linux

Do Not Collect System Performance Data From Guest OSes

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This is post #12 in my December 2013 series about Linux Virtual Machine Performance Tuning. For more, please see the tag “Linux VM Performance Tuning.” Fans of the 12th Doctor Who have often heard the phrase “the Doctor lies.” The explanation for his lies is that, because he skips around in time, he knows things that others cannot know yet. Hypervisors are like that, too. Guest OSes don’t know that they aren’t the only OS on the hardware, and the hypervisor lies to them about things like CPUs, RAM, and things like system timers because, like the Doctor, the hypervisor is skipping a VM forward in time. And that’s the rub – only the hypervisor knows what the truth is. …

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Adjust vm.swappiness to Avoid Unneeded Disk I/O

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This is post #11 in my December 2013 series about Linux Virtual Machine Performance Tuning. For more, please see the tag “Linux VM Performance Tuning.” The Linux kernel has quite a number of tunable options in it. One of those is vm.swappiness, a parameter that helps guide the kernel in making decisions about memory. “vm” in this case means “virtual memory,” which doesn’t mean memory allocated by a hypervisor but refers to the addressing scheme the Linux kernel uses to handle memory. Even on a physical host you have “virtual memory” within the OS. Memory on a Linux box is used for a number of different things. One way it is used is internally for buffers for things like network …

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Zero Out Free Space

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This is post #10 in my December 2013 series about Linux Virtual Machine Performance Tuning. For more, please see the tag “Linux VM Performance Tuning.” When we talked about the rationale behind storing logs centrally one big reason was thin-provisioned virtual disks. Those disks grow over time because filesystems on a virtual machine currently have no way to tell the underlying storage that they’re done using certain blocks on disk. There is a way to make these VMs thin again, and I wrote about it as step 9 in my guide to preparing Linux Template VMs. In short, we run a script on the VM that writes zeroes to most of the free space on the VM: #!/bin/sh # Determine …

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Disk Partition Alignment Is Still Important

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This is post #9 in my December 2013 series about Linux Virtual Machine Performance Tuning. For more, please see the tag “Linux VM Performance Tuning.” I have written about this almost yearly (beginning all the way back in 2006), but even now I routinely run across something, like a virtual appliance, that has poor partition alignment. What’s the big deal? In short, misaligned I/O is killing your disk performance. Blame Logical Block Addressing, or LBA. Back in the day, a BIOS interacted with drives by knowing the exact geometry of the drive, namely how many cylinders, heads, and sectors were on a disk (CHS). Unfortunately that limited the size of the drives that could be used, and ignored some basic …

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Store Logs Centrally

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This is post #7 in my December 2013 series about Linux Virtual Machine Performance Tuning. For more, please see the tag “Linux VM Performance Tuning.” It is universally true that over time a thin-provisioned virtual machine will slowly expand on disk. This happens because of the way files are deleted from disks. When files are written to disk the storage subsystem writes those blocks out, and the thinly-provisioned disk file grows a bit. But when a filesystem deletes a file all it does is update its own internal storage map to “forget” about the file. Nothing ever removes the data from the disk itself (which is why undelete utilities work, and why there are stringent procedures for securely erasing a …

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Use elevator=noop For Linux Virtual Machines

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This is post #6 in my December 2013 series about Linux Virtual Machine Performance Tuning. For more, please see the tag “Linux VM Performance Tuning.” Modern operating systems are fairly modular, and often have different modules to deal with memory, network I/O, and CPU scheduling. Disk I/O is no exception under Linux, as there are usually four different schedulers a sysadmin can choose from. Red Hat had a nice write-up on these a few years back and it remains relevant today: The Completely Fair Queuing (CFQ) scheduler is the default algorithm in Red Hat Enterprise Linux 4. As the name implies, CFQ maintains a scalable per-process I/O queue and attempts to distribute the available I/O bandwidth equally among all I/O …

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One Workload Per Virtual Machine

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This is post #2 in my December 2013 series about Linux Virtual Machine Performance Tuning. For more, please see the tag “Linux VM Performance Tuning.” Back when the computing world was built on bare metal we often tried to squeeze more than one workload on a physical machine. We did this to save money. Servers are expensive so we wanted to make the most of each one. What we didn’t account for was how complicated things could get. Applications fought with each other over system libraries and DLLs. Security was complicated. And, most relevant to this series, performance tuning became this multivariate dance involving resource limits and other arcane system witchcraft. In the end we saved a little money on …

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For The Best Performance Use a Recent Operating System and a Recent Hypervisor

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This is post #1 in my December 2013 series about Linux Virtual Machine Performance Tuning. For more, please see the tag “Linux VM Performance Tuning.” Just like in construction, performance tuning is most effective when you have a good foundation. If the underpinnings of your efforts are weak you won’t be able to build a skyscraper, just a small office building. While that’s still better than nothing, there is often a lot to be gained by using the newest versions of your OS & hypervisor of choice. Some quick examples: Windows Server 2012 R2 added Virtual Receive-side Scaling, which allows the load from network traffic to be processed by multiple virtual CPUs. VMware vSphere 5.5 added support for latency-sensitive VMs, …

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VMware vCenter Server Appliance & NTP

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If you’re trying to configure NTP on the VMware vCenter Server Appliance (vCSA) 5.1 builds 799730, 880472, or 947940 according to the official documentation you might be seeing what I’m seeing: vcenter:~ # yast2 ntp-client add server=0.us.pool.ntp.org Error: Cannot update the dynamic configuration policy. vcenter:~ # yast2 ntp-client enable Error: Cannot update the dynamic configuration policy. This appears to be a SuSE bug. Seems serious but it isn’t, the commands actually do complete correctly. If you want to check the work just use the command: cat /etc/ntp.conf to check for lines starting with “server” near the bottom. /sbin/chkconfig ntp on will enable the service at boot, and /etc/rc.d/ntp start will start it immediately if it isn’t started. /usr/sbin/ntpq -p will …

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Preparing Linux Template VMs

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Dan over at Bashing Linux has a good post on what he does to prep his template VMs for use with Puppet. He’s inspired me to share how I prepare my Linux VMs to become a template. He’s got a few steps I don’t have, mainly to prep for Puppet, and I have a few steps he doesn’t have. One big difference is that I don’t prepare my template images for a particular configuration management system, but instead bootstrap them once they’re deployed. Why? I use my templates for a variety of things, and sometimes the people who end up with the VMs don’t want my management systems on them. It also means I have to handle some of what …

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