Quick HOWTO: Flush DNS Cache in Linux

By Parthiban Ponnusamy


nscd (Name Service Cache Daemon) daemon provides caching service for the name service requests in Linux.

To configure the nscd caching service, edit /etc/nscd.conf

To Flush the DNS Cache in Linux server:

# /etc/init.d/nscd restart  
           OR 
 # service nscd restart


Hope this helps..


Quick HOWTO: Change I/O Scheduler in Linux

By Parthiban Ponnusamy



I/O schedulers in Linux


noop - It can be helpful for devices that do I/O scheduling themselves, as intelligent storage, or devices that do not depend on mechanical movement
cfq - A fairness-oriented scheduler. It tries to maintain system-wide fairness of I/O bandwidth
deadline - A latency-oriented I/O scheduler. Each I/O request has got a deadline assigned.
as (anticipatory) - conceptually similar to deadline, but with more heuristics to improve performance (It may decrease performance in some cases)

Here is the procedure to change the default I/O scheduler in Linux.


Dynamically setting the default I/O scheduler to a Particular Disk:

Example: 
echo "scheduler_name" > /sys/block/<Disk_Name>/queue/scheduler

To set I/O scheduler to all the Disk drives on the Linux server:

for disk in `ls -1 /sys/block |egrep '^emc|^sd'`;
do
echo "deadline" > /sys/block/$disk/queue/scheduler;
done

To verify the settings:
for dsk in `ls -1 /sys/block |egrep '^emc|^sd'`;
do
echo -e "$i\t\c";
cat /sys/block/${dsk}/queue/scheduler;
done
Permanently set the default I/O scheduler in Linux via Grub menu:

Implement permanent setting by adding “elevator=noop” to the default stanza in the /boot/grub/menu.lst file

1. Create backup 
cp -p /boot/grub/menu.lst /boot/grub/menu.lst-backup
2. Update menu.lst 

Example:
kernel /vmlinuz-2.6.16.60-0.91.1-smp root=/dev/sysvg/root splash=silent splash=off showopts elevator=noop