What Is It Like Migrating Mission Critical Servers from Paid Linux (RHEL) to Free Linux (CentOS)?
Q: When we spoke several months ago you were considering a move from Red Hat to a free binary clone of RHEL (Red Hat Enterprise Linux) like CentOS. Are you still going forward with that, and if so, why?
A: Yes. Weve been using Red Hat 7.3 for five years on about three dozen production servers, but now we are migrating to CentOS 5. We have a large commercial Web site serving tens of millions of page views per month and generating a lot of advertising revenue for our company. We dont like change for its own sake. When we get production servers up and running successfully from a stable OS image, and we see that they are doing their job without too many problems, we dont have any incentive to touch them. Its in our interest to change them as little as possible. However, that does create a problem, because the versions of the software you use can get really old. Thats what happened to us with Red Hat 7.3. Weve been using it for years and it worked fine during all that time. But eventually new versions of the applications we use like Perl and Apache came along that work better with more recent versions of Linux.
Q: What will the migration process look like?
A: Our problem is that its been so long since we updated that now its a big leap and a lot of work to move to a more up-to-date OS. In our case we will be going all the way from Red Hat 7.3 to CentOS 5, which is the equivalent of RHEL5. But we are not going to migrate all of our machines at once. Some of our servers are fairly old, some are recent. We operate by setting up a stable production image on one server and booting the other servers over the network from that server. So getting the image right is very important. We tried CentOS 4 (equivalent to RHEL4) on one of our machines and had some problems. Then we figured out that there might be a fix for those problems in CentOS 5. We tried that and so far things seem to be working well on our test machine. So we are about to roll it out to one of our production machines, where we will observe it for a while to see if all is well. If so, we will gradually roll it out to our other production machines, about 35 servers in all. But one issue we have is that the older machines will require a different kernel due to their hardware. So we will have to experiment with that to get it right, and that will involve another round of QA before we can roll CentOS out to those machines.
Q: Did you consider going with RHEL itself instead of a clone like CentOS?
A: Yes. Since weve used Red Hat for a long time naturally we looked at RHEL before choosing CentOS. But Red Hats subscription prices, which can run more than $1,000 per server per year, seemed really high for our needs. We just didnt see enough value in paying them for support. We recognize the role that Red Hat plays in the industry. They are the ones who build the distribution, test it, patch it and support it. CentOS just tracks what Red Hat does. They download the Red Hat source code from Red Hats Web site and then compile it into their own binary distribution, which they are perfectly entitled to do because RHEL is under the GPL. So they offer exactly the same software but without the Red Hat trademarks. They dont contribute anything themselves. But from our point of view thats all we need.
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