11.1 Planning for Disasters and Everyday Needs

Developing an effective backup strategy is an ongoing process. You usually inherit something when you take over an existing system and start out doing the same thing you've always done when you get a new system. This may work for a while, but I've seen companies try to retain their centralized, hordes-of-operators-based backup policies after they switched from a computer room full of mainframes to a building full of workstations. Such an attempt is ultimately as comical as it is heroic, but it all too often ends up only in despair, with no viable policy ever replacing the outdated one. The time to develop a good backup strategy is right now, starting from however you are approaching things at the moment.

Basically,backups are insurance. They represent time expended in an effort to prevent future losses. The time required for any backup strategy must be weighed against the decrease in productivity, schedule slippage, and so on if the files are needed but are not available. The overall requirement of any backup plan is that it be able to restore the entire system or group of systems within an acceptable amount of time in the event of a large-scale failure. At the same time, a backup plan should not sacrifice too much in the way of convenience, either in what it takes to get the backup done or how easy it is to restore one or two files when a user deletes them accidentally. The approaches one might take when considering only disaster recovery or only day-to-day convenience in isolation are often very different, and the final backup plan will need to take both of them into account (and will accordingly reflect the tension between them).

There are many factors to consider in developing a backup plan. The following questions are among the most important:

  • What files need to be backed up?

    The simplest answer is, of course, everything, and while everything but scratch files and directories needs to be saved somewhere, it doesn't all have to be saved as part of the system backups. For example, when the operating system has been delivered on CD-ROM, there is really no need to back up the system files, although you may choose do so anyway for reasons of convenience.

  • Where are these files?

    This question involves both where the important files are within the filesystem and which systems hold the most important data.

  • Who will back up the files?

    The answer may depend on where the files are. For example, many sites assign the backup responsibility for server systems to the system administrator(s) but make users responsible for files that they keep on their workstation's local disks. This may or may not be a good idea, depending on whether or not all of the important files really get backed up.

  • Where, when, and under what conditions should backups be performed?

    Where refers to the computer system on which the backup will be performed; this need not necessarily be the same as the system where the files are physically located. Similarly, in an ideal world, all backups would be performed after hours on unmounted filesystems. That's not always practical in the real world, however.

  • How often do these files change?

    This information will help you decide both when and how often to perform backups and the type of schedule to implement. For example, if your system supports a large, ongoing development project, the files on it are likely to change very frequently and will need to be backed up at least daily and probably after hours. On the other hand, if the only volatile file on your system is a large database, its filesystem might need to backed up several times every day while the other filesystems on the same system would be backed up only once a week.[1]

    [1] In actual fact, a database is often backed up using a facility provided by the software vendor, but you get the idea here.

  • How quickly does an important missing or damaged file need to be restored?

    Since backups protect against both widespread and isolated file loss, the timeframe in which key files need to be back online needs to be taken into account. The number of key files, how widely spread they are throughout a filesystem (or network), and how large they are will also influence matters. Your system may only have one irreplaceable file, but you'll need to plan very differently depending on whether it is 1 KB or 1 GB in size. Note that losing even a single 1 KB file can wreak havoc if it's the license file without which the central application program won't run.

  • How long do we need to retain this data?

    Backups protect current data from accidents. As such, they are normally needed or useful only for a relatively short period (months or a year or two) In contrast, most sites also need to create permanent archives of important "point-in-time" data, for example, the software and data used to prepare a tax return. These need to be saved for an indefinite period: many years or even decades. While the requirements are similar, the goals are different enough that you are unlikely to be able to rely on your regular backups for archival purposes. Thinking about this kind of data and how to create and store it must be part of every effective backup plan.

  • Where should the backup media be stored?

    Recent backups are generally kept close to the computer for quick restoration. Long-term backups and archives should be stored in a secure offsite location.

  • Where will the data be restored?

    Will the backup files be used only on the system from which they were made, or is there an expectation that they could be restored to a different system in an emergency? If multisystem compatibility is ever important, it needs to be taken into account in designing the backup and recovery plan. For example, you might need to ensure that any compression scheme in use on one system can be decoded by the other target systems (or avoid using any vendor-specific formats). Other examples of this sort of issue include access control list data that might be backed up along with files and backups of a filesystem from a system that is larger than the maximum filesystem size on the target system.



Virtually all Unix documentation recommends that filesystems be unmounted before a backup is performed (except for the root filesystem). This recommendation is rarely followed, and in practice, backups can be performed on mounted filesystems. However, you need to make users aware that open files are not always backed up correctly. It is also true that there are circumstances in which events in an active filesystem can cause some files or even the entire backup archive itself to be corrupt. We will consider those that are relevant to the various availablebackup programs as we discuss them.

11.1.1 Backup Capacity Planning

Once you have gathered all the data about what needs to be backed up and the resources available for doing so, a procedure like the following can be used to develop the detailed backup plan itself:

  1. Begin by specifying an idealbackup schedule without considering any of the constraints imposed by your actual situation. List what data you would like to be backed up, how often it needs to be backed up, and what subdivisions of the total amount make sense.

  2. Now compare that ideal schedule to what is actually possible in your environment, taking the following points into consideration:

    • When the data is available to be backed up: backing up open files is always problematic the best you can hope for is to get an uncorrupted snapshot of the state of the file at the instant that the backup is made so, ideally, backups should be performed on idle systems. This usually translates to after normal working hours.

    • How many tape drives (or other backup devices) are available to perform backups at those times and their maximum capacities and transfer rates: in order to determine the latter, you can start with the manufacturer's specifications for the device, but you will also want to run some timing tests of your own under actual conditions to determine realistic transfer rates that take into account the system loads, network I/O rates, and other factors in your environment. You will also need to take into account whether all the data is accessible to every backup device or not.

    At this point (as with any aspect of capacity planning), there is no substitute for doing the math. Let's consider a simple example: a site has 180 GB of data that all needs to be backed up once a week, and there are 3 tape drives available for backups (assume that all of the data is accessible to every drive). Ideally, backups should be performed only on week nights between midnight and 6 A.M. In order to get everything done, each tape drive will have to back up 60 GB of data in the 30 hours that the data is available. That means that each tape drive must write 2 GB of data per hour (333 KB/sec) to tape.

    This is within the capabilities of current tape drives when writing local data.[2] However, much of the data in our example is distributed across a network, so there is a chance that data might not be available at a fast enough rate to sustain the tape drive's top speed. Some backup programs also pause when they encounter an open file, giving it a chance to close (30 seconds is a typical wait period); when there are a lot of open files in a backup set, this can substantially increase how long the backup takes to complete.

    [2] In practice, of course, you would also need an auto loading tape device (or someone to change tapes in the middle of the night).

    In addition, we have not made any allowances for performing incremental backups (discussed below) between full backups. Thus, this example situation seems to strain the available resources.

  3. Make modifications to the plan to take into account the constraints of your environment. Our example site is cutting things a bit too close for comfort, but they have several options for addressing this:

    • Adding additional backup hardware, in this case, a fourth tape drive.

    • Decreasing the amount of data to be backed up or the backup frequency: for example, they could perform full backups only every two weeks for some or all of the data.

    • Increasing the amount of time available/used for backups (for example, performing some backups on weekends or doing incremental backups during the early evening hours).

    • Staging backups to disk. This scheme writes the backup archives to a dedicated storage area. The files can then be written to tape at any subsequent time. Disks are also faster than tape drives, so this method also takes less time than directly writing to tape. It does, of course, require that sufficient disk space be available to store the archives.

  4. Test and refine the backup plan. Actually trying it out will frequently reveal factors that your on-paper planning has failed to consider.

  5. Review the backupplan on a periodic basis to determine if it is still the best solution to your site's backup needs.

11.1.2 Backup Strategies

The simplest and most thorough backup scheme is to copy all the files on a system to tape or other backup media. A full backup does just that, including every file within a designated set of files, often defined as those on a single computer system or a single disk partition.[3]

[3] For the purposes of this discussion, I'll focus on per-disk partition backups, but keep in mind that this is not the only reasonable way of organizing things. I'll also refer to "backup tapes" most of the time in this chapter. In most cases, however, what I'll be saying will apply equally well to other backup media.

Fullbackups are time-consuming and can be unwieldy; restoring a single file from a large backup spanning multiple tapes is often inconvenient, and when files are not changing very often, the time taken to complete a full backup may not be justified by the number of new files that are actually being saved. On the other hand, if files are changing very rapidly, and 50 users will be unable to work if some of them are lost, or when the amount of time a backup takes to complete is not an issue, then a full backup might be reasonable even every day.

Incremental backups are usually done more frequently. In an incremental backup, the system copies only those files that have been changed since some previous backup. Incrementals are used when full backups are large and only a small amount of the data changes within the course of, say, one day. In such cases, backing up only the changed files saves a noticeable amount of time over performing a full backup.

Some Unix backup programs use the concept of a backup level to distinguish different kinds of backups. Each backup type has a level number assigned to it; by definition, a full backup is level 0. Backing up the system at any level means saving all the files that have changed since the last backup at the previous level. Thus, a level 1 backup saves all the files that have changed since the last full (level 0) backup; a level 2 backup saves all the files that have been changed since the last level 1 backup, and so on.[4]

[4] Not all backup commands explicitly use level numbers, but the concept is valid for and can be implemented with any of the available tools, provided you are willing to do some of the record keeping yourself (by hand or by script).

A typical backup strategy using multiple levels is to perform a full backup at the beginning of each week, and then perform a level 1 backup (all files that have changed since the full backup) each day. The following weekly backup schedule summarizes one implementation of this plan:

Monday: Level 0 (full)
Tuesday-Friday: Level 1 (incremental)

A seven-day version of this approach is easy to construct.

The primary advantage of this plan is that only two sets of backup media are needed to restore the complete filesystem (the full backup and the incremental). Its main disadvantage is that the daily backups will gradually grow and, if the system is very active, may approach the size of the full backup set by the end of the week.

A popular monthly plan for sites with very active systems might look something like this:

First Monday: Level 0 (full)
All other Mondays: Level 1 (weekly incremental to previous Level 0)
Tuesday-Friday: Level 2 (daily incremental to previous Level 1)

This plan will require three sets of backup media to do a complete restore (the most recent backup of each type).

In deciding on a backup plan, take into account how the system is used. The most heavily used portions of the filesystem may need to be backed up more often than the other parts (such as the root filesystem, which contains standard Unix programs and files and which therefore rarely changes). A few parts of the system (like /tmp) need never be backed up. You may want to create some additional filesystems that will never be backed up; anyone using them would be responsible for backing up his own files.

You should also consider performing a full backup whether the schedule calls for it or not before you make significant changes to the system, such as building a new kernel, adding a new application package, or installing a new version of the operating system. This may be one of the few times that the root filesystem gets backed up, but if you ever have a problem with your system disk, you will find it well worth the effort when you can avoid a significant amount of reconfiguration. Unattended backups

The worst part of doing backups is sitting around waiting for them to finish. Unattended backups solve this problem for some sites. If the backup will fit on a single tape, one approach is to leave a tape in the drive when you leave for the day, have the backup command run automatically by cron during the night, and pick up the tape the next morning.

Sometimes, however, unattended backups can be a security risk; don't use them if untrusted users have physical access to the tape drive or other backup device and thus could steal the media itself. Backups needed to be protected as strongly as the most secure file on the system.

Similarly, don't do unattended backups when you can't trust users not to accidentally or deliberately write over the tape or other rewriteable media (ejecting the tape after the backup is completed sometimes prevents this, but not always). You also won't be able to use them if the backup device is in heavy use and can't be tied up by the backup for the entire night. Data verification

In many cases,backups can simply be written to media, and the media can go directly to its designated storage location. This practice is fine as long as you are 100% confident in the reliability of your backup devices and media. In other cases, data verification is a good idea.

Data verification consists of a second pass through the backed-up data, in which each file is compared to the version on disk, ensuring that the file was backed up correctly. It also verifies that the media itself is readable.

Some sites will choose to verify the data on all backups. All sites should perform verification operations on at least a periodic basis for all of their backup devices. In addition, as they age and wear out, many devices begin to produce media that can only be successfully read in the drive that produced it. If you need backups that will be readable by devices or systems other than the one that originally wrote on the physical media, you should also periodically verify the backups' readability by examining them on the target devices and systems. Storing backup media

Properly storing thebackup tapes, diskettes, or other media once you've written them is an important part of any backup plan. Here are some things to keep in mind when deciding where to store your backup media:

  • Know where things are

    Having designated storage locations for backups makes finding the right one quickly much more likely. It is also important that anyone who might need to do a restore knows where the media are kept (you will want to take a vacation occasionally). Installation CDs, bootable recovery tapes, boot diskettes, and the like also ought to be kept in a specific location known to those people who may need them. I can assure you from personal experience that a system failure is much more unpleasant when you have to dig through boxes of tapes or piles of CDs looking for the right one before you can even attempt to fix whatever's wrong with the system.

    Another aspect of knowing where things are concerns figuring out what tape holds the file that you need to restore. Planning for this involves making records of backup contents, which is discussed later in this chapter.

  • Make routine restorations easy

    Backups should be stored close enough to the computer so that you can quickly restore a lost file, and tapes should be labeled sufficiently well so that you can find the ones you need.

    Ideally, you should have a full set of tapes for each distinct operation in your backup schedule. For example, if you do a backup every day, it's best to have five sets of tapes that you reuse each week; if you can afford it, you might even have 20 sets that you rotate through every four weeks. Using a single set of tapes over and over again is inviting disaster.

    Labeling tapes clearly is also a great help in finding the right one quickly later. Color-coded labels are favored by many sites as an easy yet effective way to distinguish the different sets of tapes. At the other extreme, I visited a site where the backup system they developed prints a detailed label for the tape at the conclusion of each backup.

  • Write-protect backup media

    This prevents backup media from being accidentally overwritten. The mechanism for write-protection varies with different media types, but most mechanisms involve physically moving a plastic dial or tab to some designated position. The position that is the unwriteable one varies: floppy disks, optical disks and DAT (4mm) tapes are writeable when the tabbed opening is closed, while 8mm tapes and removable disks are writeable when it is open.

  • Consider the environment

    Most backup media like it cool, dry, and dark. High humidity is probably the most damaging environment, especially for cartridge-enclosed media, which are easily ruined by the moisture condensation that accompanies temperature drops in humid conditions. Direct sunlight should also be avoided, especially for floppy disks, since most plastic materials will deform when subjected to the temperature within the trunk of a car or the enclosed passenger compartment on a hot summer day. Dust can also be a problem for most backup media. I've had lint make floppy disks unreadable after taking them home in my coat pocket (now I put them in a zip-top plastic bag first).

    The fact that backup media prefer the same environment used for many computer rooms does not necessarily mean that any or all backup media should be stored in the same room as the computer. Doing so runs the risk that a major problem will destroy both the computer and the backups. Backup tapes are actually more sensitive to some types of problems than some computer components. For example, if a pipe bursts above the computer room, the computer may suffer only minor damage, but your backup tapes will usually all be ruined if they get wet.

    If the tape storage area differs in temperature from the computer area by more than a few degrees, allow the tapes to acclimate to the computer temperature before writing to them.

    Magnetic interference is also something to think about. One of this book's technical reviewers relayed a story about "an entire backup library that kept getting wiped out on a nearly daily basis. Turns out that the tapes were in a secure location but placed against a wall that was shared with a freight elevator. The magnetic fields and such caused by the moving lift caused all that nice magnetic tape storage to become erased. Funny but cautionary."

  • Handle media properly

    Some media have special requirements that you'll need to take into account. For example, floppy disks and zip disks ideally should be stored upright, resting on a thin edge rather than stacked on top of one another. Similarly, cartridge tapes like to be stored with the spools vertical (perpendicular to the ground, like a car's tires) with the edge that contacts the drive heads down (so gravity pulls tape away from the spools). When you're counting on media to preserve important data, humor them and orient them the way they prefer.

  • Take security into account

    In every location where you store backup tapes, the usual physical security considerations apply: the tapes should be protected from theft, vandalism, and environmental disasters as much as is possible. Off-site and long-term storage

Off-site backups are the last barrier between your system and total annihilation. They are full backup sets that are kept in a locked, fireproof, environmentally-controlled location completely off site. Such backups should be performed on unmounted filesystems if at all possible.

Preparing backups for off-site storage is also one of the few times when simply making a backup is not enough.[5] In these cases, you also need to verify that the backup tape or diskette is readable. This is done by using an appropriate restore command to list the contents of the tape or diskette. While this will not guarantee that every file is completely readable, it will improve the odds of it considerably. Some backup utilities provide a full verification facility in which the entire content of each file in a backup set is compared with the corresponding file on disk; this is the preferred method of checking critical backups. In any case, backups should be verified in the best way available whenever the integrity of the backup is essential.

[5] Another such time is when you are rebuilding a filesystem.



For data meant for permanent archiving, you should create and verify two sets of backup media with the idea that the redundant copy can be used should the first one fail. The media should also be checked periodically (annually or possibly biannually). When a particular media item fails and they all will eventually a new copy should be made from the other one to replace it.

You should also make sure that you have at least one working drive of the type that you are using for permanent storage media. For example, if you have an archive of 8 mm tapes, you will need to always have working 8 mm tape drives to read them. This will continue to be true if your primary backup medium changes. Similarly, you must maintain whatever software programs and other running environment is required to use the data for it to be of any use.

Finally, tapes should be rewound or retensioned regularly (perhaps twice a year) to maintain readability. Given this requirement, tapes are being superceded by CDs as permanent storage media.

When Being Compulsive is Good

It's very easy to put off doing backups, especially when you are responsible only for your own files. However, performing backups regularly is vital. Basically, it's a good idea to assume that the next time you sit down at the computer, all your disks will have had head crashes. Keeping such a catastrophe in mind will make it obvious what needs to be backed up and how often. Backups are convenient for restoring accidentally deleted files, but they are also essential in the event of serious hardware failures or other disasters. Catastrophes will happen. All hardware has a finite lifetime, and eventually something will fail.

Given this reality, it is obvious why an almost drone-like adherence to routine is an important attribute for an effective system administrator. Planning for worst-case scenarios is part of the job. Let them call you compulsive if they want to; one day, your compulsiveness also known to many as carefulness will save them, or at least their files.

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

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