asg11.htm

apache技术手册

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<FONT SIZE=4 COLOR="#FF0000"><B>Magneto-Optical Disks</B></FONT></CENTER></H5>

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<P>Magneto-optical disks have a plastic or glass substrate coated with a compound that, when heated to its Curie point, allows a magnetic source to realign the polarity of the material. Once the material cools, its polarity is frozen. The material can be repolarized by a subsequent write operation.

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<P>Data is read by a lower-intensity beam, and the polarization pattern is interpreted as a byte stream. A wide variety of these devices are commercially available, ranging in format from 5 1/2 inches to 3 1/2 inches. These devices can store information ranging in size from 128MB to more than 2GB.

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<P>Read speed on these devices is as fast as that on a hard disk. Write operations usually take a little longer, but are still faster than write operations on a slow hard disk. Media reliability is very high.

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<FONT SIZE=4 COLOR="#FF0000"><B>Quarter-inch Cartridge Tapes</B><B> (QIC)</B></FONT></CENTER></H5>

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<P>A QIC tape is a low-end, PC-market backup storage solution that uses .25-inch tape. Some vendors, including IBM, are pushing the format to store up to 1600MB by using a .315-inch format; these tapes are commonly known as Travan tapes.

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<P>Standard QIC tapes can hold anywhere from 11MB to 150MB, and are usually designated as QIC-11, QIC-24, and QIC-150, depending on the amount of storage space they provide. Storage space in megabytes is indicated by the number following the QIC portion of the designation.

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<P>Sometimes tapes created on one vendor's drive are not readable by another vendor's drive. This is due to byte ordering and other special formatting issues. Within a vendor, tapes are usually backward compatible, meaning that you may be able to read lower-density tapes on a higher-density drive; however, you should verify this before you upgrade to a new drive in the same product line.

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<FONT SIZE=4 COLOR="#FF0000"><B>Travan Tapes</B></FONT></CENTER></H5>

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<P>Travan tapes are similar in size to QIC tapes, but store anywhere from 120MB to 1600MB per tape depending on the type of tape drive mechanism used. Travan tapes are compatible with the QIC tape formats, making them attractive if you have legacy QIC tapes.

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<FONT SIZE=4 COLOR="#FF0000"><B>Write-Once CD-ROM</B><B>s</B></FONT></CENTER></H5>

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<P>New technology and price drops have made the write-once CD-ROM a popular choice with multimedia enthusiasts. Write-once CD-ROMs use a technology similar to a CD burner. CDs created by write-once CD-ROMs are not as rugged as pressed CDs, but will last forever if you take care of the disk. These disks are compatible with any desktop system that has a CD-ROM, which has helped in making this a popular Write Once Read Many (WORM) format. Current capacity is about 600MB. Recording speed is slow. New formats for CD-ROMs that are currently in the works will yield 17GB storage, making it a very interesting solution to backup and archival tasks.

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<FONT SIZE=4 COLOR="#FF0000"><B>Iomega Jaz and Zip drives</B></FONT></CENTER></H5>

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<P>Zip is a popular removable disk drive. They are very inexpensive: around $200 for the drive and $10 to $15 per disk. Each disk holds about 100MB. They are available in SCSI and parallel flavors.

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<P>Jaz drives are a higher-performance, higher-capacity version of the Zip drives. Jaz drives are a bit more expensive, about $599, and require a SCSI interface. Disks cost more than $99 and pack 1GB of fast storage space. You can back up 1GB of information in about 5 minutes on PC platforms. This is a hot product.

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<FONT SIZE=4 COLOR="#FF0000"><B>Nine-Track Magnetic Tapes</B></FONT></CENTER></H5>

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<P>This is an old format of tape written at 800, 1600, or 6250BPI (bits per inch) density. This format is not in great use today except by old mainframes.

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<FONT SIZE=4 COLOR="#FF0000"><B>4mm DATs</B></FONT></CENTER></H5>

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<P>A DAT, which stores 1.3GB of information on a 60-meter tape, was originally designed for the audio market. Digital data storage (DDS), the computer version of DAT, provides the smallest storage solution of all. You can store about 2GB of data per cartridge. Drives with hardware compression can store up to 8GB. DDS is the preferred tape backup system for most UNIX users.

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<FONT SIZE=4 COLOR="#FF0000"><B>8mm Cartridge Tapes</B></FONT></CENTER></H5>

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<P>8mm cartridges (also known as &quot;Exabytes&quot; for the company that first produced them) are the same size as their video counterparts. Many Administrators purchase high-quality &quot;video grade&quot; tapes instead of the premium data versions. Drives can store 2&#150;5GB, and versions boasting hardware compression can pack up to 10GB into a single tape. Because fewer tapes are needed, this is a very convenient format. Next to the 4mm format, this is the best storage solution.

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<FONT SIZE=4 COLOR="#FF0000"><B>A Backup Strategy</B></FONT></CENTER></H4>

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<P>Doing incremental backups under UNIX means using the dump utility. This utility is powerful, but somewhat dangerous. Using it incorrectly can cause serious problems. However, dump can handle backups that span multiple tapes. If you can fit your entire backup onto a single tape, you'll be able to automate backups. Just start up the backup, and let it run.

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<P>Web sites have a slightly different usage pattern from your typical server. Because Web sites don't have users creating files all the time, the filesystem doesn't change very often (unless your server provides some sort of intranet application that uses a database for persistence, or you want to back up your logfiles).

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<P>A <I>production</I> <I>server</I>, the server people connect to in order to obtain information, is very different from a <I>development</I> <I>server</I>. Development servers contain an individual's work. They should be backed up often! Incremental backups should be used to minimize the media and time required to perform them.

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<P>If you have been following my suggestions, you will probably agree that the server documents (.html files) should reside on a separate disk. If you cannot afford another disk, a separate partition may offer the same benefits. <I>Partition</I> is just a fancy word for a smaller logical unit (smaller disk) of a big disk. Partitioning a large disk can offer many advantages:

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<LI>Partitions are smaller in size, more likely to fit into a single tape, and much faster to back up because they contain less data.

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<LI>Partitions protect your system from a runaway program that fills up a disk.

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<P>On the negative side, if you fill up a partition, UNIX doesn't provide you with a way to enlarge or shrink it. What you choose is what you live with. Don't go partition-crazy either; if you have too many little partitions, you'll probably find that some of your partitions need more space. A two-partition scheme works well. It is a good idea to partition a disk so that the base operating system fits easily into one partition and allows 15 to 25 percent of the partition space for future growth. This is the <I>system</I> partition. The second partition is allocated to a single user area. Any customizations or added third-party software should go there. If you cannot afford a second disk for user-generated files, you can store them in the second partition as well.

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<P>Separate disks or partitions help the backup process because both can be dumped separately to tape. If you are unable to partition or have multiple filesystems, your backups will take a little longer.

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<P>If you follow this scheme, you'll only need to back up software and configuration files that you add or modify, instead of having to back up the whole system. Likewise, the user partition can be backed up separately.

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<P>My strategy is to back up production servers at well-defined times:

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<LI>Right after the system software is installed, but before anything is configured.

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<LI>After the server software is installed and configured.

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<LI>Any time additional software is installed and configured.

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<P>If your operating-system software distribution comes on a CD-ROM or some other easily installed medium, the first backup you make after installing your system software doesn't need to go to tape. If you send it to /dev/null, you'll be able to create a backup set that doesn't include your system's software distribution. This will set the beginning of time for the dump utility to operate. The backup level for this dump should be at level 0; it must include everything in the newly installed system.

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<P>If the installation of basic software is problematic, you should probably direct the backup to a tape and save it for future disaster recovery. If you need to reinstall your distribution software, simply restore it to a clean disk instead of rebuilding a kernel or something else.

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<P>Subsequent backups should be performed at level 9; this will effectively back up everything that has been modified in the machine since the first backup was made.

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<P>To restore files stored in a tape created with the dump utility, use the restore program.

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<FONT SIZE=4 COLOR="#FF0000"><B>The </B><B>dump</B><B> Command</B></FONT></CENTER></H5>

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<P>The dump command uses the following syntax:

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<FONT COLOR="#000080">/usr/etc/dump [<I>options</I> [<I>argument</I> ...] <I>filesystem</I>]</FONT></PRE>

<P>The dump command tracks the scope of a backup by assigning each backup a level. Levels range from 0 to 9. Level 0 copies the entire filesystem. Subsequent dump levels copy only files that have changed since the most recent dump with a lower-level number.

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<P>If you dump a disk at level 5, it includes all files that have changed since the date of the last dump with the next lowest level (in this case, last level 4 dump, or lower if no dump level 4 was performed). When executed with the u flag, dump tracks the backup level along with the date and time the backup was performed.

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<P>The typical strategy is to begin with a level 0 dump and then make incremental backups at regular intervals. Level 0 dumps should be performed with extreme care; they should be run with the machine in single-user mode, and fsck, the filesystem consistency checker, should run before the dump to verify that the filesystem is consistent. This is important because most of the files you restore will come from dump level 0 tapes!

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<P>One disadvantage worth noting is that dump is unable to back up single directories or files. It can only be used to back up an entire filesystem. To back up individual files or directories, use the programs tar or cpio. For information on how to use these programs, refer to your UNIX documentation.

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<P>Note that tapes created under one hardware/software configuration are not usually portable to other operating systems or drives. On some environments, even tapes created with older versions of the dump program are unreadable by newer versions of the restore program.

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<FONT SIZE=4 COLOR="#FF0000"><A NAME="I2"></A><A NAME="I3"></A><A NAME="I4"></A><A NAME="I5"></A><A NAME="I6"></A><A NAME="I7"></A><A NAME="I8"></A><A NAME="I9"></A><B>dump</B><B> Examples</B></FONT></CENTER></H6>

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<P><A NAME="I10"></A>The following command will create a level 0 dump (specified by the 0) of the specified disk, /dev/<I>rsd0a</I>:

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<FONT COLOR="#000080">dump 0u /dev/<I>rsd0a</I></FONT></PRE>

<P>Replace <I>rsd0a</I> with the appropriate name for your raw disk device. This dump will include all the files found on the device. If you specify the u option and the backup finishes successfully, dump will remember the date, time, and level of this backup. This effectively sets a reference point that can be used to evaluate which files need to be dumped next time.

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<P>To create a partial backup that includes only files modified since the last backup of a lower level, in this case 0, use a level 9 dump:

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<FONT COLOR="#000080">dump 9u /dev/<I>rsd0a</I></FONT></PRE>

<P>By default, dump writes its output to the default tape unit, in this case /dev/<I>rxt0</I>. The /dev/<I>rxt0</I> is a rewinding interface to the tape drive. When the backup is finished, the tape will be automatically rewound. To finish the backup and prevent the tape from rewinding, use the nonrewinding device. On my system, this is called /dev/<I>nrxt0</I>. For the name of this device, please check the dump UNIX manual page. To specify a different backup device or file, use the f option:

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<FONT COLOR="#000080">dump 9uf /tmp/backup /dev/rsd0a</FONT></PRE>

<P>The preceding command will create a file called backup in the /tmp directory. To redirect dump's output to stdout (standard output), specify a - instead of a filename:

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<FONT COLOR="#000080">dump 9uf - /dev/rsd0a | gzip &gt; /tmp/backup.gz</FONT></PRE>

<P>In the preceding example, output from dump was sent to stdout and piped to the gzip program to be compressed. If your tape doesn't provide hardware compression, using the preceding command can be an effective way of increasing tape capacity.

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<P>For complete information on the myriad of options that dump provides, read your system's documentation.

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