A High-Availability Cluster for Linux
The resynchronization (mirroring back) procedure was implemented using rsync, which uses a lock file to disallow any mirroring to another node when a node failure is sensed. The lock file is checked for existence by sync-app before any files are mirrored. This prevents node A mirroring to node B, while node B is mirroring the same files to node A.
If preferred, clusterd could be used with a shared and/or distributed storage device by removing the resynchronization function and by not using sync-app, although I have not tried this.
To test server failure, I had to simulate the failure of every interface on the cluster. In each case, the cluster took the expected action and shut down the correct server. In the case of the inter-node/heartbeat network failing, the nodes simply carried on normal operation and notified the administrator of the failure. On a point-to-point network of this nature, it is almost impossible to determine which NIC is at fault. I simulated various network switch failures and power supply failures. The results were all as expected. After a node was put into standby (single-user) mode, I had to manually remove a standby lock file in order to fully bring up the node again. If a node recovered and entered a network runlevel while the standby lock file still existed, the remote node immediately put the node back into standby mode to prevent an IP and MAC address clash on the LAN.
Mirroring was tested over a period of several months, and I found that the nodes could typically compare 6GB of unchanged data in approximately 50,000 files in under 45 seconds.
After catastrophic node failure (I pulled the power plug from the UPS), recovery time for the node was around 10 to 15 minutes for fsck disk checking, and a disk resynchronization time of around three minutes (9GB of data). This represented a cluster services downtime of around three minutes to the LAN clients.
Failover delay from when a node failed until the remote node fully took over was typically 60 to 80 seconds. The effect on users depended on the service: Sendmail, IMAP4, http and FTP simply refused connection for users for the duration, whereas Samba sometimes momentarily locked up a Windows PC application when files were open at the point of failure. radius and dhcpd caused no client lock-outs, probably because of their UDP implementation.
On the whole, the cluster provides us with much better system availability. It is a vast improvement over the single server, as we can now afford to do server maintenance and upgrades during working hours. We have not yet had any catastrophic failures with the new Dell servers, but the test results show a minimal downtime of less than two minutes while a node takes over. We have saved large amounts of capital by implementing a simple high-availability cluster without the need for expensive specialist hardware such as dual ported RAID.
This clustering solution is certainly not as advanced as some of the commercial clusters or as thorough as some of the upcoming open source Linux-HA project proposals; however, it does sufficiently meet our needs.
The system has been in full-time production operation since September 1998. We have over 30 LAN clients using the cluster as their primary “server”. The system has proven to be reliable. The company sees the server as a business-critical system, and we have achieved the objectives of high availability.
Practical Task Scheduling Deployment
July 20, 2016 12:00 pm CDT
One of the best things about the UNIX environment (aside from being stable and efficient) is the vast array of software tools available to help you do your job. Traditionally, a UNIX tool does only one thing, but does that one thing very well. For example, grep is very easy to use and can search vast amounts of data quickly. The find tool can find a particular file or files based on all kinds of criteria. It's pretty easy to string these tools together to build even more powerful tools, such as a tool that finds all of the .log files in the /home directory and searches each one for a particular entry. This erector-set mentality allows UNIX system administrators to seem to always have the right tool for the job.
Cron traditionally has been considered another such a tool for job scheduling, but is it enough? This webinar considers that very question. The first part builds on a previous Geek Guide, Beyond Cron, and briefly describes how to know when it might be time to consider upgrading your job scheduling infrastructure. The second part presents an actual planning and implementation framework.
Join Linux Journal's Mike Diehl and Pat Cameron of Help Systems.
Free to Linux Journal readers.Register Now!
- Stunnel Security for Oracle
- SourceClear Open
- Murat Yener and Onur Dundar's Expert Android Studio (Wrox)
- SUSE LLC's SUSE Manager
- My +1 Sword of Productivity
- Managing Linux Using Puppet
- Google's SwiftShader Released
- Non-Linux FOSS: Caffeine!
- Parsing an RSS News Feed with a Bash Script
- Doing for User Space What We Did for Kernel Space
With all the industry talk about the benefits of Linux on Power and all the performance advantages offered by its open architecture, you may be considering a move in that direction. If you are thinking about analytics, big data and cloud computing, you would be right to evaluate Power. The idea of using commodity x86 hardware and replacing it every three years is an outdated cost model. It doesn’t consider the total cost of ownership, and it doesn’t consider the advantage of real processing power, high-availability and multithreading like a demon.
This ebook takes a look at some of the practical applications of the Linux on Power platform and ways you might bring all the performance power of this open architecture to bear for your organization. There are no smoke and mirrors here—just hard, cold, empirical evidence provided by independent sources. I also consider some innovative ways Linux on Power will be used in the future.Get the Guide