Backups: Slow and Painful

So in my previous post I showed you how to do a a file copy and I mentioned a bit by bit copy. File copies are cool because they copy only the files. Bit by bit copies copies all of the data and includes the zeros. So if you are backing up a 40 gig drive that has 8 gigs of data a file copy will copy the 8 gigs of files where as a bit by bit copy will copy all 40 gigs. Also this means that the output file will be the size of the source drive.

That’s some of the down sides, but here are some good points. A bit by bit copy can copy things like your file system, your master boot record and your partition table. In some cases, you might find it faster to do a bit by bit copy because a file copy must insert files in to the destination file system. On some systems with millions

or more small files a file copy will be much slower. You can also set the destination to another drive and produce an identical copy of the original without having to partition, format, and then copy the data.

That said here is how you can make a quick backup of your system. I will assume your machine uses sda as its root drive. Please keep in mind that your source drive and your destination file must not

be on your source drive.

~ # dd if=/dev/sda of=/mnt/backups/sda.dd

Once this is complete the /mnt/backups/sda.dd file should

be an exact copy of the source drive. It is also possibly to target specific partitions of a drive by setting the if=/dev/sda to if=/dev/sdaX, where X is the partition number of your drive.

Backups: Quick and Dirty

During my career as a systems administrator I’ve learned two basic ways to perform backups. There are basically two different ways to backup a system; file level backups are where you specifically backup the files from the system. The other

way is a bit by bit copy of the storage device. Remember it is important to either be in single user mode or boot to a live CD.

A file copy:

~ # df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda4 145G 100G 39G 73% /
udev 10M 192K 9.9M 2% /dev
shm 1.3G 12K 1.3G 1% /dev/shm
/dev/sda1 38M 25M 12M 68% /boot

These are the file systems I am working with. As per my previous post, it is important to note that traditional copying won’t get everything. We will use the method I mentioned in my previous post to get all the data. We’ll also be backing up to /mnt/backups. For backups, I’m going to use the rsync command. It is pretty powerful and it only copies files that don’t already exist at the destination.

miar mnt # mount -obind / /mnt/root

Once the root file system is mounted on /mnt/root you can back

that up to /mnt/backups. I’ve chosen to put it in a sub folder called root.

miar mnt # rsync -avHSx –numeric-ids –progress /mnt/root/ /mnt/backups/root

Once this complete, and it may take some time, it will be time to go to the next file system. In my above example, /boot is the only other file system that needs to be backed up. The shm and udev are dynamic file systems and are created upon boot. They do not need to be backed up.

miar mnt # rsync -avHSx –numeric-ids –progress /boot/ /mnt/backups/boot

Once this command

finishes your backups are complete or you need to repeat the steps for /usr, /var, /local and or others.

Naming: Where Dev Became Unmounted

Can’t open ‘/dev/console’: No such file or directory

I find myself running against computer problems on a daily basis and on occasion problems I’ve run up against before. I

created an rsync of a Linux machine and got this message booting up. The symptom was it stopped displaying text after it tried to boot the Linux kernel. It eventually booted and I was able to log in. I didn’t understand why it was complaining about /dev/console missing because it was there.

Some problems are just very low level and you have to take that aproch at them. On most Linux systems /dev is mounted by some means. On modern systems it is by udev, on embedded systems it

might be mdev and on older systems it might be devfs. Some systems don’t mount dev and rely on the user to manage devices manually. If you need to copy

root while you have other file systems mounted over it, the bind option of mount will be useful for you. Here is an example.

mount -obind / /mnt/root

Now the content of your root device is located in /mnt/root without the other file systems that are overlayed on top of it. I hope this helps someone who is wondering the Internet. Similar errors are that the system can’t find random, urandom or null.