Tales From the Server Room: Zoning Out
Sometimes events and equipment conspire against you and your team to cause a problem. Occasionally, however, it's lack of understanding or foresight that can turn around and bite you. Unfortunately, this is a tale of where we failed to spot all the possible things that might go wrong.
It was 2006, and we were just getting our feet wet with piloting a new server architecture for our company. We'd just received our first fully populated Hewlett-Packard blade chassis (a P-Class chassis with eight dual-core blades, for those of you who're savvy with that type of gear), a new EMC Storage Area Network (SAN) and three VMware ESX licenses. We had just finished converting a fair amount of the development network over to the VMware environment using a Physical-to-Virtual (P2V) migration, and things were going quite well. Matter of fact, many of the people in the company didn't quite understand exactly the improvements we were making to the systems, but they did notice the performance boost of going from machines that were something like single-processor Pentium 4-class servers with IDE disks to a dual-core Opteron where the storage was backed by the speed of the Fibre Channel SAN. In all, things were going quite well, and the feedback we'd received to date fueled a rather rapid switch from the aging physical architecture to a much faster virtual machine architecture.
Before we dive into the story, a couple bits of background information will become very important later on. As I said, we'd received eight dual-core blades, but only three of them at that time were set aside for VMware hosts. The rest were slated to become powerful physical machines—Oracle servers and the like. All these new blades were configured identically: they each had 16GB of RAM, two dual-core Opteron processors, two 300GB disks and Fibre Channel cards connected to the shiny new EMC SAN. With respect to the SAN, since we were devoting this SAN strictly to the blade servers, the decision was made not to add the complexity of zoning the SAN switch. (Zoning a SAN switch means that it is set up to allow only certain hosts to access certain disks.) The last tidbit relates to kickstart.
Both Kyle and I have written a few articles on the topic of kickstarting and automated installation, so by now you're probably aware that we're fans of that. However, this was 2006, and we both were getting our feet wet with that technology. We'd inherited a half-set-up kickstart server from the previous IT administration, and we slowly were making adjustments to it as we grew more knowledgeable about the tech and what we wanted it to do.
[Kyle: Yes, the kickstart environment technically worked, but it required that you physically walk up to each machine with a Red Hat install CD, boot from it, and manually type in the full HTTP path to the kickstart file. I liked the idea of kicking a machine without getting up from our desks, so the environment quickly changed to PXE booting among a number of other improvements. That was convenient, because those blades didn't have a CD-ROM drive.]
Getting back to the story...we'd moved a fair amount of the development and corporate infrastructure over to the VMware environment, but we still had a demand for high-powered physical machines. We'd gotten a request for a new Oracle database machine, and since they were the most powerful boxes in the company at the time, with connections to the Storage Area Network, we elected to make one of the new blades an Oracle box.
As my imperfect memory recalls, Kyle fired up the lights-out management on what was to be the new Oracle machine and started the kickstart process, while I was doing something else—it could have been anything from surfing Slashdot to filling out some stupid management paperwork. I don't remember, and it's not critical to the story, as about 20 minutes after Kyle kickstarted the new Oracle blade, both of our BlackBerries started beeping incessantly.
[Kyle: Those of you who worked (or lived) with us during that period might say, "Weren't your BlackBerries always beeping incessantly?" Yes, that's true, but this time it was different: one, we were awake, and two, we actually were in the office.]
Trouble in Paradise
We both looked at our BlackBerries as we started getting "host down" alerts from most of the machines in the development environment. About that time, muttering could be heard from other cubicles, too: "Is the network down? Hey, I can't get anywhere." I started getting that sinking feeling in the pit of my stomach as Kyle and I started digging into the issue.
Sure enough, as we started looking, we realized just about everything was down. Kyle fired up the VMware console and tried restarting a couple virtual machines, but his efforts were met with "file not found" errors from the console upon restart. File not found? That sinking feeling just accelerated into free-fall. I started looking along with Kyle and realized that all the LUNs (disks where the virtual machines reside) just flat out stopped being available to each VM host.
[Kyle: It's hard to describe the sinking feeling. I was relatively new to SAN at the time and was just realizing how broad a subject it is in its own right. SAN troubleshooting at a deep level was not something I felt ready for so soon, yet it looked like unless we could figure something out, we had a large number of servers that were gone for good.]
I jumped on the phone and called VMware while Kyle continued troubleshooting. After a few minutes on the line, the problem was apparent. The LUNs containing the virtual machines had their partition tables wiped out. We luckily could re-create them, and after a quick reboot of each VM host, we were back in business, although we were very worried and confused about the issue.
[Kyle: So that's why that sinking feeling felt familiar. It was the same one I had the first time I accidentally nuked the partition table on my own computer with a bad dd command.]
Our worry and concern jumped to near-panic when the issue reared its head a second time, however, under similar circumstances. A physical machine kickstart wound up nuking the partition table on the SAN LUNs that carried the virtual machine files. We placed another call to VMware, and after some log mining, they determined that it wasn't a bug in their software, but something on our end that was erasing the partition table.
A Light Dawns
Kyle and I started to piece together the chain of events and realized that
each time this occurred, it was preceded by a kickstart of a blade server.
That led us to look at the actual kickstart control file we were using,
and it turned out there was one line in there that caused the whole problem.
clearpart --all --initlabel would erase the partition table
on all disks attached to a particular host, which made sense if the
server in question had local disks, but these blades were attached to the
SAN, and the SAN didn't have any zoning in place to protect against this.
As it turns out, the system did exactly what it
was set up to do. If we had
placed the LUNs in zones, this wouldn't have happened, or if we'd have
audited the kickstart control file and thought about it in advance, the
problem wouldn't have happened either.
[Kyle: Who would have thought that kickstart would become yet another one of those UNIX genie-like commands like dd that do exactly what you say. We not only placed the LUNs in zones, but we also made sure that the clearpart directive was very specific to clear out only the disks we wanted—lucky for us, those HP RAID controllers show up as /dev/cciss/ devices, so it was easy to write the restriction.]
We learned a couple things that day. First was the importance of zoning your SAN correctly. The assumption we were operating under—that these boxes would all want to access the SAN and, therefore, zones were unnecessary—was flat out wrong. Second, was the importance of auditing and understanding work that other sysadmins had done prior and understanding how that work would affect the new stuff we were implementing. Needless to say, our SAN always was zoned properly after that.
Bill Childers is the Virtual Editor for Linux Journal. No one really knows what that means.
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