This chapter discusses methods used to provide specific kinds of backup services. The topics of this chapter are:
The chapter begins with a discussion of the concept of fault tolerance. A fault can be the loss of a device or component, or a disaster that shuts down your operation. Making your network fault tolerant means to take steps that allow you to maintain service when small problems crop up, and to restore service quickly when large problems occur.
The chapter changes topic to discuss RAIDs. The acronym is defined differently by different sources. It is often defined as your text does it: Redundant Array of Independent Disks and Redundant Array of Inexpensive Disks are both acceptable, and both wrong in different ways. The disk drives may not be inexpensive, and some RAID configurations do not allow you to think of the drives as independent. What both mean is several hard drives that are linked together for improved performance. The improvement varies with each design.
The text discusses mirroring and duplexing in more detail, listing some pros and cons for each.
Compared to the two scenarios above, striping with parity provides less loss of storage space. Duplexing and mirroring sacrifice one drive for each drive in use. Striping uses parity information for recovery, not a full copy of the data, so you "lose" one drive for each array, not one for one.
Active Directory and NDS replication
A partition is defined as a subsection of a Directory, whether it is an eDirectory or an Active Directory. In an Active Directory network, every domain controller in a given domain holds a copy of the same information as every other domain controller. When changes are made, they replicate those changes to the other domain controllers holding their Active Directory.
In a Novell network, we divide the Directory Tree into partitions by using containers as markers of where partitions start. A partition must have a container at the top, and may contain other containers as well. The partition is referred to by the name of the highest container in it. In the diagram below, the first partition we see is the [Root] partition. It is drawn so that it contains the [Root], the highest container object in the Tree. (This is not the only way to partition a Tree, just an example. The [Root] object need not be in partition by itself.)
Any partition also contains all objects inside the containers it contains, unless another partition is made as a child of the first one. For example, the [Root] partition is the default partition in an NDS Tree. If it had been left alone, the [Root] partition below would have contained all objects in the Tree. However, a child partition was created: the EMA partition. We refer to the EMA partition as the child of the [Root] partition, since the EMA partition branched from the [Root] partition. This also makes the [Root] partition the parent partition of the EMA partition. In this example, the EMA partition also has two child partitions: the NYC partition and the Tokyo partition.
The topmost container in a partition is called the partition root.
This is true for any partition. In the diagram above, we see that there
are four partitions. The first is the [Root] partition, which contains
[Root] only. The other three partitions are named for their partition
root objects, the three containers at the top of each one. Note the naming
It may be clearer if you think of the phrase "partition root" as really meaning the "partition's root", the place where we drew a line in the Tree and said it begins a new partition.
Now for the concept of Replicas. A Directory partition contains
a lot of information, and it would be a shame to lose it, so Novell invented
four kinds of replicas, most of which are copies of a given partition.
Changes to data come in two types: simple changes and complex changes. A simple change could be a change that affects the data in one object. This change needs to be replicated to all copies of the NDS partition that the object exists in. A simple change is replicated easily, and the replicas synchronize quickly. A complex change would affect multiple objects, such as creating a new partition from two smaller ones. Much data has to be replicated so synchronization may take much longer.
Three utilities are used for maintenance of NDS:
System backups and UPS usage
Making backup copies of data can be done several ways. The RAID 1 concept of mirroring a drive is one example. Another is making backups on tape, CDs, or other media.
Four backup strategies, or schedules, are often encountered. You
should know them. First some terms:
This needs more explanation. Assume we use a tape drive to make backups. In a Full backup strategy, the entire target is backed up to tape every time we make a backup tape. This strategy consumes the most time and the most tapes to carry out a backup. To restore, we simply restore the most recent tape(s). This is the least time consuming strategy for restoring, but the most time consuming for creating backups.
The second method, Incremental backup, means that we start with a Full backup of the target, and then each successive backup tape we create only backs up the elements that are new or changed since the last backup was created. This means that successive backups will not always be the same length. Therefore, this is the least time consuming backup, but the most time consuming restore. To restore, we must first restore the last Full backup made, and then restore EVERY tape made since then, to ensure getting all changes.
The third strategy, Differential backup, also starts with a Full backup tape. Then each successive tape made will contain all the files changed since the last Full backup was made. This means that we will have to restore only one or two tapes in a restore operation. If the last tape made was a Full tape, we restore only that one. If the last tape made was a Differential tape, we restore the last Full tape, then the last Differential tape.
In both Incremental and Differential backup strategies, you will typically use a rotation schedule. For example, you could have a one week cycle. Once a week, you make a Full backup, then every day after that you make the other kind you have chosen to use: Incremental or Differential.
To keep them straight in your mind, remember that:
The time required to create backup tapes should be considered along with the time to restore a backup. When you consider the two concepts as two sides of the answer to a question (What method should I use?), the answer may be the most common choice: Differential. It is the best compromise in terms of backup time versus restore time. Note also, that all three standard methods require a full backup on a regular cycle. The recommendation is usually to run a Full backup tape weekly.
Whichever backup strategy you use, you should consider keeping one set of backups in secure location at your site (handy and protected) and another set in a secure location at a distant site. Consider the potential disasters that could occur at your location (fire, flood, tornado, hurricane, vandalism, etc.) and decide how to protect your backups and how far away the other sets should be.
A true disaster recovery plan will include access to a site to use if your data center becomes unusable, unavailable, or nonexistent. The text describes such alternate sites as falling into three categories:
The text suggests some hardware that is useful in maintaining system up time and data reliability. An Uninterruptible Power Supply (UPS) is typically recommended for all servers. Various models of UPS are available with differing capabilities. The main differences between them are how many minutes of battery power they can supply to your system in case of a power failure, and how many watts they can deliver for that period of time.