by Pedro M. S. Oliveira | Jul 13, 2010 | Linux
Linux is not Windows and if reboot fail you usually still connect by SSH and do something. This commands will show you how to remotely hard reboot machine.
Hard reboot mean that shutdown scripts will not run and machine reboot immediately without syncing hard disk drives, shutdown applications etc, it’s like hitting the reset button on your server.
echo 1 > /proc/sys/kernel/sysrq
echo b > /proc/sysrq-trigger
This commands enable sysrq and after this calls fast reboot. If you want to force shutdown machine try this.
echo 1 > /proc/sys/kernel/sysrq
echo o > /proc/sysrq-trigger
This came handy, when I had a server that had some IO error and it can no longer read from disk, only few cached binaries into memory kept it running (kernel, SSHD, bash), I could still access the machine via SSH but can no longer do anything, forcing the reboot as mentioned above was my only resort, and it worked like charm…
by Pedro M. S. Oliveira | Aug 21, 2009 | Linux
Sometimes things go wrong. We simply can’t avoid it, a simple power failure can harm your data and corrupt your system.
One of these day in a normal work day one small server I maintain add an hard disk failure (yes it’s true, it happened again for the 3rd time this month). In this system I don’t have a RAID setup so the data was lost, well no prob I thought in the end all day is on my backuppc server.
BackupPC is one of my favorite tools, it’s great to manage, easy and very flexible, I’m not going the write about using backuppc to backup data as there are plenty of docs and mailing lists out there that can give you excellent how to(s) on the subject.
Booted with OpenSuSE 11.1 DVD and selected rescue mode.
On the command prompt and using fdisk /dev/sda I partitioned the drive like the old one (both drives were sata II), but Linux is so flexible that you don’t even need to do that.
Usually I like to use a volume manager (lvm) but I was short on time and will so just created 3 partitions /sda1 (150MBfor /boot), sda2 (4GB for swap ) and sda3(100GB for /), leaving unpartitioned the rest (400GB), I’ll be using the free space to create volumes afterwards and then move the data there.
Then formated the partitions:
After this I mounted the filesystems like this:
mount /dev/sda3 /mnt
created boot in /mnt – mkdir /mnt/boot
mount /dev/sda1 /mnt/boot
so now we need to get all the data in the file system… and this is the tricky part we need a ssh server to do this (we can use nfs or http download and then untar, but I still like sshd method better, it uses rsync so the transfer is really fast.)
To do this you need to set up a ssh server from a minimalistic boot system. This isn’t hard just follow the steps:
First give this machine your old ip address ex.: ifconfig eth0 126.96.36.199
Create sshd certificate, remember this certificate is just temporary so you can restore your backup. You may delete it afterwards. To create it just type:
ssh-keygen -t rsa -f /mnt/ssh_host_rsa_key -N “”
start sshd by typing:
/usr/sbin/sshd -h /mnt/ssh_host_rsa_key
This will start up sshd with all the default options.
Now just give a password to your user root or you won’t be able to login:
Add the backuppc public ssh key from the backup server to /root/.ssh/authorized_keys on the restore machine.
Finally accept host key on the backuppc key (you may do this by entering on the backuppc server and access the restore machine, it will ask you to had the machine key. Just accept it.) Then copy it to the backuppc user know hosts file ex.:
tail -n ~/.ssh/known_hosts >> ~backuppc/.ssh/known_hosts
Finally your done. If you find this large and complicated don’t think it’s like that, by now you may have configured and entire ssh daemon by hand.
Go to the BackupPC console, choose your host, select the backup you want and just press restore.
On the method choose rsync but on the destination dir choose /mnt. Go out and take a coffe the restore can take a while. After it’s done all you need is to reconfigure grub and maybe /etc/fstab.
Now that the restore is done just check if /mnt/etc/fstab reflects the partition scheme, change accordantly if it doesn’t.
Finally we need to setup grub edit /mnt/boot/grub/menu.lst and check if your root partition is on the right place.
Before you can run grub-install you need to mount 2 special partitions /dev and /proc, how do you do this on a mounted and running system? The answer:
mkdir /mnt/proc; mkdir /mnt/dev; mkdir /dev/sys
mount -o bind /proc /mnt/proc
mount -o bind /dev /mnt/dev
and finally the last command:
if you got and ok just reboot your system, don’t forget to eject the dvd before system boots again.
I think this was my largest post, hope you find it useful