitigates the unmounting and remounting media
between requests. The cache is managed as part of the archive file system.
5.4.3 Prefetching
Prefetching assumes that a request for a particular piece of data will be followed by a subsequent request for
nearby or related data. For example, a user who opts to retrieve a particular TIFF from a DVD may also want
the next or previous TIFF, or may want all the TIFFs in a particular folder. The prefetching application retrieves
those additional files automatically based on rules. Prefetching can also be based on analysis of work to be
performed, so that data that may be required are cached to a high performance magnetic disk before the work
is assigned.
5.4.4 Disk optimization
Disk optimization refers to storing similar documents together on the same physical media. There are two
primary reasons for doing this. The first is performance related: servicing requests from a single media
reduces the need to unmount and remount media between requests, dramatically speeding response times.
The second relates to the need to manage documents through their lifecycle and then destroy them once that
lifecycle has expired. It is simple to destroy an entire media and all the documents on it that expire at the
same time, compared to managing documents with different retention periods. This is discussed in further
detail in 6.8.
5.4.5 Copy to magnetic
Some optical libraries include magnetic storage as well. This magnetic storage can be used to hold the
contents of several media or even all the media mounted in the library, thereby providing access to the
information stored on the library at much higher speeds. This is often used when there is a regulatory
requirement to store information on non-rewritable media; the data is stored on WORM-type optical media, but
a higher volume of requests can be serviced faster through the local magnetic copies.
5.5 Software support for optical libraries
In attaching an optical library (jukebox) to a computer system, there are three functions that the system should
be able to perform. First of all, the robotic function of the jukebox should be controlled. This is generally
accomplished via device drivers, which are very low-level software programs that send commands to the
jukebox to do operations like retrieve a cartridge and load it in a drive. The next function needed is the ability
to format data for storage on the media. This layer of software is typically referred to as a file system. Finally,
in a comprehensive system, there may be a requirement for jukebox management. This could include ejecting
an infrequently used cartridge to make room for media which stores more active data.
The device driver layer of software is usually customized to the individual jukebox, but there are two
approaches which can be taken to the file system and library management software.
One approach to reading and/or writing data to optical media is to implement a proprietary system, not only to
read and write the actual data but also to interface with the computer. The advantage to this approach is that
the system is usually streamlined to provide maximum throughput and performance without affecting the local
or wide-area network. The disadvantage of this approach lies in the fact that it is a proprietary system thereby
limiting future changes or modifications.
Another approach typically implemented by organizations choosing to use optical storage media eliminates
the potentially very significant issue of using a proprietary methodology and utilizes ISO/IEC 13346 also
referred to as “UDF”. This is a technology International Standard supported by almost all optical storage
vendors/solutions used to read/write data to/from the optical media. ISO/IEC 13346 describes in great detail
the methodologies to be used when reading or writing optical media, thereby ensuring the data can be
retrieved at a later date by any software package supporting the industry standard.
6 Implementation strategies
6.1 General
The implementation of an optical-disk-based system can be difficult if not approached in a careful manner.
Since a one-size-fits-all approach is not prudent or practical, a complete and detailed analysis of the process
that is to be image-enabled should be performed. This analysis should provide information that will be used in
the selection of the type of optical media, the capacity per disk, the total system capacity, and required system
response. Together with the initial selection of optical storage capabilities, the imaging analysis should provide
information enabling the design of an upwardly scaleable system. For example, if the current and near-future
retrieval and storage requirements are small, a stand-alone optical system could provide sufficient capacity.
As the need for additional data storage increases, that initial stand-alone drive could be inserted into a
jukebox that would provide greater storage and retrieval capabilities. However, if the initial and near-future
storage and retrieval capabilities are large, as it is in most cases, the user can purchase multiple optical drives
and mount them into a jukebox that will handle the greater capacity. In any event, the buyer should estimate
future growth and storage requirements and address the question of hardware compatibility and
interoperability in light of future needs.
The first step in the analytical process is reviewing the existing flow of data throughout the organization that is
being image-enabled. This information provides se
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