For the longest time, tape drives were a block to me. Although I understood
the basic concept (writing to a tape similar to a music cassette), it took me
quite a bit of time before I felt comfortable with them.
device has the potential for either saving your data or opening up career
opportunities for you to flip burgers, knowing how to install and use them is an
important part of your job as a system administrator.
Because the tape device
node is usually read/write, regular users can also back up their own data with
The first tape drives supported under Linux were quarter-inch
cartridge tapes, or QIC tapes. QIC is not just an abbreviation for the size of
the media; it is also a standard.
In principle, a QIC tape is like a
music cassette. Both consist of a long, two-layer tape. The "backing"
is usually made of cellulose acetate (photographic film) or polyester (1970s
leisure suits), polyester being more common today. The "coating" is
the actual media that holds the magnetic signals.
The difference is in
the way the tapes are moved from the supply reel to the take-up reel. In
cassette tapes, movement is accomplished by a capstan and the tape is pinched
between two rollers. QIC tapes spread the driving pressure out over a larger
area by means of a drive belt. Additionally, more care is taken to ensure that
the coating touches only the read/write heads. Another major difference is the
size. QIC tapes are much larger than cassette tapes (and a little bit smaller
than a VHS video tapes).
Initially, the QIC tape was 300 feet long and
held approximately 30Mb of data. This was a DC300 tape. The tape that next
appeared was a DC600, which was 600 feet long and could hold about 60Mb. As with
other technologies, tape drives got better and longer and were able to hold more
data. The technology advanced to the point where the same tapes could be used in
new drives and could store as much as twice as much as they could before.
There are currently several different QIC standards for writing to tape
drives, depending on the tape and tape drive being used. Older, 60Mb drives used
a QIC-24 format when writing to 60Mb tapes. Newer drives use the QIC-525 format
to write to several different kinds of tapes. As a result, different tapes yield
different capacity depending on the drive on which they are written.
example, I have an Archive 5150 tape drive that is "officially"
designed to work with 150MB tapes (DC6150). However, I can get 120Mb from a
DC600. Why? The DC600 is 600 feet long and the DC6150 is only 20 feet longer. A
tape drive designed to use DC600 tapes only writes in 9 tracks, however, and a
tape that uses DC6150s (like mine) writes in 15 tracks. In fact, you can use
many different combinations of tapes and drives.
One thing I would like
to point out from a technical standpoint is that there is no difference between
150Mb and 250Mb QIC drives. When the QIC standard was enhanced to include
1000-foot tapes, 150Mb drives automatically became 250Mb drives. (I wish I had
known this before I bought so many DC6150 tapes. Oh, well, live and learn.)
A similar thing happened with 320Mb and 525Mb tapes. The QIC-320 standard
was based on 600-foot tapes. However, the QIC committee decided to go with the
QIC-525 standard based on 1000-foot tape. Thats why a 600-foot tape writing with
the QIC-525 standard writes 320Mb.
Notice that this entire time, I never
referred to QIC--02 tapes. Thats because QIC-02 is not a tape standard, but a
An interesting side note is just how the data is
actually written to the tape. QIC tape drives use a system called
"serpentine recording." Like a serpent, it winds its way back and
forth along the length of the tape. It starts at one end and writes until it
reaches the other end. The tape drive then reverses direction and begins to
write toward the other end.
Other common tape drives are QIC--40 and
QIC-80 tape drives, which provide 40Mb and 80Mb, respectively. These provide an
inexpensive backup solution. These tape drives are connected to standard floppy
controllers and, in most cases, the standard floppy cables can be used. The size
of the tapes used for this kind of drive is about the same as a pack of
Aside from using the same type of controller, QIC-40/80 tape
drives are similar to with floppy drives in other ways as well. Both use
modified frequency modulation (MFM) when writing to the device. Sectors
are assigned in similar fashion and each tape has the equivalent of a file
allocation table to keep track of where each file is on the media.
QIC-40/80 tapes must be formatted before they are used, just like
floppies. Because the size of data storage is substantially greater than for a
floppy, formatting takes substantially longer. Depending on the speed of the
tape drive, formatting can take up to an hour. Pre-formatted tapes are also
available and, like their floppy counterparts, the prices are only slightly
higher than unformatted tapes.
Because these tape drives run off the
floppy controller, it is often a choice between a second floppy drive and a tape
drive. The deciding factor is the floppy controller. Normally, floppy
controllers can only handle two drives, so this is usually the limit.
However, this limit can be circumvented if the tape drive supports
soft select (sometimes called "phantom select"), whereby the
software chooses the device number for the tape drive when it is using it. The
ability to soft select depends on the drive. Though more floppy tape drives
support this capability, many of the older drives do not. I will get into more
detail about this in the second part of the book when I talk about installing
and using tape drives.
On larger systems, neither QIC nor mini-tapes can
really handle the volume of data being stored. While some QIC tapes can store up
to 1.3Gb, they cannot compare to digital audio tape (DAT) devices. Such devices
use Digital Data Storage (DDS) media. Rather than storing signals similar (or
analogous) to those coming across the bus,
DDS stores the data as a series of
numbers or digits on the tape, hence, the name "digital." The result
is much higher reliability.
Physically, DATs are the smallest tapes that
Linux supports. The actual media is 4mm, so DATs are sometimes referred to as
Hewlett-Packard DATs can be divided into multiple logical
tapes. This is useful when making backups if you want to store different file
systems to different "tapes" and you don't want to use any extra
physical tapes. Device nodes are created to represent these different logical
drives can quickly scan for the location of subsequent partitions (as
they are called), making searches much faster than with backups to single tapes.
One thing to watch out for is that data written to DATs are not as
standardized as data written to QIC tapes. Therefore, it is possible that data
written on one DAT
drive cannot be read on another.
There are two reasons
for this problem. This first is the blocking
factor, which is the minimum space
each file will take up. A 1Kb file with a blocking
factor of 20 will have 19Kb
of wasted space. Such a situation is faster in that the tape drive is streaming
more, though there is a lot of wasted space. DAT
drives use either a variable
fixed block size. Each drive has a default blocking
factor that is determined by
the drive itself.
Another problem is data compression, which, if it is
done, is performed at the hardware level. Because there is no standard for data
compression, it is very unlikely that two drives from different manufactures
that both do data compression will be able to read each others tapes.
Keep in mind that that's not all. There are many more standards that I
didn't list here. One place to start is the QIC consortium's home page at
www.qic.org, which lists dozens of tape standards and associated
Before you buy a tape drive, be sure to find out how easy it is to get the tapes
and how expensive they are. I bought a tape drive once that was fairly
inexpensive, but the tapes were hard to find and more expensive than others.
Eventually, I had to special order them from a distributor on the other side of
the country, because my local vendor stopped carrying them (I was the only one
who used them). The initial cost might have been more for a different tape drive, but
I would have saved in the long run.
If you have a lot of data to backup, tape loaders can be a real time saver.
In essence, a tape loader is a single tape drive with the ability to store
multiple tapes. Because the mechanism can load any tape you choose, they
function similarly to music jukeboxes. As a result, tape loaders are sometimes
called tape jukeboxes.
Most of the tape loaders I have seen come with either five or seven slots.
You can fill up all of the slots on Monday and write to a different tape each
day of the week. Although this saves time, I would still recommend taking the
tape out every day and storing it separately from the machines.
Even so, I still feel it is a time saver to fill the loader once on Monday
for the week, particularly if you have a large pool of tapes. For example, in
one company, we had enough tapes for a couple of months worth of backups. Our
backup software keep track of which tapes were in the drive as well as on which
tape any given file resided. We checked once on Monday to see what tapes were
needed for the week, filled up the loader and then simply removed each tape as
it was used.
On Friday, we did a full backup of every file on the system. This required the
loader be filled up completely, since we had some much data. Therefore, having the
loader was a necessity for the weekend backups. Therefore, we simply used the
available functionality during the week.
As the company and quantity of data grew, we eventually needed more tapes
than could fit in a single loader. That meant we had to get a second loader for
that machine. Although most of the more advanced backup packages can handle
loaders not all of them work well with multiple loaders. Therefore, you should
check in advance before buying something that cannot grow with you.