Facts About Tape Technology
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[I] - In its basic form tape technology is linear in fashion. Later technologies employ serpentine recording where the tape is written/read in multiple passes moving forward and in reverse. There are also helical scan formats, such as DDS, 8mm and AIT , which are written diagonally across the tape.
[I,S] - Tapes must be read from beginning to end, with few exceptions. There is provision on AIT technology to pre-format the tape into sectors which are similar to volumes on a hard disk. This allows files to be written into different sectors or desired points throughout the tape.
[S] - Tape read/write speeds are limited by the tape technology (firmware/drives), and other factors such as firmware or software in adapter cards or drives can reduce the maximum performance.
[S] - The biggest cause for lower performance in tape restoration speeds (outside of the issues above) is due to block sizing. Small block sizes require a disproportionate amount of processor cycles and demands large cache buffers. Different backup software packages use differing block sizes; even the same software on a different platform may have a different default block size. Most recent software packages have user-configurable block sizes.
[I] - Tape technology is for the most part platform independent (Windows/VMS/UNIX), with the largest exception in the mainframe arena.
[P,S] - Tape errors are typically categorized as logical errors or physical errors, although they often show them selves as I/O errors. Latter technologies such as 3590 can be software write protected which puts the cartridge into read only mode after a number of write errors are encountered.
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[P,S] - 3590/92, 9840, LTO, SDLT, 3570, and some others have active servo tracks which are used to align the heads and the data tracks. These servo tracks can be the cause of many types of problems, which manifest themselves as failures of the error recovery process "ERP" errors. This happens when the drive read-back does not match up with the expected results. In such a circumstance the drive tries to read that portion of tape over again several times prior to issuing a failure. Defective servo tracks can cause the tape to go "off track" and fail to read enough of the track that it should, hence the read errors.
[P,S] - 99% of backup software will write and/or read data up to an error point and then fail. This is desirable when writing data, but when reading data, it prevents access to other data in non-error areas on the tape. Most enterprise software solutions will write to a tape until an error is found. Once an error is detected, the system will write protect the tape and continue onto another tape.
[I] - To catalog the data on the tape, it must be read in its entirety.
[I] - Different tape cartridges of the same tape type DO DIFFER on the amount of data the tapes will hold. This poses potential issues for duplicating tapes. For example, duplicating one DLT8000 format tape to another may fail, as the source tape may contain 40.4GB of data and the destination tape may only have room for 40.1 GB. Cartridge-based tapes do not have a physical end of tape marker. The point where a tape stops recording data is based on a calculation which uses the hub speed to determine the circumference of the tape wind. Lots of components contribute to variation in this calculation, including tension, drive speed variations, along with tape thickness. This leads to varying results in the amount of data that can be written to a tape.
[I] - Based on the issue above, it is often advisable to step up to a higher capacity cartridge when duplicating tapes. For example, DLT 7000 should consider going to DLT 8000.
[I] - Tape compression is a factor of the data. Some types of data will readily compress where as others will not. Most drives use the same or very similar software algorithm, LDZ1. Hardware only effects compression in as much as what version of the compression algorithm that is loaded, if any.
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[I,S] - "Enterprise" backup software packages are highly user configurable, so no two installations of the same software package may be alike. What makes them different from each other is the practice of multiplexing a data stream across many drives, or backing up many servers simultaneously to the same drive. Additionally, the use of backup agents which provide incremental or brick by brick backups of mail or database files will differ in the format of the data written to the tape.
[I,S] - Per the above comment, by performing non-native tape restoration of data, meaning restoration of data by means other than the software that was used to create the data on the backup tape, there is often a slow ramp-up period to match the non-native restoration software to the source tape data.
[I] - eMag updates its non-native restoration software with every instance and configuration seen by our processing engineers. This ensures that no ramp-up development effort is required the second time the configuration is seen.
[S] - Non-native restoration of data is almost always faster, as there are no operating system issues or backup software index/database issues. This allows Windows backups to be restored to UNIX environments and vice-versa.
[S] - Restoration of data from backup tape often occurs at a much slower rate than the backup process that wrote data to the tape. This fact is not well known and serves as the leading source of frustration for users requiring data restoration. The reasons for the slower restores could be that unwanted data from other servers is interleaved with the wanted data, requiring more data to be read than is required. Additionally, the need for the backup software to check with internal catalogues causes drives to start and stop versus simply streaming the data.
[I] - Tape technology is far more portable than hard disk technology, which is why it continues to thrive while disk-based storage costs decrease.
[I] - The price per MB or tape storage is much less than with hard disk technology, especially considering some of the newer, high-capacity tapes.
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[P] - Data recovery from overwritten tape differs significantly from hard disks in that data written to tape is linear and not random as it is on hard disks. Recovery is only available for the data that has yet to be overwritten (if any). Another difference between tape and disk is that to erase a file on disk only the directory entry in the file allocation table is deleted .This leaves the data in place awaiting overwrite or recovery.
[S] - Modern, high-capacity tape formats can take up to 24 hours (and beyond) to restore one tape, due to one or more of the following issues: block size, firmware, lack of processing power, memory, cache, adapter cards, and operating system. The headline data transfer rate of any format normally allows for a drive to be read / written in 3-4 hours.
[I] - Tape storage capacities have taken a significant and rapid leap in the past year, advancing to upwards of 600GB native capacity, 1.2TB compressed (Certance just released their Ultrium 3 drive).
[I] - At present, only a handful of tape technology conforms to SEC and NASD requirements as a WORM (write once, read many) media. StorageTek has their Vault Safe and IBM has their 3592 WORM tape available. Additionally there are also now AIT and LTO WORM tape.
[P,S] - Tape media is susceptible to tearing, breakage or other issues due to media characteristics, such as the thickness (thinness), edge damage, drive wear and tear, and even impact issues due to the media being dropped.
[P,S] - The leading causes of physical damage to tape media are poor maintenance on tape drives (little to no cleaning) and poor physical handling by operatives or robots.
[P,S] - Lack of maintenance on tape drives can lead to an increase in logical errors during the restoration of data from tape. For example, data written to a tape with a misaligned drive head may or may not be able to be read by the same drive, but almost never will be able to be read by a different tape drive without employing modification to that drive's physical characteristics (which should be left to recovery professionals).
[S] - Tape backup sets (i.e. more than one tape per backup) must be restored sequentially. As such, it is important to know the order of the tapes in the backup set. Without being told the order, it must manually be determined, which will slow the restoration effort.
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