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Figure 1


An Economical Scheme for Quasi Real-Time Backup

Leo Liberti and Franco Raimondi

Backup is the most important systems administration task and often neglected by sys admins. Restoring is another daunting task and, during the lengthy restore process, the main server is unable to serve, further pressuring the admin. With freeware, inexpensive PC hardware, and high-quality UNIX-like operating systems, it is possible to have a couple of standby servers ready to supply the services when the main server crashes. One such redundancy scheme is described in the article "Quick Network Redundancy Schemes" (Leo Liberti, Sys Admin, April 2001: But what use is a backup server if the data it is supposed to serve is all stored on the crashed main server?

The solution is a real-time backup system on the standby servers so that when the main server is unreachable, the standby server automatically takes its place and is ready to supply the service while the admin works with the crashed main server. Unfortunately, real-time backup is a black beast in every way: it requires complicated (proprietary) software, costly hardware, and various locations, all linked by high-speed dedicated connections (see "Real-Time Remote Data Mirroring" by Primitivo Cervantes, Sys Admin, July 2001:

In this article, we will discuss the implementation of a quasi real-time backup scheme for high-availability systems that updates the standby server's data disks every given time interval, down to cron's minimum granularity (one minute), or alternatively in a "respawn"-like continuous fashion. Because user data tends to gather in large quantities, and for security considerations, things are not as simple as running rsync as often as possible. Our system relies on the replication of the relevant network segments to supply a lot of bandwidth for the quasi real-time backup tasks, and for this backup to happen in an isolated logical network.

How the System Works

This section provides the overall description of the system. We envisage two computers -- A and B. Computer A is the main server, and B is the standby backup server that provides the necessary redundancy. Refer to my previous article on network redundancy schemes to set up a standby server. We intend to keep B's data disks constantly synchronized with A's data disks.

There are a lot of ways to transfer data over a network, most notably NFS, FTP, rcp, rsync. All of these methods have advantages and disadvantages. The main disadvantage with NFS is that it requires a persistent network connection between client and server. Because we are devising a solution for situations where the server has crashed, it may lead to problems (e.g., broken NFS connections are infamous for requiring long timeouts when servers are down). FTP is difficult to automate, and FTP clients usually cannot deal with incremental transfers (i.e., where only the differing parts of a file are transferred in order for the file to be updated). rcp suffers from security problems (as all r- services do), and it has the same drawback as FTP in that it cannot deal with incremental transfers. For all these reasons, we are going to work with rsync.

Although rsync is slightly less standard than NFS, FTP, and rcp, it is widespread in most UNIX-like operating systems. In any case, it is easy to download and compile (see The advantages of rsync include:

  • It is easy to run rsync on top of SSH (secure shell), so that transfers can be secure.
  • It supports incremental file transfers. Because we intend to continuously run rsync, it is crucial that the amount of transferred data be kept to a minimum.

Running rsync on the existing local network is not wise. For one thing, taking steps towards prevention of disaster does not mean that disaster will actually occur, so clogging the local network with data that may never be used is not the best idea. Also, the amount of transferred data may be very large. This, of course, depends on the network usage and cannot be generalized, but we don't want a self-inflicted denial-of-service so that when the network is under a heavy load, the amount of traffic doubles because of our real-time backup. Furthermore, because we are handling user data, running continuous transfers on the local network may expose it to packet sniffing. Finally, using a separate network adds resiliency to the system; the main network goes down but network backups are still occurring.

This leads to the conclusion that data backup should take place over a dedicated cable. Even though a network segment between computers A and B must already exist on the local network, we are going to replicate the network segment using two additional network cards and a piece of network cable. Figure 1 shows the proposed network layout. The data backup will take place on the replicated network segment.


We suggest using the same type of network hardware for the replicated segments as that of the local network. If the local network runs at 10 Mbps, it does not make sense to install two top-of-the-range 1-Gbps Ethernet cards on the segments. On the other hand, if you choose to install 10-Mpbs replicated network segments within a faster network, data to be backed up may arrive at a faster pace than the segment capacity allows.

We run a 100-Mbps local network at Ipnos, so we installed two PCI 100-Mbps network cards on computers A and B with a crossover UTP cable between them. We recommend against using a hub or a switch for point-to-point Ethernets such as our replicated network segments, because in addition to the added cost with no added benefit, network devices add a small performance penalty to network transfers when compared with transfers over the raw cable.

With respect to the choice of hard disk storage, it is better to install disks of the same make, model, and size on each of the servers. However, this requirement is not absolute. It suffices to keep an eye on the disk capacity of the smallest hard disk.

Network Configuration

There are basically two possibilities for integrating these network segments into the existing network setup:

1. Let them participate to the same local network, letting ARP and static routing take care of packets.

2. Create a separate network for each of the segments.

The advantage of the first choice is the ease of setup. However, because we want to keep an eye on security issues, we want to enforce a definite logical separation between the segments and the local network. This is not the only advantage of logical network separation -- extended resilience in case of failure of the local network and greater ease of use with rsync are other advantages. Therefore, we recommend using a separate network for each the replicated network segments.

Suppose then that the local network is, computer A (the main server) has IP on device eth0, and computer B (the standby server) has IP on device eth0. (We assume the operating system on the computers is Linux; just change the device name accordingly if this is not the case). After installing the second network card on each of the two computers, we configure the network segment with on device eth1 for computer A, and on device eth1 for computer B. Refer to Linux NET-3-HOWTO or Net-HOWTO if you have problems in setting up the network segments. Remember a crossover UTP cable must be used when connecting the two network cards on the segment.

Necessary Network Services

If the local network is completely separated from the rest of the Internet, or if it is guarded by a strict, well-administered firewall, we can use rsync client and daemon to transfer files. These are the steps to follow:

1. Check that rsync is present on computers A and B. If not, download and install it.

2. Install the rsync daemon service on computer A by adding the line rsync stream tcp nowait root /usr/bin/rsync rsync --daemon to the file /etc/inetd.conf on computer A.

3. Copy the following rsync daemon configuration file to /etc/rsyncd.conf on computer A, and give it file permissions 600.

# on computer A
uid = root
gid = root
use chroot = no
max connections = 0
syslog facility = local5
pid file = /var/run/
read only = true
hosts allow =
hosts deny =
# path points to the user data directory
path = /home
comment = user data
auth users = realtime
secrets file = /etc/rsyncd.secrets
4. Reconfigure the running inetd daemon on computer A. As root:

A# killall -HUP inetd
5. Create the secrets file on computer A. As root:

A# echo realtime:mypasswd>/etc/rsyncd.secrets;chmod 600 /etc/rsyncd.secrets
6. Set up accessing rights on computer B. As the user that will run the rsync client (may be root):

B# echo mypasswd > ~/.rsync-pwd ; chmod 600 ~/.rsync-pwd
7. Create the script that will read the data on computer A and transfer it to computer B. Copy the following lines to the file /usr/local/sbin/realtimesync on computer B, and give it file permissions 755.

# on computer B
rsync -a --password-file ~/.rsyncd-pwd rsync://realtime@ $1

If we need a secure connection over the replicated network segment, we can run rsync over SSH. Follow these steps:

1. Download, install, and configure SSH on computer A so that unchallenged root logins are allowed from computer B (use the segment's IP,, and see sshd man page, searching for the terms "PermitRootLogin", "IgnoreRhosts", "RhostsAuthentication"). If you don't want to allow unchallenged root logins from anywhere, create a user that has the right to read all data files that are going to be backed up and allow unchallenged root logins from that user instead.

2. Install rsync on both computer A and B (if it's not already there), and the SSH client on computer B.

3. Create the script that will read the data on computer A and transfer it to computer B. Copy the following lines to the file /usr/local/sbin/realtimesync on computer B, and give it file permissions 755.

# on computer B
rsync -a -e ssh $1

To check that everything has been successful, try running the script /usr/local/sbin/realtimesync from computer B, and verify that it returns the complete list of data files in the /home directory on computer B.

Performing the Backup

Having put everything in place, we are now ready to perform the backup by running realtimesync on computer B, either in a cron job or in a continuous fashion. In each case, we wrap the script in another script, which checks that the previously launched script has finished running. Create the script /usr/local/sbin/run_realtimesync on computer B as follows, and give it permissions 755:

# on computer B
ps ax | grep /usr/local/sbin/realtimesync | grep -qv grep || \
    /usr/local/sbin/realtimesync /home/
Notice that the argument /home/ specifies the directory on computer B to where data from A will be mirrored.

For a cron job, choose a time interval between 1 and 10 minutes (or even higher if you like, but anything higher than 10 minutes will make this real-time backup scheme sound more like seldom-time). For a 3-minute time interval, edit the following line in the cron system using the crontab command:

  */3 * * * * root /usr/local/sbin/run_realtimesync
For a continuous execution, modify the run_realtimesync script:

# on computer B
while true ; do
ps ax | grep /usr/local/sbin/realtimesync | grep -qv grep || \
   /usr/local/sbin/realtimesync /home/
sleep 10
Launch this script from one of the initialization files in the init.d directory (look in /etc, /etc/rc.d or /sbin), taking care to add an ampersand to launch in background:

run_realtimesync &
The sleep 10 command has the purpose of avoiding the overloading computer B with an excessive checking using ps. It is the number of seconds to wait before trying to spawn another backup process. You can modify this parameter to as low as 1-second intervals.

Integration in Network Redundancy Schemes

The ideas described in this article can be effectively used in conjunction with the quick network redundancy scheme described in the "Quick Network Redundancy Schemes" article. In this section, we discuss some of the issues arising from the combination of the two schemes.

In the aforementioned article, network redundancy was obtained by having two or more computers monitoring each other continuously. When one of the computers failed to respond, another one would take its IP address by means of IP aliasing. It would then spawn the necessary daemons to supply the services which were originally supplied by the crashed machine.

Diagnosing the Network Problem

In the above-cited article about redundancy schemes, the standby server continuously monitors the main server by sending ping probes. When the ping stops responding, the main server is assumed to be down. However, we can now use the replicated network segment to further diagnose the problem by having computer B continuously ping'ing computer A on both network interfaces. If both ping attempts fail, we can assume computer A is down. If the local network ping fails but the replicated segment is alive, we can wait some time and try ping'ing again before taking action; it may be that the local network is under heavy load.

Securing Communication

Some of the strategies computer B uses to try to resume service on computer A (described in the above-cited article) rely on remote unauthenticated root login. By running these commands on the replicated network segment, it is less likely that somebody on the local network could break into the network connection and steal data.

Automatically Curing Network Woes

If one of the ping probes fails (say the one on the local network), we can take automated remote action by using the replicated network segment. For example, we can trigger an automatic reboot to see if things improve. Otherwise, we might try to restart the network configuration. Likewise, if the replicated network segment fails, we can take the same actions on the local network.

Bring Interfaces Down Prior to Takeover

When computer B has decided that computer A is really not able to provide the service, it takes over by grabbing its IP number (as described in the previous article about redundancy schemes). If the replicated network segment is still alive, it is a good idea to use SSH or RSH to bring computer A's local network interface down prior to taking its IP. This way, even if the interface on computer A starts working again, there will never be the problem of having two interfaces with the same IP address over the same local network.

Stopping Real-Time Backup Services

Before computer B effectively takes over the service from computer A, all real-time backup services must be stopped. This can be obtained by inserting the commands:

  • chmod 644 /usr/local/sbin/realtimesync
  • chmod 644 /usr/local/sbin/run_realtimesync
  • killall run_realtimesync
  • killall realtimesync

in the "take action" section of the monitor scripts (see previous article), so that the real-time synchronization scripts are not executable anymore.

After the Main Server Has Resumed Service

Suppose computer A has failed, and computer B has taken over. Now the sys admin is left with the task of resuming services on computer A. There must be some interruption of service during the "restore data" phase for computer A (the main server), but because we can't lose any user data, all data gathered on computer B during computer A's downtime must be transferred to computer A in an autonomous fashion (i.e., without any more user data being stored on computer B). We always see this stage (resuming services on computer A) as safer if conducted by a systems administrator, rather than relying on automated procedures. Ideally, these should be the steps:

1. Mend computer A and, while it's disconnected from the local network, test it, and make sure it works.

2. Bring down the local network interface on computer A, but make sure the replicated network segment is working.

3. Suspend customer services on computer B and put up a banner on the Web site informing customers that service will be resumed as soon as possible (of course you will have advised them about this downtime by email at least two days prior).

4. Use FTP, NFS, or what have you to transfer the data files from computer B to computer A. (Do not attempt to use rsync because the rsync daemon on computer A is configured for read-only operation. However, you may still use rsync on RSH or SSH.)

5. Bring down the local network interface on computer B corresponding to computer A's IP address.

6. Bring up the local network interface on computer A (the service has now been resumed and your customers can start working immediately).

7. On computer B, resume the monitoring services (by relaunching the monitor script), and then remove the Web site banner advising of the downtime. Resume the real-time backup services by setting permissions 755 to the scripts /usr/local/sbin/run_realtimesync and /usr/local/sbin/realtimesync. If you were using real-time backup through the cron daemon, you are done. If you were using continuous spawning of realtimesync, you must restart the modified run_realtimesync script manually.


We have described procedures that allow for quasi real-time data backup services between a main server and a standby replacement server. The procedures are based on replicating the network segment between the two servers. Unlike commercial full-fledged real-time backup services, which rely on costly hardware and proprietary software, the procedures described here require no further cost to implement than that of a couple of spare network cards and a crossover cable.

Leo Liberti graduated in Mathematics from Imperial College, London, UK, in 1995. He then received an M.Sc. in Mathematics from Turin University and went on to do more research at Imperial College and some part-time work as a systems administrator. He is a co-founder of IrisTech S.r.l. (Italy -- and Ipnos (UK --

Franco Raimondi graduated in Physics from Milan University, Italy, in 1997. He is a co-founder of IrisTech S.r.l. and Ipnos. He has recently started a Ph.D. on Intelligent Software Agents at the Department of Computing at Imperial College.