At home I have a NAS with 3 disks in a RAID 5 configuration that exposes an iSCSI volume that I use from Windows machines. I chose iSCSI so I could format it with NTFS and use advanced file system features that the NAS doesn’t support natively. Although NTFS does not support multiple clients connected to the same iSCSI volume at once, if I want multiple machines to use it simultaneously, I just connect it to one machine and share it out over Windows File Sharing / Samba / SMB.

However, although Windows 7 ships with iSCSI support, it makes it awkward to transfer it from one machine to another in the LAN, which is sometimes useful. If I’ve used the disk at all, it will inevitably decline to disconnect it with the following error message:

The session cannot be logged out since a device on that session is currently being used.

Even when I’ve closed all apps that could possibly be using the disk, the message persists. But it turns out there’s a simple way around this:

1. Open up Control Panel -> Administrative Tools -> Computer Management, and in the left pane click on Storage -> Disk Management.
2. Right-click the iSCSI disk (the disk itself, all the way to the left, not the logical partition) and choose “Offline”. This disconnects the disk. It is not the least bit deterred by applications actively using the disk (they will receive an error if they attempt to continue reading it).
3. Proceed to disconnect the iSCSI volume normally. It will succeed without any delay.

When later re-connecting the disk back to the original machine, you must return to Computer Management, right-click the disk, and choose Online before it is accessible again. Remember when connecting to an iSCSI disk to not add it to your “Favorite Targets” because this tends to lead to multiple machines accidentally connecting to the same iSCSI disk at the same time later on.

I have an Ubuntu 11.04 VM running in VMware on my Windows machine, and twice this morning after locking my computer and going away for a few minutes, I came back to discover the VM had powered off. At first I suspected a system crash, but there were no errors in the system logs.

It turns out that the way I was locking my computer was to press CTRL+ALT+DEL, then press ENTER to choose “Lock this computer”. In VMware 7.1.4, the CTRL+ALT+DEL keypress is also sent to the VM when it has focus. In Ubuntu 11.04, this key is bound by default to the “Log out” action, which brings up the “Log out” window. If you leave the Log out window up for 60 seconds, it automatically shuts down the system.

The solution: go in Ubuntu to System->Preferences->Keyboard shortcuts, scroll down and click on Desktop->Log out, and press backspace to remove the key binding (or select another one of your choice). Another alternative is to use WINDOWS+L to lock your computer, which should be easier anyway.

The Ubuntu Linux package repository is a great resource, giving easy access to an enormous number of free applications, but on the other hand requires an Internet connection and quite a bit of bandwidth to use, especially for packages with many dependencies. This is problematic in any scenario where you’re offline (e.g. on a plane), on a low-bandwidth link, or on a link with high bandwidth cost. The best solution to this is to plan ahead: go to a place with low-cost bandwidth, like many cities in the US and Europe, and set up a local Ubuntu mirror on your local storage, as described below. If you can’t go there yourself, get a friend with a low-cost bandwidth link to put a mirror on an external storage device (a 2.5 inch external hard drive,a Bluray disc, a thumb flash drive, etc.) and drop it in the mail.

Using debmirror to produce the mirror:

1. debmirror is designed for producing mirrors of Aptitude repositories like those of Debian and Ubuntu. Here’s the command I use, which has more than I need, strictly speaking (make sure to “mkdir ubuntu-packages” first):

   debmirror –progress –state-cache-days=365 –host=source.mirror.hostname.goes.here.org –method=rsync –root=ubuntu –dist=maverick,maverick-backports,maverick-proposed,maverick-security,maverick-updates –section=main,restricted,multiverse,universe –arch=i386,amd64 ubuntu-packages/ –ignore-release-gpg

   Here are the most important options to change:

   • Replace “source.mirror.hostname.goes.here.org” with a suitable mirror host from the Mirrors of Ubuntu list. I recommend you choose one with a high-speed link near your geographical location that has “rsync” next to it. It’s okay to choose a mirror which is not up-to-date – we’ll fix that in step 2.
   • Set “–method” to one of the methods supported by your server (http, ftp, or rsync). rsync is best if supported; http and ftp work but are much slower at transferring large numbers of small files.
   • Use “–arch=i386″ to include 32-bit binary packages, “–arch=i386″ to include 64-bit binary packages, and “–arch=i386,amd64″ for both
   • Add “–nosource” if you don’t need source packages (you don’t, unless you’re planning to recompile packages from scratch).
   • In “–dist” feel free to remove maverick-backports and maverick-proposed if you’re only interested in mainline Ubuntu packages and not more cutting-edge recent packages.

   
   

   The initial download may take a few days, depending on your bandwidth. You can interrupt the download at any time with CTRL+C and resume it later by re-running the command.

   You may receive “key could not be found” errors. You can safely ignore them. If you want to get rid of them anyway, see the FILES section at the end of the debmirror man page.

2. After you eventually finish, you can modify the mirror host and set it to a different host labelled “Up to date” on the Mirrors of Ubuntu list and run the command again. This will ensure you have the most recent versions of all packages.
3. You can re-run the debmirror command later to update an existing mirror with new updates. Be warned that once you’ve downloaded a mirror, you have to return yourself or your external storage to the high bandwidth environment to update it (or else you have to download them normally) – attempting to download updates over an expensive connection will quickly prove more costly than downloading packages you need directly. This can be a downside, but having stale versions for a little while sure beats having no versions.
4. Remember, once you’ve downloaded a local repo, the bandwidth cost of giving a copy to a friend is zero! Ask anybody who might want one.

At the present time, all packages including i386 binaries, x86-64 binaries, and sources comes to about 89 gigabytes. Just including binaries for your platform will probably be about 30 GB, small enough to fit on a Bluray or 64 GB flash drive (it might fit on 32 GB but that might be cutting it a bit too close).

Configuring your installation to use the mirror (the following steps require root access):

1. Mount or copy the package mirror at some location like “/usr/packages”.
2. Make a backup of your /etc/apt/sources.list file as /etc/apt/sources.list.remote. Modify your /etc/apt/sources.list to replace every HTTP or FTP URL with “file:///usr/packages” (or whatever your local path is).
3. Do an “apt-get update”, then go ahead and use all your package manager software normally to upgrade or install packages. It will appear to skip the download step and proceed directly to installation.

Another reason you might want a local mirror is if you administer a number of Ubuntu installations, each with different package requirements. This might be several VMs on your own machine, you and your roommates’ machines at home, sharing over wifi with your neighbors, or all the machines in a small office. However, be warned that if your repository is not accessed frequently enough, the bandwidth it costs to update it will never justify the bandwidth you save by using it.