Write barriers have gotten some discussion in the Linux world recently which is a good thing due to a lack of documentation about what write barriers are and when they should be used. Let's first start with the assumption that a user wants file system integrity over performance. This often means turning off write caching on all disks. The reason is that file systems rely on the disks not reordering write requests in order to maintain the integrity of the file system. Of course, turning off write caching harms write performance substantially so another option was invented: write barriers.

I suppose that growing a RAID 1 array is not very common but I had a client that wanted to go from a 2 disk array to a 3 disk array so that one disk could be rotated out periodically and kept off-site. Here were the steps:

• mdadm --add /dev/mdX /dev/sdXX
  • This adds the partition to the array as a hot spare.
• mdadm --grow /dev/mdX -n 3
  • This increases the number of disks from 2 to 3.

A new kernel, 2.6.24-17, has been released for Ubuntu 8.04 (Hardy Heron) in the hardy-updates repository. This kernel may fix at least 1 xen bug (bug 218126, bug 204010) which I’m anxious to test. I’ve held off upgrading any Xen user domains until these were resolved. I think it would be a good time to at least test an upgrade, and since virtualization makes it easy to rollback I think a test is in order. I also noticed that Ubuntu 7.10 got some new kernel modules in the gutsy-updates repository. Not sure what the changes are but it’s something to be aware of.

I’ve been doing a lot of research on storage solutions for a couple of projects. I’ve been delving into RAID setups for large numbers of disks and trying to determine in what situations a hardware RAID controller is appropriate instead of Linux software RAID. I’ve also been considering various nested RAID levels and their value.

Yes Linus Torvalds announced the release of the 2.6.25 kernel today. I’ve been trying to confirm whether the TASK_KILLABLE patches have been included. It has been mentioned in the comments of the LWM article announcing the 2.6.25 release but the Kernel Newbies 2.6.25 page only lists:

“softlockup: automatically detect hung TASK_UNINTERRUPTIBLE tasks”

Honestly after reading about the other big changes this is the one that has the most effect on my work. The addition of LatencyTop support should help improve desktops running Linux and I look forward to playing with it. But most of the other changes do not address issues that I am experiencing while the TASK_KILLABLE patches most certainly do. I will have to do more research on the state of these patches; perhaps they don’t seem like a big deal to other people but I’m surprised that such an important change wouldn’t be mentioned.

I really do think that ip is a big improvement from ifconfig, route, ifup, ifdown, and the other commands it replaces. Still I seem to have trouble remembering which commands you need to use the word “dev” and which ones you don’t and similar minor semantic issues (like parameter ordering). So I decided to try and compile a list of commands I commonly use to be used as a reference for studying.

I just did some reading on unkillable processes. I've seen this happen a few times before and wondered what caused it. I experienced it again today when using KVM. Apparently when processes go into the "D" state they cannot be killed because they are usually waiting for some IO to complete. I have not found any way to fix these processes while a system is running though I continue to investigate. I did find that the 2.6.25 Linux kernel will add a new TASK_KILLABLE state to help alleviate this problem. Hopefully software including KVM will be quickly updated to take advantage of this new state which functions just like TASK_UNINTERRUPTIBLE but allows the process to be killed.

I noticed that I had an field in top labeled "st" and that it was busy once I switched some Xen domains to be file backed (tap:aio) instead of LVM backed (phy). Anyway, after doing some investigation I found that "st" means "steal_time" and is "the cpu time spent in involuntary wait." This is time spent executing other virtual domains in Xen or other virtualization applications. I couldn't find the details when searching for information about the top command (top being one of those difficult search terms). But since top gets its values from /proc/stat I was able to use that to aid my research. I ended up finding the details at http://www.linuxhowtos.org/System/procstat.htm and http://kerneltrap.org/node/8235.

I was noticing that kfreebsd-amd64 and kfreebsd-i386 are now architectures in the Debian unstable repositories. I’ve always been interested in running a GNU operating system with an alternative kernel. Of course, so far nothing has been stable enough to warrant leaving Linux. And besides, Linux does have the best mascot of all the kernels. The Linux kernel is also largely responsible for the rapid growth of the free software movement. Still, the idea is appealing and here’s where things stand with other kernels.

During some recent testing of traffic shaping I found that ingress traffic policing was important to maintaining low latency for high priority traffic such as VOIP. Unfortunately the traffic policy abilities of the Linux kernel are crude and limited to simply dropping packets over a given rate for specified conditions. This keeps latency low but requires the dropped packets to be retransmitted thus wasting bandwidth. IMQ has been one way of addressing it. Anyway I noticed that new IMQ patches were available for more recent Linux kernels and iptables versions. However, at the moment I’m more interested in trying the successor to IMQ, IFB (Intermediate Functional Block), which is supported in Ubuntu 7.10 (Gutsy Gibbon).

New kernel releases have been released by both the Ubuntu and Debian teams. Both address CVE-2008-0600 which could allow an attacker to escalate their privileges. The Debian vserver kernels have also been updated to address CVE-2008-0163 which allows users in one vserver to access information in another. For Ubuntu this comes just a week after the last Linux kernel security update.

• DSA-1494-1 linux-2.6 — missing access checks
• USN-577-1: Linux kernel vulnerability
• USN-574-1: Linux kernel vulnerabilities
• Patching CVE-2008-0600, Local Root Exploit

Seems like if you click "Exit" in cssh it ends your entire desktop session. That doesn't seem like a good thing to me.

Also, when playing with the HFSC traffic shaper on a Debian server I managed to lock the system up 3 times (mostly when changing the maximum latency parameters). I think that indicates a pretty serious kernel bug, which is a shame because HFSC is by far the best Linux traffic shaper.

• The project is not dead.

Excellent. For the rest of the news (good and bad) read ZFS on FUSE/Linux - Status Update.