Mainframe Tape Details:

Physical Recording Issues
Logical Recording Issues
Data Issues
 

Physical Recording Issues:

Media:

9-track tapes:

9-track tapes are half-inch tape spooled on a large, open, reel.  This tape is recorded with 9 tracks across the width of the tape; 8 of the tracks form one byte of data, and the 9th track records a parity bit (an error checking bit).  There are four recording densities: 800, 1600, 3200, and 6250 BPI (bits per inch). Each of these densities uses a different recording method.  Because there is one byte of data recorded across the width of the tape, the byte density is equal to the bit density (although there is overhead in the CRC and gaps).  i.e., a 1600 BPI (bits per inch) tape records 1600 bytes per inch of tape.

9-track tapes come in three common sizes, 600', 1200', and 2400', but are also available in 100', 300', and 3600' tapes.  The block size has a great effect on the amount of data a 9-track tape will hold.  With a large block size, 1600 BPI 2400' tapes hold up to 42 MB, and 6250 BPI 2400' tapes hold up to 160 MB.  9-track drives never compress the data before writing it to tape.

3480, 3490, 3490E tapes:

3480 tapes are also half inch tape, inside a hard, protective case that is about 4" x 5" and 1" thick.  When inserted into the drive the tape leader is pulled out of the case and wound onto a take-up spool inside the drive. By putting the take-up reel inside the drive, the cartridge is kept small.  The standard 3480 contains 656 feet of tape, recorded with 18 tracks across the tape, at a recording density of 18,000 FCI (flux changes per inch). As with 9-track tapes, the block size determines the recording capacity, but 210 MB is the stated capacity with large blocks.  3480 tapes are intended by IBM to be a replacement for 9-track tape, and as such are usually found on similar equipment and recorded in similar ways as 9-track.  3480 drives do not compress the data before writing it to tape.

3490 is 3480 with compression.  The media and number of tracks are the same as 3480. The drive compresses the data before writing it to the tape.  The compression ratio depends greatly on the data content, but 2:1 is considered typical, so these tapes are usually considered to hold about 400 MB. Some binary files may not compress at all, but we have also seen 1800 MB on a 3490 tape.

Despite the name similarity to 3490, 3490E is quite different.  It is recorded with 36 tracks across the tape, not 18 as in 3490, has a higher bit density of 36,000 ftpi, and uses thinner and longer 1100 foot media. Like the 3490, it uses data compression for a typical capacity of 1600 to 2400 MB per tape.

Most 3490E drives can read 3480 and 3490 tapes, but can't write them, because of the thinner recording heads for the 36 tracks.   However, Overland Data's 3490E drives can write 3480 & 3490, and these are re-marketed under DEC and IBM brand names.

3480 media was not originally intended to be used in 3490E drives, so it was not certified (tested) for 36 tracks. Because the 36 track drive writes to untested areas of an 18 track tape, the error rate can be excessive if you use an 18 track tape in a 36 track drive. But it happens so frequently, that recently manufactured 3480 media is certified for use in 3490E drives. Notice, however, that the 3480 media is shorter than the 3490E media, so it has less capacity.

We strongly discourage the use of a 3480 tape in a 3490E drive. There is no way to know if a particular tape was certified for 36 track use, and we have seen many 3480 tapes recorded in 36 track mode that were just not readable.

If the recording density isn't marked on the outside label by the operator who wrote the tape, there is generally no way to tell if one of these tapes is recorded in 3480, 3490, or 3490E density without trying the tape in a drive. However, it's very unlikely a 3490E tape would be recorded in 18 track mode.

3570 tapes:

IBM 3570 tapes are center load, fast access, moderate performance tapes encased in an elongated cassette.  Unlike the 3480 tapes, the 3570 has two spools in the cartridge, and the media never leaves the cartridge housing. 

3570 B and C tapes have the same capacity, but different data transfer rates, and work in the "B" and "C" model drives, respectively. 3570 CXL is an extra length tape for the model "C" drive only. 3570 B and C tapes have a native capacity of 5 GB and a compressed capacity of about 15 GB. 3570 CXL has a capacity of about 7 GB, or 21 GB with 3:1 compression.

3590 tapes:

IBM's new 3590 is similar to 3490. The half-inch tape is contained in a cartridge the same size as a 3480, but the tape itself is very different. The 3590B drive writes 128 tracks, 32 tracks at a time, in a serpentine pattern, and has an uncompressed capacity of 10 or 20 GB, depending on the tape length, and a typical compressed capacity of 20 or 60 GB, although we've seen well over 100 GB on one tape.

There is a 3590E drive which is similar to the 3590B and uses the same media, but writes 256 tracks, for double the capacity of the B drive.  In July 2002 IBM announced a 3590H model which writes 384 tracks across the tape, again using the same media as the 3590B and E models. The 3590H has a capacity of 30 to 180 GB, depending on tape length and compression.
 

Blocks, Gaps, and File Marks:

Most tape drives have an upper limit on the block size they can handle, and some have a minimum block size. Nearly all drives can accept up to 32K bytes per block, with 64K being common. Newer drives like the 3590 are capable of larger block sizes, up to 2 MB (2048 KB), but most tapes are still written with blocks of 32K or less.

Each block contains CRC (cyclic redundancy check) bytes after the data. These CRC bytes are error detection and correction data which allow the tape drive to read data from a tape which contains some "dropout", or lost bits, and correct for the missing bits. Without these CRC bytes most tapes would not be readable with 100% accuracy.

Each block is followed by a "gap" (basically unrecorded tape) to separate it from the next block. A group of blocks comprise a tape "file" (usually comparable to a disk file), and each tape file is followed by a "file mark" (also called a "tape mark"), which is a special magnetic pattern recorded on the tape to denote the end of a file. By convention, two file marks in a row denote the end of recorded data on the tape, or logical EOT (end-of-tape). However, newer drives utilize a special magnetic pattern to mark the end of data on the tape.

The gaps use a significant amount of tape, 0.3" or 0.6" per gap on 9-track tape, for example. So the block size, and hence the number of gaps, determines the amount of data you can fit on a 9-track or 3480 tape. If you make the blocks large, say around 16KB per block, you can fit more data on a tape. To demonstrate how much of a difference the block size makes in the capacity of a tape, a 2400' 1600 BPI 9-track tape with a block size of 80 bytes only holds about 3.5 MB, compared to 41 MB with an 8000 byte block size. So, if you record one 80 byte record per block the tape will hold about 44,000 records, but if you record 100 records per block the tape will hold over 500,000 records.
 
 

Logical Recording Issues:

Tape Blocks:

A better method is to make the size of the tape block some multiple of the record size, say 10.  Then exactly 10 records fit in the first block, 10 in the second, and so on.  This is the method most often used for mainframe tapes, in two variations: "fixed-block" and "variable-block".

Fixed Block:

Most data on mainframe systems is stored in fixed length records .  When writing these records to a tape, the operator normally specifies a block size that is some multiple of the record size, such as 10, 20 or 100 records per block.  This results in a tape where all the blocks except the last are of the same size (fixed block).  The last block is often a "short block" because there are not enough records remaining to fill another complete block.  For example, a file of 93 records of length 250, which is "blocked 10" (meaning 10 records per block), will have 9 blocks of 2500 bytes (10 x 250), and one short block of 750 bytes (3 x 250). The last block is usually permitted to be shorter than the others, so it can contain only the remaining records. However, some systems or operators will pad the last block with dummy data (usually spaces or 9s) to make it the same size as the other blocks.

Notice that "fixed block" when used to describe mainframe tapes does not mean the blocks can only be one certain size, constrained by the drive; rather it means the blocks are all the same size.  The size of these blocks can be anything within the system limits, usually between 1 byte and 32,768 bytes. Furthermore, each tape file can use a different block size.  (There are some kinds of tape drives, notable QIC drives, that are constrained to writing blocks of only 512 bytes, and these drives are often called "fixed block mode" drives. This is a different issue than fixed-block mainframe tapes.)

Variable Block:

This generic term means writing data of variable length records to a tape with variable length blocks.  There are several methods to do this, but these three are common:

IBM Variable Block:

This is a standard method specified by IBM.  Each record varies in size and is prefixed by a 4 byte binary length code, called a "Record Descriptor Word" (RDW), giving its size (including the 4 byte RDW).  The tape blocks will be of variable size, and will also begin with a four byte length code, called a "Block Descriptor Word" (BDW), giving the size of the block (including the 4 byte BDW).  The operator specifies the maximum allowed block size.  Records are then assembled in memory until no more can fit in the block without overflowing the maximum size.  The size of the block is then calculated from the total of all the records plus the 4 bytes for the BDW (block length code).  The system then writes the BDW, plus all the records, to a tape block.

The tape block is then composed of a 4 byte BDW (block size code), followed by several records, each of which is preceded by a RDW (record length code).  This method allows you to write records of varying size to tape without spanning blocks.  To retrieve the records you reverse the process.

PC (UNIX, Windows, Macintosh) applications normally can't understand this format, so when we convert a variable-block tape, we convert the data to a PC file format, typically records delimited with a line-feed, carriage-return, or carriage-return / line-feed pair.

ANSI Variable Block:

This is similar to the IBM VB format, but the length codes and a few other things are different.

ANSI calles the record length code a "Record Control Word" (RCW), and the block length code a "Block Control Word" (BCW).  These length codes are written as ASCII numeric characters, as opposed to the IBM RDW and BDW which are written in binary format.

Blocks of Varying Size:

Undefined Format:

One of the standard IBM and ANSI tape types is an "undefined" format.  This allows you to write anything to a labeled tape.  There is no requirement for any kind of structure to the data, as there is in FB and VB tapes.  Undefined format is generally used to write unstructured data to a tape.  AS-400 "Save" tapes are written this way.

Labeled and Unlabeled Tapes:

Unlabeled Tape:

This is a method of writing a tape where only the raw data file is written to the tape; it has no labels or other overhead on the tape -- only the raw data. The tape may be fixed block or variable block, but most are fixed block.  The data may be in EBCDIC, ASCII, or binary.

If you inspect an unlabeled tape you will just find block after block of your data file, and nothing else.  As such, there is no information recorded on the tape to denote the file name or anything about the file; all that information is passed on the paper label on the reel, or other documentation.  If the tape contains multiple files, the order of the files and their record and block sizes will have to be listed on separate documentation.

Labeled Tapes:

To overcome the lack of information on unlabeled tapes, "Labeled tape" was invented.

IBM Standard Label (SL):

The user's data is written in the same manner as an unlabeled tape, but each data file on tape has a "header label" written before it, and a "trailer label" written after it.  Each of these labels is actually a small file written to the tape in a very specific format, containing information about the data file it is associated with. Among other things, these labels contain the DSN (Data Set Name, or file name) of the tape file, the record length, block length, creation and expiration dates, and the format of the file (fixed, variable, or undefined).  If a file spans multiple tapes these labels indicate that, and also the file segment on the current tape.  IBM Standard Labels are recorded in EBCDIC, and the data file should also be in EBCDIC.  The beginning of each tape also contains a "volume label".

The DSN (Data Set Name - the file name) is up to 17 characters in length, and is the right 17 characters of the disk file name, including the separators (periods) in the disk file name.  If the disk file name is over 17 characters, the left most characters are lost.

ANSI Standard Labels:

These are very similar to IBM SL tapes, but both the label and the data are in ASCII.  There are some differences in the labels and file handling, especially for multi-volume tapes, but that's beyond the scope of this discussion.
 

Putting it all together:

•       Unlabeled Fixed-block EBCDIC
•       Unlabeled Fixed-block ASCII
•       IBM Standard Label Fixed-block EBCDIC
•       ANSI Standard Label Fixed-block ASCII
•       IBM Standard Label Variable block EBCDIC

Other tapes:

Many tapes, such as UNIX tar and cpio, and many backup formats, will combine all the disk files into one big tape file.  That tape file contains information about each disk file, followed by the file, then immediately followed by another file.  There can be thousands of disk files in one tape file.  Such programs will commonly write blocks of some binary multiple (512 or 10,240 bytes, etc.) because that's usually faster.  There will be a file mark (tape mark) only at the end of the big tape file.  If you inspect the tape you will find only one file, written to blocks of the same size throughout the tape.

There are some other interesting formats: DEC VMS Backup wraps all the disk files up into one big "saveset", but then writes that saveset to a standard label fixed-block tape.  The tape labels are normal, but only VMS Backup can understand the contents of the saveset and extract the individual disk files.

Likewise, IBM OS-400 writes an entire library or "object" (folder) to a tape that has standard IBM labels, but Undefined (U) format. Only another AS-400 can understand the format.
 

Data Issues:

Additional Information

For more articles on data conversion, see our TechTalk Index.

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