DATMAN TECHNICAL BULLETIN #035
From: Kan Yabumoto tech@datman.com
To: DATMAN user
Subject: What is the expected life of a DDS tape?
Date: 1999-12-01
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The questions we hear quite often are:
How many times can I use a tape cartridge safely for backup?
How many weeks can I use the same tape for daily backup?
What is the life expectancy of the tape media?
What does the manufacturer say?
Hewlett Packard, the leader in the DDS technology states in
its manual the following recommendation.
All DDS-certified media is electronically checked to withstand at
least 2,000 passes. Under optimum envionmental conditions
(50% relative humidity, 22 degree Celcius), this is equivalent to
about 100-150 backup operations. This takes into account that
an area of tape may have several passes during a backup, either
because streaming cannot be maintained, or because the backup
software package releatedly accessess certain areas of tape.
Hewlett Packard also suggests a limit of 75 backup operations
when the operating environments are less than ideal; for example,
sustained low humidity, a very slow host computer which causes
more streaming failure (the machine cannot keep up with the
optimal I/O speed of the tape device), backup software package
which causes frequent accesses to a certain area of tape.
In another HP literature, the phrase used are more conservative;
only use each tape for a maximum of 2,000 passes on any part
of the tape.
A closer look:
The key number seems to be 2,000 passes over the same spot.
Let us examine the mechanism of the head drum and tape contact
more closely.
Most people are familiar with the Video Cassette Recorder (VCR,
both the Betamax and VHS) which uses helical scan recording
where a head drum is mounted with a slight angle (ca. 6 degrees)
relative to the tape. While the tape makes a slow horizontal motion,
the spinning head passes over the tape surface diagonally. The high
relative speed of the head against the tape makes good magnetic
signal. Although the head surface is very well polished, the
drum-to-tape contact is always a source of the surface wear.
One good thing about the fast rotating head drum is that it
drags air near the surface of the drum just like the surface of a
hard disk; a thin layer of air creates the so-called air-baring
effect which reduces the direct contact between the drum and tape.
In order to avoid an excessive surface wear of one region of the
tape, when the linear motion of the tape halts, the drive will
automatically stop the head spin within several seconds.
The QIC/Travan tape which employs the so-called Serpentine format
is much simpler in operation. Because the recording head does not
move, when the tape stops no more wear and tear. This article
examines the case of the tape drives with helical scan technology.
Let us analyze exactly what happens when a DDS (or 8mm) tape goes
through the recording head. Since the head drum is spinning
diagonally and the contact area is certainly greater than the
QIC/Travan case, even one pass must undergo repeated passages over
the spinning drum. In the case of the standard DDS drive, the tape
wraps one quarter (90 degree) of the circumference of the drum
(30 mm in diameter). That is, the tape and drum surface have a
contact area whose horizontal lengfth is 23.6 mm. The DDS-1
standard tape moves at a linear velocity of 8.15 mm/sec which
makes a given point on tape experience a contact to the drum
for 2.89 seconds. Since the head spins at 2000 rpm (33.3
revolutions per second), during this period, the head drum spins
about 96 times.
The case with DATMAN:
As a file system, usage of DATMAN may certainly increase the
number of accesses to the tape which increases the wear of
the tape media. If we follow Hewlett-Packards'guideline, the
area of the tape which undergoes the most frequent head passes
is the auxiliary partition (Partition 1, or P1). In P1,
DATMAN records the quick-mount catalog which is an exact
replica of the most recent catalog which is recorded at the
furthest end of the current data region.
BOT P1 (QuickMount Catalog) Partition 0 EOT
+----------------------------+-------------------------------------+
| P1 Catalog | free space | P0 Catalog | free space |
+----------------------------+-------------------------------------+
BOP-1 EOD-1 BOP-0 EOD-0
BOT: Beginning of Tape
EOT: End of Tape
BOP: Beginning of Partition
EOD: End of Data
When the tape is loaded (before a volume mount action takes
place) at first, the drive will always position the tape at BOP-0
(beginning of the main partition). When the volume is mounted
by DATMAN, it will rewind the tape to BOT and make access to the
Quick-mount catalog at P1. When the user makes read accesses to
the volume, the tape will travel to the user-file region which is
situated between BOP-0 and EOD-0. Before the tape is ejected,
it will perform a rewind at first. The simplest case of a complete
read session therefore involves a minimum of four passes over the
P1 region. When a session involves only with directory accesses
without any seek and read access to the user file contents, the
number of passes over the P1 region is also 4 passes, the same as
the other case.
In the case of a write access (typically for an incremental backup),
the situation is similar for the initial mounting procedure. But
once the write access to the tape takes place, the entire P0
region experiences two passes without an explicit backup verify
pass (four passes with a verify pass). Since a write session
requires a catalog update at P1 before rewinding the tape at
the end, the P1 region undergoes 6 times (50% increase in pass
count).
Of course, repeated accesses for user files which are a direct
result of DATMAN's increased convenience over the traditional
backup-only usage. Add two additional passes over a given
tape position for each read access. When the user files
are accessed in a semi-random access sequence, the number of
passes will dramatically increase. An intelligent file access
order which minimizes the number of passes is highly desirable
not only from the access speed point of view, but from the
media wear concerns.
DCOPY and XXCOPY on DATMAN volume:
When a large number of files are accessed, it becomes critically
important that the order of file accesses on tape be sorted by the
position of the files on tape so that the direction of tape motion
remain the same. That is exactly what DATMAN's companion file-copy
utility program, DCOPY (and the generic version, XXCOPY) does.
For the best performance, longer cartridge life and the drive life,
we strongly recommend the use of DCOPY and XXCOPY.
If use other file management programs to access the DATMAN volume,
make sure you select "unsorted" order" if possible. The DATMAN
file system always provides the list of files within a directoy
by the order of file position on the tape.
The worst enemy to the tape cartridge and the drive is Microsoft's
Explorer which makes an arbitrary assumption that all drives
are created equal. It mercilessly rearranges the order of file
accesses to typically an alphabetical order. The icon display
(usueally for .EXE, and .DLL) files on tape would make mostly
unnecessary file accesses which are bad to the longevity of the
cartridge and the drive as well as and the user's mental health.
For this reason, DATMAN has a feature to block almost all such icon
data access by Explorer (you may disable this feature if you wish).
Conclusion:
Under normal circumstances, the very conservative 2,000 passes
is still a very large number. Just consider how long it takes
to make 2,000 passes on a given spot on the tape. Almost all
tape drives spend anywhere from 10 to 20 seconds to reverse the
direction of the tape motion and to restart a tape motion.
That is, if one can write a special program which simply performs
2,000 direction changes in tape motion, the program would still
spend up to 11 hours without doing much else. In more realistic
working environments, making 100 passes over a particular spot
on tape takes many hours. It is this relatively slow speed in
the direction changes that almost automatically makes it very
difficult to reach the maximum number of passage on a given point.
The life expectancy of a cartrifge will become an issue only
when you rotate a very small number of cartridges over and over,
months after months. In that case, we suggest you retire the
tape after a few hundred backups. If you do a daily back up,
5 days a week, it is almost a year and that's conservative.
For typical file archiving applications, the limit of 2,000 passes
is a very large number to reach.