Fixing Time Sync problem in Xen DOMU on Oracle VM Server

A 0 in the following suggests syncing time with DOM0:
/proc/sys/xen/independent_wallclock

However, when DOMU is unable to keep time sync with DOM0, the following may be tried:
echo 1 > /proc/sys/xen/independent_wallclock

If you have a ntpd setup elsewhere:
/usr/sbin/ntpdate <ntpd_server>

To update over the network:
rdate -s http://time-a.nist.gov

To manually set the date:
date -s "<date>

Updating Time Zone in Linux

Following should work on RHEL, Oracle Linux, Centos, Ubuntu, and similar may work on others:
Update /etc/sysconfig/clock with the desired timezone from /usr/share/zoneinfo
Backup current localtime:
mv /etc/localtime /etc/localtime.o
cp /usr/share/zoneinfo/ /etc/localtime

If the command is available, the above can be achieved by using:
system-config-date

Simple Oracle listener.ora

$ORACLE_HOME/network/admin/listener.ora:
listener= (ADDRESS_LIST =
(ADDRESS=(PROTOCOL=tcp)(HOST=)(PORT=1521))
)

SID_LIST_listener = (SID_LIST=
(SID_DESC=(SID_NAME=)(ORACLE_HOME=))
)


--

$ lsnrctl <start|stop|status>

Drop missing data file on ORA-01110

* May not be appropriate in production env *

Sometimes in test env, if a datafile, which is no longer required, is deleted without first dropping the tablespace, then to cleanup:

startup mount;
alter database datafile '' offline drop;
alter database open;
drop tablespace including contents;

Configuring HugePages with Oracle JRockit JVM

Making JRockit JVM use Linux hugepages requires the following 2 configurations:

[1] Mount hugetlbfs
mount -t hugetlbfs nodev /mnt/hugepages
chmod 777 /mnt/hugepages

[2] Use JVM options
-XXaggressive: usually auto enables hugepages, and will use hugepages, if enough available
-Xlargepages: explicitely enables hugepages, and will use if enough hugepages are configured on the system
-Xlargepages:exitOnFailure=true: Will exit if could not use hugepages for any reason

Configuring RSH without Password

Tried on RHEL and Oracle Linux:

[1] Install rsh-server*.rpm from the Linux distribution for your system or using yum.

[2] Add the following to /etc/securetty:
rsh
rexec
rlogin

[3] Edit the rsh, rexec, and rlogin files in /etc/xinetd.d/ and change value of disable from yes to no

[4] /etc/init/d/xinetd restart

[5] Add list of hosts to /etc/hosts as:


[6] Add the hosts to ~/.rhosts as:


[7] chmod 600 ~/.rhosts
Also, ensure .rhosts is owned by the right user:group. Otherwise use chown as well.

[8] Repeat steps 1 to 7 on all hosts

Now, you should be able to rsh among the hosts on which you have setup rsh-server correctly.

Mounting RamFS, TmpFS, ISO image

Mount Ramfs
mount -t ramfs -o size=1g ramfs /mnt/ram

Remember that specifying size with ramfs does not limit the /mnt/ram. The ramfs is bound by the free RAM on the system, after which the system will start swapping and performance will be severely degraded.

Mount Tmpfs
mount -t tmpfs -o size=1g tmpfs /mnt/tmp

Mount ISO
mount -o loop,ro <img>.iso <destination>

[Ref] Nice article on Ramfs and Tmpfs available on thegeekstuff

SSH without Password

In order to ssh from Host1 to Host2 without password, the following needs to be done:

user1@Host1:~> ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/user1/.ssh/id_rsa):
Created directory '/home/user1/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/user1/.ssh/id_rsa.
Your public key has been saved in /home/user1/.ssh/id_rsa.pub.

If ~/.ssh doesn't exist on Host2 as user2, create one:

user2@Host2:~> mkdir -p .ssh
user2@Host2:~> chmod 700 .ssh

Append user1's public key to authorized_keys and authorized_keys2 in user2@Host2:.ssh/:

user1@Host1:~> cat .ssh/id_rsa.pub | ssh user2@Host2 'cat >> .ssh/authorized_keys'
user1@Host1:~> cat .ssh/id_rsa.pub | ssh user2@Host2 'cat >> .ssh/authorized_keys2'

user2@Host2:~> chmod 640 .ssh

Now ssh from user1@Host1 to user2@Host2 should be possible without password:
user1@Host1:~> ssh user2@Host2\

Avoid Host Verification

ssh -o "StrictHostKeyChecking no" user@host

Debugging

Set LogLevel to DEBUG in /etc/ssh/sshd_config
/etc/init.d/sshd restart
Then try to ssh, debug messages will be logged to /var/log/secure
PS: Once you have resolved the issue remember to switch back LogLevel to INFO and again restart sshd.

Editing Xen System.img

This post is useful when you want to change system configuration files of a stopped guest VM without botting up the guest first. This method can be used especially when your guest VM crashes or hangs during boot up due to a configuration error.


The following steps have been tested on Xen 4 and Oracle VM 2.2 as well for a Linux guest VM. From DOM0: 


Check FS Type
fdisk -u -l System.img

If FS type is ext3, you can directly mount a partition using lomount, otherwise follow the mounting LV's steps.

Mount Ext3 Partitions

Linux system.img with ext3 partitions can be mounted as:
lomount -diskimage System.img -partition 2 /mnt

Mounting LV's

# Mounting guest's root partition locally
# Find a free loop device
loop_dev=`losetup -f`

# Now bind the image file to that loop device
losetup ${loop_dev} System.img

# Next, scan the loop device for partitions
kpartx -av ${loop_dev}

# If /dev/mapper doesn't list LVs for the partitions from kpartx, find LV:
vgscan
vgchange -ay sysvg

# Mount the desired LVM
mount /dev/mapper/loop0p2 /mnt

LVs UnMounting
# After editing in /mnt, unmount and remove partitions:
umount /mnt

# Disable the LV
vgchange -an /dev/mapper/loop0p2

# Remove the discovered partitions
kpartx -dv ${loop_dev}

# Delete the loop device
losetup -d ${loop_dev}