S 4.169 Use of appropriate archival media
Initiation responsibility: Head of IT
Implementation responsibility: Head of IT, Administrator
The permanent electronic archiving of documents requires the use of suitable data media (archiving media). The following questions should be considered when selecting the archiving media:
- What data volume is to be archived?
- What access times are required on average?
- What is the number of simultaneous accesses on average?
- What retention periods are to be covered by the archiving medium?
- Is audit-proof storage of data required?
The following sections describe typical archiving media and their fields of application. Magnetic, magneto-optical or optical storage technologies are usually used for the data media. The advantages and disadvantages of the technologies are described in the relevant sections.
All archiving media described are susceptible to physical damage, for example, due to
- water
- fire or generated heat
- scratching of the medium by the drive due to contamination or dropping
- crumpling or tearing of the medium in the tape drive
- sabotage and theft
For this reason, archiving media must be stored carefully and protected against the impacts above. In addition, unauthorised access to the data media must be prevented. For this purpose it is recommended to apply the safeguards for protection of the data media described in module S 2.5 Data media archive and S 2.7 Protective cabinets, depending on the specific operational scenario of the electronic archive.
Digital magnetic systems
In magnetic storage systems, a storage effect is achieved by targeted local modification of a magnetised base medium. The magnetisation can be detected by a read device and the stored data can thus be read. By repeated application of a magnetic field the stored data can be changed. This is carried out in a targeted manner by using a read/write device or in an untargeted manner by strong external magnetic fields (e.g. electromagnetic fields near transformers or large coils). This can also occur unintentionally.
Magnetic data media are susceptible to attacks by means of strong magnetic fields which affect the storage medium. Since the magnetic base medium is typically manufactured as a composite material of plastics and a metallic (magnetisable) coating, long-term changes are to be expected, even when handled carefully. These can be caused, for example, by degradation (due to plasticisers in plastics), swelling (dissolution of plastic and metal layers) or oxidation (of the metal layer).
Due to the technology used, magnetic storage media are also always rewritable and erasable and are thus, without additional backup procedures, in principle only suitable for short-term archiving, where no protection against changing or rewriting documents has to be provided by the medium. Typically, this rules out the use as archiving medium, if audit-proof archiving is required. However, magnetic systems can be used for data backups and as cache media.
Auditing acceptability can be achieved with considerable effort by the use of cryptographic methods which allow for the detection of changes to the data (e.g. signing).
Typical magnetic storage media are hard disks, diskettes, and (magnetic) tape media.
- Diskettes
Diskettes, which are currently offered in dimensions of 3.5 inch, previously also 5.25 inch and larger, have a small capacity of 1.44MB. The use of diskettes as archiving media is only recommended for very small archives where no audit-proof (write-protected) archiving is required.
- Hard disks
In hard disks the storage medium and the write/read drive are typically accommodated together in one unit. This makes them susceptible to mechanical failures such as, for example, of the drive mechanism. The physical containment allows for a more dense arrangement of the magnetic media while protecting them against dust particles so that in contrast to diskette drives, hard disks have multiple write/read units.
Hard disks typically have a high capacity and short access time with a high transfer rate. Due to the storage technology used they are not suitable for permanent, audit-proof storage of documents. However, hard disks are used as data media for the archive system itself and in cache systems. - Magnetic tapes
A magnetic tape has a length of magnetic tape wound on a reel which is usually sequentially moved past a read-write head. The magnetic tape and write-read unit are typically not connected to each other.
For technical reasons, magnetic tapes have a very long access time and very low transfer rate. Their storage density and space requirement, however, is similar to hard disks.
- Magnetic tapes are suitable for storage of large amounts of data which only need to be accessed rarely and sequentially. They are therefore suitable for backups where medium-term, but not long-term stability is expected. As, in principle, magnetic tapes, too, can be overwritten, deleted or changed due to accidental impact of magnetic fields, they are not suitable for audit-proof storage of data.
The following table gives a short overview of the suitability of magnetic storage media for electronic archiving:
| Medium | Format and capacity | Standard | Use |
|---|---|---|---|
| Diskette | 3.5 - 5.25 inch,up to 1.44MB | de facto | Short-term storage for very small archives, not audit-proof |
| Hard disk | 2.5 - 5.25 inch,more than 100GB | Manufacturer standards | Short-term storage for small archives and cache systems, not audit-proof |
| Magnetic tape | more than 80GB | Manufacturer standards | Medium-term storage of medium-sized archives, not audit-proof |
Table: Suitability of magnetic storage media
Digital optical systems
In optical systems, a storage effect is achieved by targeted modification of the optical behaviour of a base medium. Data is typically stored by modification of the base medium by creating or simulating indentations (pits) in a flat base surface ("land") which result in a different optical behaviour of a precisely focused laser beam during reading. This allows for the interpretation of bit patterns.
Whereas the reading process is typically the same for all optical media (the wavelength of the laser used can be different though), there are major technological differences in the storage process.
- CD-ROM
CD-ROMs (Compact Disk Read Only Memory) are generated mechanically by stamping with a master data medium. Typically, it is not possible to subsequently change the data stored on a CD-ROM (WORM). However, the production of such data media is only cost-efficient in case of large quantities. Such data media are not suitable as archiving media, since typically only a very small quantity is produced in electronic archives, which is not cost-efficient. However, there are exceptions, such as "volume archives" as a supplement in widely distributed magazines.
Various standards have been defined for CD-ROMs ensuring extensive manufacturer support. Their storage capacity is typically up to 650MB. - CD-Recordables (CD-Rs)
In contrast to CD-ROMS, CD-Rs have an additional layer (typically cyanine or phthalocyanine) in which a light reflection can be generated by selectively creating marks with a laser ("burning"), so that a similar optical effect as with a CD-ROM is achieved during reading of the data media. The storage capacity is typically up to 700MB. Once the marks have been "burnt" they can no longer be deleted. The advantage over mechanical stamping of the data carrier is the individual adaptability. The disadvantages are the following:
- CD-Rewritables (CD-RWs)
Similar to CD-Rs, CD-RWs use an intermediate layer which, however, consists of a more advanced material (silver, indium, antimony, and tellurium) whose reflectivity can be changed in a targeted manner. This depends on the intensity of the laser used. CD-RWs are therefore rewritable and erasable, which may also occur unintentionally in case of a defective drive. They are therefore not suitable for archives where audit-proof storage of data is required. The storage capacity is typically up to 700MB.
Analogous to the CD-R, the CD-RW technology shows some weaknesses when considered for archiving:
- If there is a fault in the burn process, it is possible to simulate a light reflection, occasionally generated because the reaction in the intermediate layer is only temporary. CD-RWs therefore have to be checked after a few days to verify that this is not the case.
- There is a small residual risk with CD-RWs that spontaneous crystallisation of the surface will cause the random alteration of stored data.
- DVD
DVD media (Digital Versatile Disk) are a technological advancement of the compact disk (CD). DVDs allow for a significantly higher storage density from 4.7 to up to 17GB, depending on the manufacturer. Unlike the CD, the DVD format is not standardised, which is why different DVD versions are currently available on the market.
In some DVD versions, data can be stored on top of each other in two separate media layers which can be read separately with two differently focussed lasers (dual layer DVD).
- DVDs are currently also available as DVD-Recordable (DVD-R). It is expected that DVD-RWs will also be available on the market in the future. Analogous to the CD, the DVD-Recordable version is of particular interest, as it allows for audit-proof storage with a high storage capacity. The same restrictions regarding overwrite protection as for the CD-R must, however, be taken into account.
In addition to the widely used CD and DVD media, there are other standardised optical media which are used by manufacturers of large storage systems. The following table gives an overview of the available media formats and the associated standards:
| Format | Capacity | Standardisation |
|---|---|---|
| 3.5 inch | ANSI X3.213 | |
| CD (5.25 inch) | 650 - 700MB | ISO 9660 |
| DVD (5.25 inch) | 4.7 - 17GB | ISO 13346 |
| 5.25 inch, RW | 1 - 2.6GB | ISO 10089 |
| 5.25 inch, WORM | 1 - 2.6GB | ISO 9171, ANSI X3.191, ANSI X3.211, ANSI X3.214 |
| 12 inch | 2.6 - 16GB | manufacturer-specific, no standard |
| 14 inch, WORM | 6.8 - 25GB | ANSI X3.200 and ISO/IEC 10885 |
Table: Available media formats
The technology used for these media is generally the same as the optical method used for CD-R (DVD-R) and CD-RW (DVD-RW). The major differences lie in the processing of reliable materials and extended manufacturer warranties. These guarantee a data stability between 10 and 100 years for rewritable media and between 30 and 100 years for WORM media, depending on the manufacturer and the relevant ideal operating conditions specified.
For the WORM media described above, for technical reasons, it has so far also not been possible to rule out unused areas being subsequently overwritten. Accordingly, they, too, are not "real" write-once media, but merely non-erasable data media.
Usually, the corresponding manufacturers do not offer individual media, but complete storage solutions, mostly with automatic data media control. The storage media are then mechanically adopted to the relevant manufacturer solution and equipped with a housing allowing for their use in the corresponding robot systems (jukeboxes).
| Medium | Format and capacity | Use in archives | Auditing acceptability |
|---|---|---|---|
| CD-ROM | 5.25 inch, 650MB | not recommended | yes |
| CD-R | 5.25 inch, 700MB | small archives | yes* |
| CD-RW | 5.25 inch, 700MB | small archives | no |
| DVD | 5.25 inch, 4 - 17GB | not recommended | yes |
| DVD-R | 5.25 inch, 4 - 17GB | medium-sized archives | yes* |
| DVD-RW | 5.25 inch, 4 - 17GB | medium-sized archives | no |
| ISO 9171 WORM media | 5.25 inch, 1.3 - 2.6GB | medium-sized to large archives | yes* |
| ISO 10089 RW media | 5.25 inch, 1.3 - 2.6GB | medium-sized to large archives | no |
| 12 inch RW, manufacturer-specific | 12 inch, 2.6 - 16GB | large archives | no |
| 12 inch WORM, manufacturer-specific | 12 inch, 2.6 - 16GB | large archives | yes* |
| 14 inch media, manufacturer-specific | 14 inch, 6.8 - 25GB | large archives | unknown |
(* For technical reasons, overwriting of these media cannot be completely prevented. However, WORM media are generally considered to be audit-proof.)
Table: Overview of the suitability of optical storage media for electronic archiving.
Magneto-optical systems
Similar to optical storage systems, in magneto-optical (MO) storage systems, the stored data is read by scanning the storage media with a laser beam. However, in contrast to CD-like storage, the optical effect is not achieved by indentations in the surface of the storage medium, but by a magnet layer whose particles act as a polarisation filter during scanning and reflection of the laser beam. The polarisation of the surface can be selectively modified by applying a magnetic field to one particular region of the storage medium which is specifically heated by a laser. In a write process, the polarisation of the regions of the media surface is selectively modified.
The following table gives an overview of the available media formats and the associated standards:
| Format | Capacity | Standardisation |
|---|---|---|
| 3.5-inch format | 128 - 256MB | ISO standard 10090 |
| 5.25 inch, RW | 1.3 - 9.1GB | ANSI standard X3.212 |
| 5.25 inch, WORM | 1.3 - 9.1GB | ISO/IEC 11560, ANSI standard X3.220 |
Table: Media format
For the WORM media described above, for technical reasons, it has so far also not been possible to rule out unused areas being subsequently overwritten (burnt) without authorisation. Accordingly, they, too, are not "real" write-once media, but merely non-erasable data media.
Magneto-optical systems have a very high long-term stability (more than 30 years according to manufacturer specifications) and a high storage capacity of up to 9.1GB per medium. The following table gives a short overview of the suitability of magneto-optical storage media for electronic archiving:
| Medium | Capacity | Use in archives | Auditing acceptability |
|---|---|---|---|
| 3.5-inch format | 128 - 256MB | not recommended | no |
| 5.25 inch, RW | 1.3 - 9.1GB | medium-sized archives | no |
| 5.25 inch, WORM | 1.3 - 9.1GB | medium-sized archives | yes* |
(* For technical reasons, overwriting of the media cannot be completely prevented. However, WORM media are generally considered to be audit-proof.)
Table: Storage media for electronic archiving
Regardless of the type of the archiving medium selected, the medium should always be verified after storage. On the one hand, this should be realised by the system in order to verify that an exact image of the data to be stored was created. On the other hand, the Archive Administrator should conduct spot checks to ensure that all data intended for archiving has been archived and was not overlooked as a result of incorrect configurations.
Review questions:
- Are the archiving media suitable for the amount of data to be archived (e.g. with respect to the data volume to be archived, average access times and average simultaneous accesses to the archive system)?
- Are the archiving media suitable for long-term archiving (e.g. with respect to the auditing acceptability and service life)?