Managing FOSS for Business Results

By Elizabeth Krumbach

Built on the Free/Open Source Software (FOSS) model for cluster deployments, LinuxForce staff has been hard at work over the past months developing and deploying LinuxForce Cluster Services built upon exclusively FOSS technologies and on December 15th we put out a press release:

Announcing LinuxForce Cluster Services

In September Laird Hariu wrote the article “File Servers – The Business Case for High Availability” where, in addition to building a case to use clusters, he also briefly outlined how Debian and other FOSS could be used to create a cluster for a file server. File servers are just the beginning, we have deployed clusters which host web, mail, DNS and more.

The core of this infrastructure uses Debian 6.0 (Squeeze) 64-bit and then depending upon the needs and budget of the customer, and whether they have a need for high availability, we use tools including Pacemaker, Corosync, rsync, drbd and KVM. Management of this infrastructure is handled remotely through the virtualization API libvirt using the virsh and Virtual Machine Manager.

The ability to use such high-quality tools directly from the repositories in the stable Debian distribution keeps our maintenance costs down, avoids vendor lock-in and gives companies like ours the ability offer these enterprise-level clustering solutions to small and medium size businesses for reasonable prices.

By Laird Hariu

Introduction

You have probably heard of high availability transaction processing servers.  You have most likely read about the sophisticated systems used by the airlines to sell tickets online.  They have to be non-stop because downtime translates to lost orders and revenue.  In this article I will discuss the economics of using non-stop technologies for everyday applications.  I will show that even ordinary file sharing applications can benefit from inexpensive Linux based Pacemaker clustering technology.

Availability Goal

What is our availability goal?  Our goal should be to take prudent and cost effective measures to reduce computer downtime to nil in the required service window.  I’m not talking about 99.999 % (five 9s) up time.  This is the popular (and very expensive) claim made by high availability vendors.  I’m talking about maintaining enough up time to service the application.  Take a simple example, for office document preparation the service time window is office hours (9-5).  The rest of the time the desktop PCs can be turned off, nobody is there to operate them anyway.  You only need the PCs for 5 days a week for 8 hours a day or for 2080 hours per desktop PC per year.  This translates into an up time requirement of 24 percent.   Ideally you want the desktop PCs to be available all the time during office hours but are willing to give up availability for routine maintenance and for the infrequent breakdowns that may occur only once per workstation every five years or so.  Perhaps you have two spare desktop PC workstations for every 100.  This extra capacity allows your office workers to resume their work on a spare while their workstation is being repaired.  In this example the cost of maintaining adequate availability is the cost of maintaining two spare desktop PCs.  You might adjust this cost to account for real world conditions at the work site.  Wide swings in operating temperature or poor quality electricity supply, might dictate that you increase the number of spare PCs.  Sounds like a low stress, straightforward availability solution.

Network Effects

The problem gets more complicated when the desktop PCs are networked together and all the documents are stored on a central file server rather than on each workstation’s hard drive.  There is a multiplicative effect.  If the file server is not available then all 100 document processing PCs are rendered unavailable.  Then you have 98 (remember the 2 spares from above) workers being paid but not producing documents.  A failure during office hours can become expensive.  One hour of downtime can cost as much as $1500 in lost worker wages.  A day of downtime can cost $12,000 of lost worker wages.  How long will it take for a hardware repair person to travel to your site?  How long will it take for spare parts to arrive?  How long will it take the repair person to replace the parts?  How long will it take for damaged files to be replaced from backup by your own people?  A serious but not unlikely failure can take several days to be completely resolved.  Its not unreasonable to assume that such a $24,000 failure can occur once every 5 years.  This is a very simple example.  We are not talking about a complicated order-entry or inventory control system.  We are talking about 98 office workers saving files to a central file share so that they can be indexed and backed up.

The Effects of Time

I’m going to add another wrinkle to our office document processing example.  This file sharing setup has been in use for 4 years.  Time flies.  The hardware is getting old faster than you realize.  Old hardware is more likely to fail.  It has been through more thunderstorms, more A/C breakdowns, people knocking the server by accident and all that.  You’ve been noticing that your hardware maintenance plan is costing more every year.  How long is the hardware vendor going to stock spare parts for your obsolete office equipment?  Please forgive me for playing on your paranoia but the real world can be rude.

Time for an Upgrade

In this scenario you conclude that you are going to have to replace that file server soon.  Its going to be a pain to migrate all the files to a new unit.  I am going to have to upgrade to a new version of Windows server.  How much is that going to cost?  How much has Windows changed?  If I am going to have to go to all this trouble, why not get some new improvement out of it.  I know I can get bigger disks and more RAM (random access memory) for less money than I paid for the old server.  Whoops.  Windows is going to cost more.  I have to pay a charge for every workstation attached to it.  That CAL (Client Access License) price has gone up.  I read something about high availability clustering in Windows.  Enterprise Server does that.  Wow.  Look at the price of that!  Remember that $12,000 per day of downtime cost overhang?  It’s more of an issue now that you are dealing with an old system.

A Debian Cluster Solution

Enough of this already.  Since I asked so many questions and raised so many doubts, I owe you, the reader, some answers.  Debian Linux provides a very nice high availability solution for file servers.  You need two servers with directly attached storage and also a third little server that can be little more than a glorified workstation.  You need Debian Squeeze 64 bit edition that has the Pacemaker, Corosync, drbd and Samba packages installed for each server.  The software is free.  You pay for the hardware and a trustworthy Linux consultant who can set everything up for you.  What you get is a fully redundant quorum cluster with fully redundant storage, multiple CPU cores on each node, much more RAM than you had before and much more storage capacity.

Here are hardware price estimates:

Each file server node has software RAID 5 and each node holds 14 terabytes of disk storage.  Because it is completely redundant across nodes, total cluster storage capacity is 14 terabytes.  Performance of this unit will be much better than the old unit.  It effectively has 4 CPUs per file storage node and much more RAM for file buffering.  Software updates from Debian are free.  You just need someone to apply the security patches and version upgrades.

The best feature is complete redundancy for file processing.  In our file server example, any one of the nodes can completely fail and file server processing will continue.  Based on the lost labor time cost estimates above, this system pays for itself if it eliminates 1 day of downtime in a five year period.  You also have hardware maintenance savings of whatever the yearly charge is for your old system times 3 years because you get 3 years of warranty coverage on the new hardware.  You have the consultant’s charges for converting to the new system, but remember, you were going to have to pay that fee for a new Windows system as well.

Conclusion

I hope I have stirred your interest in Linux Pacemaker based clusters.  I have shown a file server upgrade that pays for itself by reducing downtime.  You also upgrade your file server’s performance while reducing out of pocket expenses for software and hardware maintenance.  Not a bad deal.

By Elizabeth Krumbach

There are dozens of virtualization products on the market. When we launched our first high-availability cluster in early 2008 we chose Xen due to it’s ability to run on non-virtualized hardware, support in Debian 4.0 (Etch) and general flexibility. We’ve learned a lot about the upstream of Xen, including the challenges that Debian maintainers face, and we were increasingly drawn to another free and open source virtualization technology, Kernel Based Virtual Machine (KVM). The primary downside to KVM is that it requires special CPU hardware support to run, but this hardware support is now almost ubiquitous on modern servers. KVM has the advantage of being supported upstream in the Linux kernel itself, removing the onus of difficult kernel patching from the Debian Developers and has become the supported virtualization option for Ubuntu, Fedora and Red Hat. Additionally, KVM allows the guests to run unaltered, meaning you don’t need a special kernel and can run many OSes, from Linux to FreeBSD to Windows 7.

We still work on a number of machines which lack hardware virtualization support, but as our customers upgrade hardware we’ve begun moving production virtual machines from Xen to KVM. In tackling the migrations of these production virtual machines we encountered several challenges, the major ones being:

• In Xen, partitions were created in separate logical volumes on the host and mounted by Xen itself and as a result we didn’t require Logical Volume Manager (LVM) within our Xen guests, in KVM the logical volume on the host for a virtual machine is a single disk image, not separate partitions.
• In Xen, the kernel package is not installed, in KVM it is required
• In Xen, you don’t have an independent bootloader on the OS, in KVM you need one to boot

The first step was to create a partition table on the new KVM image which is identical to the one on Xen. We wanted to use LVM within the guest, which required a Matryoshka (Russian) doll approach. First we’d create a volume group on the host to give us the typical flexibility of LVM host-side, and then we’d create one on the disk image giving us the flexibility required within the guest itself to expand any partitions. Finally we’d need to bootstrap the new system and copy the files over.

One way to go about all of this is a manual process. This solution would allow for scripting the procedure but requires a significant investment of time to get everything right so there is the least amount of down time possible. Since we only have a half dozen of these VMs to migrate in total, we looked for some way which already existed for handling all these steps in a familiar way, which is when we looked to the Debian Installer. Assuming a local mirror of core Debian packages (recommended), we could install a skeleton system on a test IP address which was properly partitioned, had LVM configured and bootstrapped in less than 20 minutes. We could then take this skeleton system that we’re sure is an functioning, bootable system and move the files over from the Xen installs, arguably decreasing our risk with each migration and the downtime required.

To get started, you’ll first need to calculate the total size of the current VM and lvcreate a slice of that size on the KVM system, then launch the Debian installer against the image using virt-install, something like:

virt-install --connect qemu:///system -n guest.example.com -r 768 -f /dev/mapper/VolumeGroup-guest.example.com -s 12 -c /var/lib/libvirt/images/debian-506-i386-netinst.iso --vnc --noautoconsole --accelerate --os-type linux --os-variant debianLenny

In this example we assume the debian-506-i386-netinst.iso is in /var/lib/libvirt/images/ and we want 768M of RAM, the information you put here is similar to the information that you would have previously defined in your /etc/xen/guest.example.com.cfg file for Xen.

Then use virt-manager to connect (we actually connected from a remote desktop running virt-manager) to the running install session (the standard Debian installer does not provide serial access) and install Debian, you will need the root password to launch the installer. Proceed to install.

When you get to the step to partition the disks, lay out the partition table to be identical to the VM you want to migrate to it except put it on LVM, put /boot on a separate partition outside of LVM. Complete the install, including installing grub.

Confirm that the system will boot and works on a test IP address and make a copy of your /etc/fstab to the host system, you’ll need this later.

You now have a skeleton install of Debian which runs on KVM with the LVM partitions you need.

To begin the actual data migration, you’ll want to mount the volumes within your new KVM disk, this can be done with the help of a great little mapping tool called kpartx. To map and activate the volume group on the guest, following these steps:

kpartx -av /dev/VolumeGroup/guest.example.com
vgscan
vgchange -ay guest

In this example we assume the Linux Volume on the host is called “guest.example.com” and the Linux Volume within the guest is called “guest”.

Now that the host can see the Volume Group on the guest, and all the Volumes in /dev/mapper/ you’ll want to mount them.

Once mounted, you’ll be able to start your rsync. To incur the least amount of downtime, you’ll want to rsync the large data partitions (like /srv, /home, /var, perhaps /usr) while your production host is still running. Note: All rsyncs completed during this process must be done with the “–numeric-ids” option so the permissions are not inherited from the host!

While you’re rsyncing the data, you’ll want to go into your Xen system and install the following packages (installing these will no impact your Xen system, it doesn’t strictly use them so it will simply ignore them):

• linux-image-2.6-686
• grub
• lvm2

When you’ve completed moving the largest portions of your Xen guest, bring down the production Xen guest (downtime starts now!) and mount the filesystems. And begin rsyncing the data over (preferably over a crossover cable for the fastest transfer, remember to use –numeric-ids in your rsync).

Once the rsync is complete, edit the following files on the KVM host:

• /mnt/guest/etc/fstab – use the version you saved to the host in a previous step
• /mnt/guest/etc/inittab – uncomment the ttyS0 line to allow for serial access from the KVM host for virsh
• /mnt/guest/etc/udev/rules.d/70-persistent-net.rules – comment out eth0 line so eth0 can come up on the new KVM MAC address

Unmount the filesystems on both sides and on the KVM side disable the volume group and use kpartx again to unmap the filesystem from the host:

sudo vgchange -an guest
sudo kpartx -dv /dev/VolumeGroup/guest.example.com

You’re now ready to boot the VM on the KVM side. This can be done with virt-manager or virsh.

Since you just moved the machines to a new server, and probably new MAC addresses, you will probably need to run the arping command to reclaim the IP address of the VM and all its service IP addresses.

Some things to confirm are working:

• Networking (confirm there are no lingering arp caching issues)
• Email (where applicable, confirm system messages etc are being sent)
• All services running (confirm key services, review monitoring dashboard, log in via ssh)
• Confirm that you have contiguous logs

Now that we’ve completed one of these migrations we have a lot of ideas about how to improve the process, including the possibility of making the whole process more scriptable, but this quick method leveraging the Debian installer for easier disk configuration and bootstrapping worked very well.

By Elizabeth Krumbach

Virtualization provides the facility to run multiple isolated computer operating systems on one piece of computing hardware. There has been a huge increase of interest in virtualization technology because recent advances in multi-core technology provide significantly more computing power in each machine with ever decreasing costs. Virtualization is one of the best ways to take advantage of these big changes in hardware.

Currently, Xen is the most mature FOSS (Free and Open Source Software) virtualization technology. Although we love the idea of KVM, since it requires a special processor extension on X86 systems, it cannot work on older hardware. So for at least another few years, we think Xen is the more flexible choice for FOSS virtualization projects.

The Xen infrastructure consists of the Xen hypervisor which “runs the show”, a domain 0 (dom0) which runs a special, privileged version of the operating system (typically Linux, but NetBSD and Solaris are also supported), and one or more domain U (domU) “guest” (or “User”) operating systems. We have found that Xen is easy to configure in many situations, but we encountered some complications in running a domU on a dom0 with a different architecture.

We recently migrated some 32-bit domUs running Debian Etch (4.0) from a 32-bit dom0 to a newer 64-bit dom0 running Debian Lenny (5.0). We did a direct move (using rsync) of the Logical Volume Manager (LVM) slices from the 32-bit dom0 to the 64-bit dom0. This means we’d now be running our 32-bit Etch domUs on a 64-bit Lenny dom0.

The first question was whether this would be possible. Absolutely! 32-bit domUs have no trouble running on 64-bit dom0s, we could even use the 64-bit Xen kernel in these 32-bit systems to avoid additional kernel installations we’d need to maintain on the dom0. The second question was whether we could properly load the 64-bit kernel modules inside our domU. Again, yes! But with a caveat: the domUs were 32-bit Etch, so the 64-bit Lenny kernel modules were not simply installable via apt. We realized that copying over the .deb package for the kernel modules and running dpkg -i --force-architecture linux-modules-2.6.26...deb would not be a maintainable way to handle the kernel module updates moving forward. So we weighed our options:

1. Serve these modules via a network file system (such as NFS) to each domU on bootup.

2. Deploy a script that would notify the domU and copy the new kernel modules .deb to it for installation. We could then install the new module package at our discretion.

We decided that the first option violated our strict security policy which calls for running as few services on the dom0 as possible. Since the second solution is scriptable and therefore automatable, it fit our vision of having easily maintainable systems regardless of the underlying complexity. So we installed the 64-bit modules prior to migration so that all the proper modules would be loaded as soon as we brought up the domU on the new Dom0. The result was a flawless migration of our 32-bit domUs to the new 64-bit dom0.