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.
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 tapeMagnetic 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.
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:
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.
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:
11.2.2 Magneto-optical disksMagneto-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.
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.
11.2.3 CDs and DVDsWriteable 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:
11.2.4 Removable disks: Zip and JazRemovable 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 disksFloppy 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.
11.2.6 Hard disksGiven 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 devicesThere 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.
11.2.8 Media LifetimeFrom 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 MediaTable 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.
11.2.10 Tape Special FilesTraditionally, 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.
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:
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 attributesOn 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 |
EAN: 2147483647
Pages: 162