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Tape 101 - The Basics of Tape Backup Media
Tape 101 - The Basics of Tape Backup Media
Today there are multiple families (DLT, LTO, 9840, etc) in existence. Tape capacity has been growing exponentially, and there are an enormous number of tapes in circulation storing an even more enormous amount of information. There have also been quite a few predictions about the death of tapes, but they're still widely in use today because of two simple reasons - they're inexpensive, and they're easily moved off-site for archiving purposes.
So What is a Tape?
- It is a sequential recording system
- It contains blocks and filemarks (tapemarks)
- A tape ends with End of Data (EOD)
- Blocks can be any size, typically 512-bytes to 256KB
- Data can only be appended to a tape, never inserted
What's a Typical Tape Made of?
Data on a Tape: The Definitions
- Bit - 1 or 0
- Nibble - 4 bits
- Byte - 8 bits 10010110
- Word - 16 bits
- Block - Series of words set into manageable chunks (512 byte block for example)
- Inter block gap - Space between blocks. The smaller the block the more the gaps, the slower the read the less the capacity
- File - A series of blocks of data that make up a file like a .txt
- Filemark - A mark the signifies the end or beginning of a section of data on a tape
- Double filemark - Typically a double filemark signifies the end of data (EOD)
Recording a Bit (binary digit)
The Recording Types Used by Tapes
- Longitudinal - Tracks written the length of the tape; takes less time (eg. 9-track, 3480)
- Serpentine - The head moves to new track positions, still longitudinal; cheap, stores a lot, takes a long time (eg. DLT, LTO)
- Helical Scan - Tracks written at an angle (about 9 degrees) to the tape (eg. 4mm, 8mm)
Some Factors to Determine in Selecting a Tape Technology
| Reasons | Good | Bad |
|---|---|---|
| Speed-Access to data | 9840 | DLT |
| Speed-Transfer of data | 3590K | DLT |
| Cost per MB of storage | DLT/LTO | 3590J |
| Frequent cartridge loads | 3590J | 4mm |
| Low cost of tape drive | QIC/4mm | 3590K |
| Rugged environment | 3590 | 8mm |
| Low cost per MB; infrequent tape use | 4mm/8mm | 9940 |
| Need to keep data secure long term | LTO | QIC |
What is on a Backup Tape
- A backup contains a snap shot of a computer at a certain point in time
- Dates of files
- It may contain files that have since been deleted
- It might contain files that have since been modified
- It may contain Windows registry and other system log files
- A single tape very often contains the complete computer, not just the data files
What is Not on a Backup Tape
- Deleted files are not backed up
- There is no slack on a tape
- There are no un-allocated sectors (free space)
Overwritten Tapes
- The part of tape overwritten must be considered lost
- Most previously written data will still be on the tape if new data is shorter
- 9-track tape marks EOD logically, and so previous data may be recovered
- On most other tapes, the drive locks out the ability to read past the end of the current data. Data is there, but cannot be accessed
- Reading past EOD requires modified tape drives - that are not commercially available
What Types of Tapes Should be Restored for Discovery Purposes
- System backups have most information
- Less Relevant - data interchange, online storage (mainframes)
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