IT Tip Collection

This is a collection of problems and their solutions that I have come across in my work.

Save diskspace:
For very large disks, you can set the 5% reserve in filesystem to 1%, sometimes regaining 40GiB of diskspace from a 1TiB drive.

tune2fs -m 1 /dev/sda1

SSH WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!

ssh-keygen -R server.example.com

Create a tmpfs of certain size:

mount -t tmpfs -o size=20m tmpfs /mnt/tmp

Creating a network tarpit for a port:

iptables -A OUTPUT -m state -p tcp --dport 23515 --state NEW,ESTABLISHED,RELATED -j DROP

Monitor process threads with top:

(Press H for showing threads)

top -p pid


Setting up netconsole:

First on receiver side:

netconsole recieve: nc -u -l -p 6666 2>&1 | tee nc.log

Then on sender side:

modprobe netconsole netconsole=[src-port]@[src-ip]/[],[tgt-port]@/[tgt-macaddr]
modprobe netconsole netconsole=@/,@10.100.150.202/00:30:48:99:99:16


partition schema copy:


sfdisk -d /dev/sda > sda; sfdisk /dev/sdb < sda;

uuid of device:


blkid


uuid of software raid device through mdadm:


mdadm --detail --scan


mdadm re-assembly:


mdadm --assemble --scan --force


Quickly create directories:


for (( i=1; i<13; i++ )); do mkdir -p /dss/sd$i; done


I need create bad block device for testing:


dd if=/dev/zero of=bad_disk.img bs=1M count=$((4 * 1024))
losetup /dev/loop0 bad_disk.img
dmsetup create bad_disk << EOF
0 2097152 linear /dev/loop0 0
2097152 2097152 error
4194304 2097152 linear /dev/loop0 2097152
EOF


This creates a file that is about 4GiB, then uses it in a loopback device with a trick to create a bad area on the “disk”. This is useful for debugging VMware, Xen and Virtualbox disk handling capabilities.