The use of tape for data storage and data recovery in the computer industry for many decades. Tape a solid and robust methods of storing code and data, together with a far lower cost of ownership than the available hard drive options.
Today the cost of the drive has dramatically, but tape storage is still considered the best available form of long-term archiving with regard to price and resilience.
Those of us who were born long before enough can remember treading cautiously machines, and speaking in low tones when the existence of the floor of the building where the computer disks were placed. Disks were unreliable, in small quantities, and expensive, around whereas Restoring data from tape was fast enough and is likely to work.
The concept of "near-line storage developed and still exists in the world of mainframe, AS/400 and large UNIX computing. Years, a request to restore a file to a message appear on the operator's computer screen to get the open tape reels marked KV19473D and load it on 15th Drive The data were from the band after only a short delay to the user.
In those days the operator was some form of robotic tape library, and the open tape reels with a tape cartridge that can be handled mechanically (as IBM 3570 and 3590, STK 9840 and 9940, and natural LTO Ultrium and DLT ). This process has evolved into hierarchical storage management or HSM and allows for "unlimited" storage (as infinite as you can afford it, space, tape drives and media).
With smaller systems, including some systems that would pretty much like today MicroVax, there was a further procedural questions use of tape storage. Partly this was due to the cost of robotic equipment, but especially as the rise of the mini-and micro-computer coincided with the beginning of the rise of lower cost more reliable hard disks and the concept of client /server and the daily band As a backup source or data required only if an error occurred and thus to avoid the hard drive data recovery work.
attempts to introduce HSM in this market, with intermediate storage such as optical disk and tape for long-term archive, came and went throughout his 90-years, but band was largely as a backup and retrieval medium.
Tape Storage How different from Disk
Apart from the substantive differences and the low-level recording technologies used by the general concepts do not differ between tape and disk. Each uses magnetism to encode data on a suitable recording medium receptive.
The real differences are in the implementation and use, and the big physical differences between the two.
The short answer to "What is the difference" is that the disk is a random access medium and tape is a sequential one. To go into greater depth, hard disks are usually pre-formatted with a known number of recordable "sectors" whereas tape is written on the fly.
The sequential access nature of the band reflects its physical nature, it is long and narrow and in order to receive some data at the end of the trip must traverse the length of the strip. With the recording on hard disk sector attracts all recorded all the drive, is the head position on the right track and wait for the data to spin past. Thus an access time of small fractions of seconds against everything except for a few minutes, you would not go far implement random access to tape.
The question of formatting, however, is far from clear. Early open roles tapes, Exabyte and quarter-inch cartridges (the older version of the SLR is often known as streamers) had clear mechanisms for cleaning the tape before the write head, so that the recording was always on blank tape.
The smaller quarter-inch cartridges, DC2000 and recently Travan, ADR and ditto, were formatted with sectors (usually during the production). The first DC2000 drives ran from the floppy disk controller in a PC and be operated as floppy disks. So in theory they were random access, but the practical access time would be very few people would live long enough to make it in this form for a considerable amount of data.
Newer tape format (SDLT, LTO Ultirum, 3590, 3570, and many others), while not already formatted with data sectors have a lot of servo data is written to them during the manufacture and, if they are deleted useless . These include servo-tracking data, which is used for data alignment in the process now that the recording densities have increased, and there is little or no space unused.
One, often undesirable feature of tape storage is the concept of "the last thing you wrote is the last thing you can restore." With a hard drive each sector is clearly addressable. If data is in Sector 79 has no impact on the sectors 78 and 80 With tape, as soon as recording has finished the drive notes that the last thing written is the new end-of-data. So, if you have a cassette with 400GB and write 2MB to the top of it is just sitting 400GB on the tape that can not be accessed without recourse to a tape data recovery service.
In
Data Recovery parlance, this is writing about or re-initialization. Do not be fooled to think that there is a chance to get the data back, actually over-written, this is the stuff of science fiction, but the rest of the inaccessible but Data can often be removed from the tape
The advantage that the band is that every file contiguously in general and there are no weaknesses in the File Allocation Tables, when access to the data.
That is all right in general, but there are no rules. Some tape-recording formats (Legato Networker, NetBackup and ARCserve among them) the data from multiple sources and they intertwine on the tape (sometimes called multiplexing, or multi-streaming). As said earlier, there's nothing to the development of random-access tape, but the shape is wrong, and it would never catch on.
However, there are compromises. IBM 3570 and 9840 STK attempt to split the difference between the two styles of recording. You use a tape cassette, so that the band is on two wheels inside the case, then more like DLT and Ultrium, where there is a single coil and the tape is to assume the new roles in the drive. The "start" of the band is actually in the middle, so that during the charging time of the cassette is midway from both ends, and the data on multiple tracks, so that the drive can be positioned and along the tape to data. So a wink in the direction of random-access and faster access time than the average of the tape when the time to restore data from a single file is still generally much more that with the hard disk.
Tape Storage Concepts
We can alongside the actual recording technique and the clock back to the 9-track? Open-inch tape reels for the terms used in data storage tape and tape data recovery . This type of tape was overwhelming during 1980 and to a certain extent, the drives that followed had to imitate the methodology, to replace them. This means that an Ultrium drive, a DLT drive and a DAT drive all the data and give it back just as the open-wheels drive, even if they are radically different recording formats.
With the open-reel tape has data on the drive as a sequence of data buffer blocks called loads. The drive would encode each with its own detection and error correction data, and with a gap between each individual. These inter-block gap is the reason why you might sometimes hear people say that they "used a bigger block size to increase capacity." On open reel tapes of the gap was a fixed size, so that the smaller the block size the greater the number of blocks needed to ensure an amount of data. The greater the number of blocks, the greater the number of gaps and capacity has been lost. Then again, with older bands ever larger the block size of the opportunity, a striking range of the band unusable, so that the whole thing was a bit hit-and-Miss.
With modern drives, the data block is only what you click on the drive, and what you get back. Internally, it is a matter of encoding and has little to do with how the data is actually stored.
The above had a few exceptions, notably the older 8mm Exabyte helical antennas scan drives. This breakdown of the data in 1024 byte sections when writing to tape and would not connected to a 1024-byte memory unit between data users blocked. The result was that when you write 1025 byte blocks on a tape then everyone was than 2048 bytes and the capacity of a tape was halved. There are exceptions to all rules.
tape concepts
Sun, tape drives, so that theoretically blank tape, have no pre-formatting, and if you have data at the beginning you have lost everything that you have overwritten and everything after the dot on At which you stop writing.
There is nothing either right or wrong with each of these, it is just they way they are. Which band gives you is high volume, low cost per gigabyte of memory, which can be found on the ground, pick up and read later. Do not try that with a hard disk and then expect to be able to simply your data.
file Marks aka Mark tape
It is a sub-divider that you do not find on a plate. A file is a data-coded patterns by the drive and used to distance to a certain position on a cassette. They want to restore data from backup 3, including the backup software can not read by backup sets 1 and 2, first, it skips file marks, and then begins to read and restore data once it has set 3rd
With 4mm DAT there is an additional type of file called mark as the mark. This allows two different types of data marker, even if only Sytos Plus, SBACKUP and some proprietary formats ever made use of this feature.
Helical scan drives, AIT, Exabyte and DAT, brand-coded file so that they can be found during the high-speed search operations. Normally, like a VCR, the tape is moving slowly while reading. It would take 2-3 hours on heading down to read the band kick in order to quickly locate the drive and can then go to the next file mark in a fraction of the time. In relation to this video is a fast-forward "and allow quick access to restore data from the tape.
Do not be fooled by the name but. They sound like little markers, if they actually can be several megabytes in size to some types of tape.
end-of-Recorded Media
When reading from a band you might encounter a condition known as "end-of-Recorded Media", sometimes referred to as "blank check". For older drives in the reception would erase the disk a length of tape afterward. Subsequent attempts would be reading in this length of the blank tape and know that they had at the end. Modern drives encode data model, similar in size to a file mark that denotes the end of the recording. Data on normal means no longer at this point, there is no way past and recovery methods and technology specialist will need to have access to this data is lost.
mainframe, midrange and some systems do not rely on the drive message that the end of data has been reached, but to their own devices. IBM Systems would encode a double file, HP systems use a triple-file mark. These patterns called logical end of the data.
These systems are still on their logical mechanism to say 'that's it', but the drive is still its own thing. At the moment, stops the recording of the EOD is written, and that is that data recovery without professional assistance.
block modes
variable block mode
Disk drives are usually formatted with recordable sectors with 512 bytes. IBM AS/400 for the use of 520 or 522 bytes. Tapes, of course, have to be different.
Modern tape drives can be either in solid blocks of variable block mode. This is to plug in systems with different pedigrees.
mainframe systems, for example, 380/390 and IBM AS/400 (OK, it is not a mainframe, but it behaves like a), writing data in blocks, the right size for their purposes. The label block at the beginning of an IBM Labeled Volume was defined than 80 bytes long, so that a 80-byte block was written to the tape. For 80 byte blocks were not a practical proposal when dealing with real bands open the actual data into larger units was limited in size only by available memory either in the system of the tape drive Formatter.
Fixed-mode recording
Smaller systems cheaper and more drives with data pretty much as they did with the hard disk. It does not matter how much data it would be written in the same size pieces and the drives in this market segment obliged. The early quarter-inch cartridge drives would only record data in sections 512 bytes. Smaller UNIX systems and PC systems have a tradition of inclusion in this kind do. The only real difference between disk and tape here is that the tape block sizes for solids shooting mode typically have to 64KB or higher.
Later drives were designed to be backward compatible with these more primitive format and with the more expensive drives that are in the variable mode, or inserted as a direct replacement for these drives and so can either be fixed or variable block-recording devices .
block numbering
Early drives on brands skipping file on position along tape, but later tape devices introduced the concept of block-numbering. So each tape block has a unique number beginning with 0
This partly explains why the band block sizes used in the course of time. The SCSI specification describes the block number with 3 bytes, maximum 16777215 blocks. 512-byte blocks, this would mean that the maximum capacity of the band would be in the region of 9 GB, not very helpful when writing on a 800GB Ultrium 3 Data Cartridge.
Recording Techniques
Three basic tape storage formats have evolved since the late 1980s.
multi-track parallel
Helical Scan
Serpentine
Although the Soil between parallel and serpentine formats has closed in recent times with drives that elements of both formats.
? Open roles - AKA known as the 9-track parallel
The drive 9 Track records of data at once to the tape surface. Reception begins at the physical beginning of the tape (PBOT) and ends at the end of the physical of the tape (PEOT). This format developed by the punch card idea with the eight-bit bytes and a parity bit. So this is one byte at a time of recording.
The capacity of these tapes is tiny by today's standards. NRZI Recording format managed to write 23MB to 800bpi on a 2400 foot band. In its heyday, with a massive 6250 bits-per-inch capacity rose to an impressive 180MB.
Helical Scan We
are all more familiar with helical antennas scan when we can realize. It is a technology that was developed for video recording (VHS and Video8) and audio (DAT).
The tape is wrapped around a cylinder in which The read and write heads. The band is moving slowly, while the cylinder rotates rapidly with every twist the data to be written and then back to check (Read-after-write).
The name Helical scan sources from the patter described by the head passing along a slow moving band as "describes a part of a helix." (it is probably a marketable name as "diagonal data")
Exabyte Corporation has the Sony Video8 8mm recorder, a SCSI interface and some additional control and came with 2 GB of data storage format, the way ahead of its competitors, albeit briefly.
HP and Sony adapted developments in the audio market with 4mm media called Digital Audio Tape, added additional error correction and came with DDS-DAT. Sony AIT later on the basis created an 8mm Helical-scan format, and even one of the STK mainframe drives used this technique. Serpentine
The name derives from the model of the recording is to the front and rear for a series of tracks that seem a bit snake-like in character (according to some imaginative marketing person).
Early drives had a couple of recording heads for the recording forward and one for vice versa. The record would drive forward, until the end of Physical-od - Tape (PEOT), to reverse the physical beginning-of-band (PBOT), then the heads position and repeat the process. Early drives recorded 4 tracks, the latest record hundreds and overlaps with the parallel recording formats through multiple tracks simultaneously.
Similarly parallel recording format drives now records along the tape forward and then reversed, so that they become almost serpentine.
In the data connection, there is the problem that physical damage effects of several places in the recording, since the drive is going on in every area of the band. Of course, this is only a problem if the tape is wrinkled or brandy, and it is an argument How likely that is, in comparison with helical antennas scan devices that are a much more complex tape path. We have no intention of affray between the exponents of the individual style of recording.
Summary
Tape still has a big role in data security and the long-term archiving of important information. As a data recovery specialist, I see both disks and tapes are not damaged, and during recovery tape comes with its own set of challenges that make it a difficult process, rarely is a band a complete failure and success of the recovery is well over 95%.
The author has been working as a data recovery engineer and software developer for the past 25 years in the UK, US, Germany and Norway and has now, along with other long standing technical experts, started a data recovery business aimed at providing a technical rather than sales led service.
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