CentOS Rescue

Rebuilt the madadm.conf and re-configured grub on MBR of sda

To rebuild the mdadm.com
==> mdadm --detail --scan >> /etc/mdadm.conf

You can copy the partition layout from existing one to the newly replaced one as follows.

==> sfdisk -d /dev/sda | sfdisk /dev/sdb

Important: This will dump the partition table of sda, removing completely the existing partitions on sdb


Reconfigure Grub via rescue mode
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Lets say the /dev/sda contains the boot partition, and sda1 mounted as /boot.

First, you have to make sure that boot flag is enabled on sda1 (There will be a * if the partition is bootable). You can mark it as bootable using 'a' with fdisk command.

#grub-install --boot-directory=/boot /dev/sda
#update-grub


You would not see any /dev/md[], /dev/sd[] or /dev/hd[] devices listed as they are not automatically mounted by the Rescue System. In some cases like MySQL crahses, you will be able to start the demand by mounting these virtual filesystems via rescue mode as follows.

mount /dev/sdb3 /mnt/sysimage
mkdir /mnt/sysimage/proc
mkdir /mnt/sysimage/sys
mkdir /mnt/sysimage/dev
mount -o bind /proc /mnt/proc
mount -o bind /sys /mnt/sys
mount -o bind /dev /mnt/dev
chroot /mnt/sysimage

Flashcahe On SSD

Git Clone Flashcache:

git clone https://github.com/facebook/flashcache.git

Install and Configure Flashcache (Note: Please remember to change the versions to match your uname -r output!)

cd flashcache/
make KERNEL_TREE=/usr/src/kernels/2.6.32-042stab075.10
make install KERNEL_TREE=/usr/src/kernels/2.6.32-042stab075.10
modprobe flashcache
Make sure its running: dmesg | tail

Making /vz flashcached:
umount /vz
Find your UUID for /vz: grep "/vz" /etc/fstab
flashcache_create -p back vz_cached /dev/sdb /dev/disk/by-uuid/replace-with-your-uuid
Comment out vz in fstab: nano /etc/fstab

Configuring Flashcache

Copy the config file: cp /tmp/flashcache/utils/flashcache /etc/init.d
Change the permissions: chmod 755 /etc/init.d/flashcache
nano /etc/init.d/flashcache

Change the following information:

SSD_DISK=/dev/sdb
BACKEND_DISK=/dev/disk/by-uuid/replace-with-your-uuid
CACHEDEV_NAME=vz_cached
MOUNTPOINT=/vz
FLASHCACHE_NAME=vz_cached
CTRL +X and save it.

Turn on Flashcache at boot: chkconfig flashcache on
Reboot the Server: reboot -n

Make sure everything looks good and run df-h.

Do a DD test to make sure flashcache is working.

First do a DD test in the root directory:
cd /
dd if=/dev/zero of=test bs=64k count=16k conv=fdatasync; unlink test

Note the output

Now do a DD test in the /vz directory.
cd /vz
dd if=/dev/zero of=test bs=64k count=16k conv=fdatasync; unlink test

Note awesomeness.

Everything is now setup as far as the base system with SolusVM OpenVZ and Flashcache!
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LVM Special Options

Create swap on a logical volume:
# lvcreate -C y -L 10G VolGroup00 -n lvolswap
Fill all the free space left on a volume group
# lvcreate -l +100%FREE VolGroup00 -n lvolmedia

Extend an existing logical volume to use the maximum space:
# lvextend -l +100%FREE VolGroup00/lvolhome

# lvextend -l +100%FREE /dev/vg0/backup -r