Bits and Chaos


Between bits and chaos, a sysadmin stands.

Red Hat acquires Kumranet

A terrific news.

After the acquisition of Xen by Citrix, another virtualization start-up become a part of a global player.

Kumranet develops KVM, which is still in its infantry but promises a lot more than Xen due to its tight kernel integration (see here for a more detailed explanation) and has also a desktop virtualization solution, that is clearly headed toward Citrix Metaframe.

So, they have the operating system, the in-kernel hypervisor, they just need a datacenter  virtualization-aware resource manager.

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NYSE will move to Linux

NY Times reports that the New York Stock Exchange is heavily investing in Linux. Tech speaking:

  • they prefer Linux over IBM-AIX and HP-UX, and they have HP hardware (Opteron blades and Integrity);

  • they will use HP OpenView for monitoring;

  • they are already using Solaris on some systems, but prefer to move to a more open system;

  • they do not like virtualization, as it introduces not negligible latency.

Using Linux (in the Red Hat or SuSE flavor, I guess. Note that Red Hat is listed on NYSE) will be of big impact to financial institutions world-wide, as the volume of data and money processed by the NYSE is simply enormous: NYSE is saying that Linux is mature enough although is not so polyshed as other Unix dialects. Some feature are missing (as an example, backup of an entire system, a la mksysb in IBM AIX) but these things can usually be managed at the infrastructure level: if you system is completely redundant, you can put offline small portions of it and do the planned maintenance tasks. It will cost money for the redundancy (which is possible you would pay in every case) but it’s entirely possibile that it would be cheaper than paying the high licenses for other unixes, especially when, in the long term you will found yourself vendor locked-in.

Latency in virtualization is a big problem, best known as os interference (see this good article from IBM) that will severely impact performances for HPC clusters and could be desruptive for financial trading systems, where you receive hundreds of different data feeds per second, and you should decide what to buy or sell according to some heuristics before your competitors. It will be interesting to see what the Red Hat MRG platform will do in this field. As usual, we are at the beginning of the open source world.

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How to manipulate the files stored inside a Xen virtual machine

Assume you have created a Xen virtual machine, and its data are stored on a file. You might want to access the files stored inside the machine (even when the machine is not running) for one reason or another:

  • inspect them;
  • change some of them, as an example you copy the virtual machine imagine file to another physical host and you need to change some settings (e.g. network-related) in order to fit the virtual machine for the new host;
  • doing some post-mortem analysis and/or recovery;

If you search on the net, you may find this document from IBM that will give you some hints, assuming that on the virtual machine you don’t use LVM. I will move a step further, with the the procedure for dealing with virtual machine image file containing a LVM partition.

For the rest of the discussion, I assume you know what a loop backed file system is and how LVM works. Instead of saying “virtual machine file”, i.e. a file on the host that is the disk of the virtual machine, I would say “virtual disk”.


1. Be sure the virtual machine it’s shutted down

This is required to avoid metadata corruption.


2. Use losetup to associate the virtual disk to a block device

$ losetup -d /dev/loop0 virtual-disk

3. Use fdisk to get information about the block device

$ fdisk -l -u /dev/loop0

Disk /dev/loop0: 8388 MB, 8388608000 bytes
255 heads, 63 sectors/track, 1019 cylinders, total 16384000 sectors
Units = sectors of 1 * 512 = 512 bytes

      Device Boot      Start         End      Blocks   Id  System
/dev/loop0p1   *          63      208844      104391   83  Linux
/dev/loop0p2          208845    16370234     8080695   8e  Linux LVM

This shows that the virtual machine has a tiny initial partition (probably the /boot) and a LVM partition.

4. Use losetup, again, to associate a specific partition of the virtual disk to a specific block device.

If you want the /boot partition:

$ losetup -o $((512*63)) /dev/loop1 /dev/loop0

$ mount /dev/loop1 /mnt

the magic here is the -o parameter, which defines the offset from the beginning of /dev/loop0: 63 sectors, as seen above from the fdisk output. If you want the files in the LVM partition, see next point.

5. Accessing LVM inside a loop device.

$ mount -o $$(208845*512) /dev/loop2 /dev/loop0

To scan the volume groups and the logical volumes, you need to tell to LVM that the /dev/loop<n> devices may contain LVM data. So, edit /etc/lvm/lvm.conf, find the line that defines the “types” parameter:

# types = [ “fd”, 16]

it’s a comment, showing the default value. Immediately after, put this line:
types = [ “fd”, 16, “loop”, 1]

and see what happens:

$ vgscan

  ACTIVE            '/dev/system/root' [14.62 GB] inherit
  ACTIVE            '/dev/system/home' [97.66 GB] inherit
  ACTIVE            '/dev/system/tmp' [512.00 MB] inherit
  ACTIVE            '/dev/system/swap' [4.00 GB] inherit
  inactive          '/dev/VolGroup00/LogVol00' [5.75 GB] inherit
  inactive          '/dev/VolGroup00/LogVol01' [1.94 GB] inherit

So we eventually can access the LogVol00 which contains data (someone tells me that LogVol01 is a swap partition):

$ vgchange -a y

 4 logical volume(s) in volume group "system" now active
 2 logical volume(s) in volume group "VolGroup00" now active

$ mount /dev/VolGroup00/LogVol01 /mnt/

Now the files of the virtual machine are accessible from the /mnt mount point.

6. Unmount all

$ umount /mnt
$ vgchange –activate n VolGroup00
$ losetup -d /dev/loop2
$ losetup -d /dev/loop1
$ losetup -d /dev/loop0

Remove the type parameter from /etc/lvm/lvm.conf if you feel you won’t need it anymore (if it’s present it does not hurt, though).

7. Some notes

  • Remember that the LVM namespace is unique across the system, so your virtual machine must use volume group name and logical volume name different from the ones of the physical host;
  • If you need to change the UUID of the virtual disk, see the from the Xen distribution for some hints.
  • A different approach, based on kpartx, is available in the Fedora Wiki.

Filed under: virtualization, ,

Remotize your desktop: NX

Sometimes, you are so lucky to encounter a software so great that you realize that the future is already here.

This is one of my all time favourite software, the NX server from Nomachine. It’s basically a secure terminal server for Linux, with an amazing speed: remotely work over an ADSL line and you barely notice the difference between the local desktop and the remote desktop.

From time to time, the experienced system administrator may find him/herself in the necessity of using a graphical tool on a remote server (the junior administrator, instead, always wants to use a graphical tool for system administration). The first way to get this is using the ssh-X command, but the speed of the remote desktop is quite low: the X Window System was designed for LAN environments, whilst in a WAN scenario round trip times are no longer negligible.

The very experienced system administrator could bypass this problem by using VNC, tunnelling it over a secure connection, and this could give a speed gain. This require some work: installing the vnc server on the remote server, securing it, definining a X startup script, creating an SSH tunnel.

This is where NX comes in place. You can install it on the server (you need the NX client, NX server and NX node pieces, they are packaged for all the most important distributions), and then by using the NX client on your client you have:

  • near local speed;
  • encryption of all traffic;
  • session suspending and resuming.

the latter being a real advantage over the traditional ssh -X solution. NX is release free as in beer, the free server can accept a maximum of two user at the same time, if you need more power and want advanced features like server statistics and node balancing you can buy the premium versions.

NX is in a market niche where the behemoth is Citrix Metaframe. To gain market share, they were smart enough to release the NX libraries with the GPL license. A completely GPL’ed server is being developed by the FreeNX project, I played with it some months ago and I found that it lacked some functionalities (as an example, there was problem with a Microsoft Terminal Server client running inside a remote desktop session), but things may have changed, so you can get it a try.

There’s only one thing to check for after the installation of the NX server: if you have a iptables firewall on the remote server with rules that governs incoming SSH traffic, be sure to add a rule like this one:

iptables -I INPUT 1 -p tcp –source –dport 22 -j ACCEPT

this is required because NX does a proxy authentication with the remote SSH server. If you omit this rule, the NX connection will stall at “Downloading session information” phase.

With such a tool, I was able to administer a scientific cluster working from home, installing software that requires a graphical screen (like Matlab). I’m projecting to install a Network Management Server like Nagios or OpenNMS and to make it listening for incoming HTTP requests only from I will connect to the remote machine via NX and then issue a remote Firefox browser to reach the local web server. With such scheme, I can connect to the remote machine using a certificate or port knocking or both, and even if the Network Management Server would experience a security flaw, it won’t be listening on the Internet.

Congratulations Nomachine! And thanks for bringing us this great tool.

Filed under: virtualization, ,

VMware Server 2.0 beta: first impressions

I’ve eagerly downloaded and installed VMware Server 2.0 beta to test it, over my Fedora Core 6 (yes, when it comes to operating systems I’m a bit conservative). The machine is a AMD 64 3000+, with 1 GB RAM, that I’ve successfully used with the VMware 1.0.x series.

Competition in the arena of virtualization is today mostly on the management and support tools, because as the hardware support increases, the relative differences between hypervisors (being there binary translator a la VMware or para-virtualizer a la Xen) will be of less importance. AMD has the nested paging feature on the Barcelona series resulting in a lowering of TLB miss count. Intel has concentrated its efforts to obtain an efficient DMA mapping: the Intel VT-d technology will reduce the perfomance hits that a guest machine experiences every time it performs a I/O operation. These technologies will be first deployed by high-end consumers, but sooner or later they will land over final user desktop, so someday we’ll play all the Windows games over a Linux machine 🙂

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