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 [email protected]/,@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.