11.2 Backup Media


When I first started working as a system administrator, 9-track tape was the only medium you'd consider using for abackup.[6] That's certainly no longer true. Today, there are many different media suitable for storing backed-up data. This section provides a quick summary of the available choices. This list includes most of the drives and media types which are in common use. The backup strategy for any particular system will often involve more than one media type.

[6] The only other possibilities were punch cards and paper tape.

Up-to-the-minute information about available backup devices and media may be obtained from http://www.storagemountain.com. There is also an excellent discussion in Unix Backup and Recovery.

11.2.1 Magnetic tape

Magnetic tape of one sort or another has been the traditional backup medium for decades. Over the years, it has taken on a variety of sizes and forms, beginning with 7-track and then 9-track tape: 1/2-inch wide tape wound around a circular reel. The introduction of plastic cartridges containing the tape and both reels was a major step forward in terms of reducing the space requirements of backup media. The first tape of this type was 1/4-inch cartridge tape (also known as QIC tape), which for a while was the medium of choice for most workstations; these tapes are still occasionally used.

Around 20 years ago, higher-capacity tapes in formats originally developed for other markets became available. 8 mm tape drives became popular in the late 1980s and are still in wide use. Originally designed for video uses, the tapes are about the size of an audio cassette. 4 mm digital audio tapes (commonly called DAT tapes although the data storage scheme is technically known as DDS) are also in wide use. DAT cartridges are about 25% smaller than 8 mm tapes.

8 mm and 4 mm tapes come in two grades, one designed for video and audio recording (respectively), and a better, more expensive grade designed for data. Be sure to purchase only data-quality tapes. Although lower-quality tapes will sometimes appear to work fine, in my experience they are much, much less reliable about retaining data (despite urban legends to the contrary).

Both of these tape types are in use today, although DAT is far more prevalent than 8 mm. Both types of tape come in a variety of lengths and corresponding data capacities. Currently, the largest ordinary 8 mm tapes are 160 meters long and hold up to 7 GB of data,[7] although there are also tapes that hold 1.2 GB (54 m) and 2.4 GB (112 m). DAT tapes correspond to various DDS levels:

[7] That is, 7 GB of bits. The amount of "data" written may be much more if the original files are compressed before or as they are written to tape. Tape drive and media manufacturers love to inflate their products' capacities by quoting maximum compressed data numbers.

DDS-1

2 and 3 GB tapes (60 and 90 m

DDS-2

4 GB tapes (120 m)

DDS-3

12 GB tapes (125 m)

DDS-4

20 GB tapes (150 m)

DDS-3 and DDS-4 use a different technology than the earlier versions.

Be aware that only the newest tape drives can support the largest tapes, but most drives provide read-only backward compatibility.

There are also several newer magnetic tape technologies. Exabyte's Mammoth-2[8] and Sony's Advanced Intelligent Tape (AIT) technologies take 8 mm tapes to much higher capacities: 20, 40, or 60 GB and 35 or 50 GB, respectively. They both use the Advanced Metal Evaporative (AME) cartridge developed by Sony (a new 8 mm format). Some Mammoth-2 drives can also read earlier 8 mm tapes, but they require an extensive clearing procedure to be performed after each instance. These are also among the fastest tape drives, with transfer rates of up to 12 MB/s for Mammoth-2 drives and 6 MB/s for AIT drives.

[8] This was preceded by the Mammoth technology, which was notoriously unreliable. Mammoth-2 initially seems to be better.

The Digital Linear Tape (DLT) technology was initially developed by Digital Equipment Corporation, but they later sold it to Quantum Corporation. This format uses cartridges similar to DEC's old TK family, which have proven themselves to be extremely reliable and long-lived. It is also a fast format, with transfer rates of up to 10 MB/s.

The high capacity of magnetic tapes make them ideal for unattended backups: you can put a tape in at night, start a shell script that puts several filesystems on one tape, and go home.

Tapes also have some disadvantages:

  • They are extremely sensitive to heat and electromagnetic fields and fail quite easily when they are mishandled. Electromagnetic fields are produced by a variety of common devices found near computers, including UPS power supplies, external peripheral devices containing their own power supplies, monitors, and speakers. Moreover, simply reading a magnetic tape also contributes to data degradation.

  • They are sequential storage devices. In order to reach a given file on a tape, you have to wind the tape to the proper point. This is more of a problem for older tapes drives; current high-end drives can reach an arbitrary point on a tape in seconds.

11.2.2 Magneto-optical disks

Magneto-optical disks have the same width and length as floppy disks but are about twice as thick and hold a lot more data. Magneto-optical disks also come in 3.5-inch and 5.25-inch versions,[9] and their current capacity ranges up to 9.1 GB. Optical disks are purported to be much more stable than any of the purely magnetic media; the stability comes from the fact that they are written magnetically but are read optically, so reading the disk has no degrading effect on the stored data. In addition, the media can also be erased and rewritten as needed. Finally, magneto-optical disks also have the advantage of being random access devices. Transfer rates for these devices peak at about 5 MB/s.

[9] You might wonder what is so magical about 3.5 and 5.25 inches. Devices of this size fit easily into the standard device bays found in PCs as well as into available storage boxes.

Current drives are still quite expensive over $2000 as are the disks themselves, but they are nevertheless very popular. As I noted in the previous edition of this book (circa 1995), "a rewriteable medium that can permanently store over a gigabyte of data in the space of a couple floppy disks probably has a future." Now it's gigabytes of data and a definite future.

There are also other optical formats used or in development by a few manufacturers.

11.2.3 CDs and DVDs

Writeable CDs and DVDs have become viable backup media due to the substantial price reductions for both drives and media. There are two types of writeable CDs, referred to as CD-R (write-once CDs) and CD-RW (rewriteable CDs). Both come in 640 MB capacity, and recently 700 MB CD-R media have become available.

Writeable DVD technology is just emerging into the general marketplace at this writing. In fact, there are several DVD recording formats:

DVD-RAM

The first available format, it is now falling out of use since it cannot be read in ordinary DVD drives.

DVD-R

Write-once DVDs (also an aging technology).

DVD-RW

Rewriteable DVDs that can be read by ordinary DVD drives.

DVD+RW

An emerging technology devised by a coalition of drive manufacturers. These drives can produce ordinary (sequential) DVDs as well as random access disks. The former are readable by ordinary DVD players (but not by recorders of the other types), although some older models may require firmware updates. DVD+RW media can hold up to 4.7 GB per side.

As of this writing, Hewlett-Packard has recently released a low-cost DVD+RW writer suitable for use on PC-based systems, so this may become a popular backup device in that market in time.

11.2.4 Removable disks: Zip and Jaz

Removable disks are fully enclosed disk units that are inserted into a drive as needed. They tend to be significantly more reliable than either tapes or floppy disks. On Unix systems, they generally behave like a hard disk, but it is also possible to treat them as a giant floppy disk. They are suitable as backup media in some environments and circumstances.

There have been a variety of removable disk technologies over the years. The Zip and Jaz drives by Iomega have come to dominate this market. Zip drives which come in 100 MB and 250 MB sizes can be used with most Unix systems. Jaz drives, which have capacities of 1 GB or 2 GB, can also be used. I had a great deal of trouble with early Jaz drives, which were designed for infrequent, intermittent backup use and consistently failed when used on even a semi-continuous basis. More recent drives are said to be better. Both drive types are available with various I/O interfaces: SCSI, USB, IDE.

11.2.5 Floppy disks

Floppy disk drives are still found on most PC-based computer systems,[10] and they do have some limited backup uses. For example, PC-based Unix versions (as well as a few running on larger systems) often use floppy disks for emergency boot devices. In addition, floppy disks can be useful for inherently limited backup tasks, such as saving customized system configuration files from the root filesystem. Standard floppy disks hold 1.44 MB, and some Unix workstations include drives that double that capacity to 2.8 MB. Occasionally, you will come across a floppy drive that also supports Super disks: media that look like floppy disks but hold 120 MB.

[10] Although this will probably no longer be true in a couple of years.

11.2.6 Hard disks

Given the low prices of harddisks these days, they may also be a viable backup target device in some circumstances. For example, some sites provide a large backup disk on the local network where users can make periodic copies of key files that they are working on. Large disks can also be used for scratch purposes, for temporary data repositories and data holding areas, and similar purposes. They can also be used as a staging area where backups are stored temporarily on the way to being written to tape or other media.

11.2.7 Stackers, jukeboxes, and similar devices

There are a variety of devices designed to make media handling more automated, as well as to store and make available large numbers of media units. For example, there are auto-loading tape drives also known as stackers which can feed tapes automatically from a stack of 10 or so. Early stackers could access the component tapes only in order, but many current devices can retrieve any desired tape.

Another type of device puts multiple drive units into a box that looks to users like a single tape drive with the combined capacity of all of its components. Alternatively, such a device can be used to make multiple identical tapes simultaneously.

Still other units combine both multiple drives and tape auto-loading capabilities. These devices are known as jukeboxes or libraries .[11] The most sophisticated of them can retrieve a specific tape and place it into the desired drive. Some of these devices include integrated bar-code readers so that tapes can be identified by their physical label rather than storage location or electronic label. Similar devices also exist for optical disks and writeable CD-ROMs.

[11] Very large libraries (greater than 500 volumes) are known as silos. The two types of devices used to be distinguished by whether or not multiple hosts could be connected, but some libraries now have this capability. Separate silos are also able to pass tapes between them.

11.2.8 Media Lifetime

From time to time, you also need to think about the reasonable expected lifetime of your backup media. Stored under the right conditions, tapes can last for years, but unfortunately you cannot count on this. Some manufacturers recommend replacing tapes every year. This is certainly a good idea if you can afford to do so. The way that tapes and diskettes are stored also affects their lifetime: sunlight, heat, and humidity can all significantly shorten it. I always replace tapes that have had read errors or other failures more than once, regardless of their age; for some people and situations, a single failure is enough. I always throw away diskettes and Zip disks at the first hint of trouble.

Even so-called permanent media like CDs actually have a finite lifetime. For example, CDs begin to fail after about 5 years (and sometimes even sooner). Accordingly, creating two copies of important data and checking them periodically is the only prudent course.

Given these considerations, your site may want to consider alternative media for off-site and archival backups. For example, manufacturers of optical disks claim a lifetime of 15 years for this media (this is based on accelerated aging tests; as of this writing, we won't know for about 8-9 years whether this is really true).

11.2.9 Comparing Backup Media

Table 11-1 lists the most important characteristics of a variety of backup media. The largest media capacity for each item shown is the biggest that was available as of this writing. These size values refer to raw data capacity: the actual amount of data that can be written to the media.

The drive price is the lowest generally available price at this time and can be assumed to use the least expensive I/O interface; you can expect SCSI versions of many devices that are also available in IDE form to cost at least 15% more (and sometimes much more). Similarly, at about $100, a USB floppy drive costs 10 times that of an ordinary one.

The media prices are the lowest commonly available when the media is purchased in large quantities (e.g., 50-100 for CDs) and in no-frills packaging (e.g., on a spindle for CDs rather than in individual jewel cases). All prices are domestic prices in the United States, in U.S. dollars, as of mid-2002.

The minimum lifetime column gives an approximate rule-of-thumb time period when you can expect some media to begin failing. Of course, individual media will fail even sooner in some cases.

Table 11-1. Popular backup devices and media

Type

Media capacity

Drive price[12]

Media price[12]

Minimum lifetime

Floppy disk

1.44 MB[13]

$10

$0.25

2 years

Super disk

120 MB

$120

$8

2-3 years

Zip Disk

100 MB

$70

$5

3-5 years

 

250 MB

$140

$12

3-5 years

Jaz Disk

1 GB

$300

$80

4-5 years

 

2 GB

$340

$100

4-5 years

CD-R

700 MB (80 minutes)

$150

$0.85

5 years

CD-RW

640 MB (74 minutes)

$150

$1

5 years

DVD-R

4.7 GB (single-sided)

$700

$8

5 years?

 

9.4 GB (double-sided)

$700

$40

5 years?

DVD+RW

4.7 GB

$600

$8

5 years?

DAT tape 4 mm DDS

4 GB (120 m DDS-2)

$550

$6

3-4 years

 

12 GB (125 m DDS-3)

$700

$12.50

3-4 years

 

20 GB (150 m DDS-4)

$1200

$26

3-4 years

8 mm tape

7 GB (160 m)

$1200

$6

2-4 years

Mammoth-2 (AME)

20 GB

$2500

$36

3-4 years?

 

60 GB

$3700

$45

3-4 years?

AIT tape

35 GB

$900

$79

3-4 years?

 

50 GB

$2600

$85

3-4 years?

 

100 GB

$3900

$105

3-4 years?

DLT

40 GB

$4000

$70

10 years

SuperDLT

110 GB

$6000

$150

10 years

Magneto-optical (RW)

5.2 GB

$2300

$65

15 years?

 

9.1 GB

$2700

$93

15 years?

Hard disk

100 GB (IDE)

N/A

$2-3/GB

5-7 years

 

180 GB (SCSI)

N/A

$10/GB

5-7 years

[12] Approximate minimum price in U.S. dollars.

[13] A few floppy drives provided by Unix vendors increase the maximum capacity to 2.8 MB .

11.2.10 Tape Special Files

Traditionally, special files used to access tape drives had names of the form /dev/rmtn or /dev/rmt/n, where n indicates the drive number. Tape drives are virtually always accessed via the character (raw) special file. Currently, special file names usually include other characters as prefixes and/or suffixes, which indicate the way the device is to be accessed: the density setting to use, whether to use the drive's built-in hardware compression, whether to rewind the tape after the operation is completed, and so on.

AIX systems also use suffixes to select whether the tape should beretensioned before use. Retensioning refers to equalizing the tension on a tape, and it consists of moving the tape to the beginning, then the end, and then rewinding back to the beginning; it's even slower than it sounds. The idea is to eliminate any latent slackness in the tape, but it is seldom necessary in practice.

Table 11-2 lists the current tape special file naming conventions for the various operating systems we are considering.

Table 11-2. Tape special file names

Unix version

Format and examples[14]

Prefixes/suffixes

man page

AIX

/dev/rmtn[.m]

/dev/rmt0.1/dev/rmt0.5

Note: Compression is enabled and disabled with the chdev command.

m:

none=rewind, no retension, low density1=no rewind, no retension, low density2=rewind, retension, low density3=no rewind, retension, low density4=rewind, no retension, high density5=no rewind, no retension, high density6=rewind, retension, high density7=no rewind, retension, high density

rmt(4)

FreeBSD

/dev/[n]rastn/dev/[e|n]rsan

/dev/nrast0 /dev/nrsa0

n=no rewinde=eject tape on close

(Density and compression are chosen with the mt utility.)

astsa(4)

HP-UX

/dev/rmt/citjd0TYPE[b][n]

/dev/c0t3d0DDSbn/dev/c0t3d0BESTbn

i=controllerj=SCSI IDn=no rewindb=use BSD-style error controlTYPE=keyword indicating tape type and/or density (e.g., BEST, DDS)

mt(7)

Linux

/dev/[n]stnx

/dev/nst0/dev/nst0m

n=no rewind

x:

none=default densityl=low densitym=medium densitya=autoselect density

st

Solaris

/dev/rmt/nx[b][n]

/dev/rmt/0lbn/dev/rmt/0hbn

b=use BSD-style error controln=no rewind

x:

none=default densityl=low densitym=medium densityh=high densityc=use hardware compression

st

Tru64[15]

/dev/[n]rmt/tapen_dm

/dev/nrmt/tape0_d2/dev/nrmt/tape0_d3

m:

0=low density, use compression1=high density, use compression2=low density, no compression3=high density, no compression(values 4-7 are also defined for some drives)

tz

[14] In all cases, n refers to the tape drive number. The examples are all for a non-rewinding tape device with hardware compression disabled using the lowest and highest density (as available).

[15] Older Tru64 systems use the now-obsolete device names of the form /dev/tz* and /dev/ta*.

Some systems provide simpler names as links to commonly-used tape devices. You can figure out which device they refer to by looking at a long directory listing. Here is an example from an HP-UX system:

crw-rw-rw- 2 bin bin 205 0x003000 Oct 7 1999 0m crw-rw-rw-  2 bin    bin    205 0x003080 Oct  7  1999 0mb crw-rw-rw-  2 bin    bin    205 0x003040 Oct  7  1999 0mn crw-rw-rw-  2 bin    bin    205 0x0030c0 Oct  7  1999 0mnb crw-rw-rw-  2 bin    bin    205 0x003000 Oct  7  1999 c0t3d0BEST crw-rw-rw-  2 bin    bin    205 0x003080 Oct  7  1999 c0t3d0BESTb crw-rw-rw-  2 bin    bin    205 0x003040 Oct  7  1999 c0t3d0BESTn crw-rw-rw-  2 bin    bin    205 0x0030c0 Oct  7  1999 c0t3d0BESTnb crw-rw-rw-  1 bin    bin    205 0x003001 Oct  7  1999 c0t3d0DDS crw-rw-rw-  1 bin    bin    205 0x003081 Oct  7  1999 c0t3d0DDSb crw-rw-rw-  1 bin    bin    205 0x003041 Oct  7  1999 c0t3d0DDSn crw-rw-rw-  1 bin    bin    205 0x0030c1 Oct  7  1999 c0t3d0DDSnb

In this case, 0m and c0t3d0BEST refer to the same tape drive and access mode (as do their corresponding suffixed forms).

The default tape drive on a system is usually the first drive in its default (rewinding) mode:

AIX
/dev/rmt0
FreeBSD
/dev/rsa0
HP-UX
/dev/rmt/0m
Linux
/dev/st0
Solaris
/dev/rmt/0
Tru64
/dev/rmt/tape0_d0

On Linux systems (and some others), the device /dev/tape is a link to the default tape device on the system. You can make the link point to whatever drive you want to by recreating the link. On FreeBSD systems, some commands use the TAPE environment variable to locate the default tape drive.

11.2.10.1 AIX tape device attributes

On AIX systems, you can use the lsattr command to view the attributes of a tape drive:

$ lsattr -E -H -l rmt0  attribute      value  description                    user_settable block_size     1024   BLOCK size (0=variable length)     True compress       yes    Use data COMPRESSION               True  density_set_1  140    DENSITY setting #1                 True  density_set_2  20     DENSITY setting #2                 True  extfm          yes    Use EXTENDED file marks            True mode           yes    Use DEVICE BUFFERS during writes   True

This 8 mm tape drive will use data compression and a block size of 1024 by default.

You must use the chdev command to change the many attributes of a tape drive (rather than having these selections encoded into the special file name as with other systems). For example, the following command changes the block size to 1024 and turns off compression and retensioning for drive 1:

# chdev -l rmt0 -a block_size=1024 -a compress=no -a ret=no


Essential System Administration
Essential System Administration, Third Edition
ISBN: 0596003439
EAN: 2147483647
Year: 2002
Pages: 162

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net