Two-Tier Model

Two-Tier Model

As discussed in Single Server Model scalability and network bandwidth are limited when working with a single server site. In a two-tier model, an intermediate backup server is used as a staging platform (see Figure 6). The advantages are twofold:

  1.  The backup is done to the first backup server (or source  server), which resides locally (and has a LAN connection), and only then  forwarded to the central backup server (or target server). This can be  done asynchronously, so that communication performance between the first  and second-level backup servers is not critical.

  2.  The backup completes  in a much shorter time as data transmission is not slowed down by tape  drive write speeds. This leads to much shorter backup windows.

You could also load balance large sites by adding additional source servers.
Figure 6. Two Tier Model

Figure 7 shows what happens with the data that needs to be backed up. In the first stage, data is moved to the source server. This happens during the period of time that we have to take our backups (referred to as backup window; see Scheduling Backups).
Figure 7. Data Movement in a Two-Tier Model

The specifications of the storage device connected to this source server should be sufficient to store all the data that is backed up. Typically, it will also be a fast device (probably a disk drive). In the second stage, data on this storage device is moved across the network to a second backup server. This normally happens after stage 1 completes (but not necessarily, however), and can be done outside of the normal backup window. The only rule here is that all data from the source servers must be moved to the target server before the backup window restarts.

This setup gives advantages with regard to scalability, since you can add as many source servers as you want. However, more intelligent software is required to manage the transfer of backed-up data both in backup mode and in restore mode. In the case of a restore operation, the user should not need to know on which backup server the data is.

Another advantage of this server storage hierarchy is that in case of a site disaster at the source server location, the backups still reside on the target server. Of course, this advantage will only be true if the target and source servers are geographically separated, and all backup data has been moved to the central server.

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