Software RAID issue after reboot of OS

After successful installation of OS, if there are some problem to boot os in one Hard-drive, as grub issue like kernel panic or grub error 17 or other issue

1. Reboot the machine in Rescue mode

Boot: linux rescue

# chroot /mnt/sysimages
OR
# mkdir /a
# mount /dev/sdaX /a
# cd /a

# cat /proc/mdstat (UU means both disk are live)

2. Run the following commands to hotadd and rebuild the array

# mdadm /dev/md0 –add /dev/sda1
# mdadm /dev/md1 –add /dev/sda2
# mdadm /dev/md2 –add /dev/sda3

# grub-install /dev/md0    OR

# grub (GRUB shell type above command to reinstall the boot loader on both drives & reboot.)

grub> device (hd0) /dev/sda
grub> root (hd0,0)
grub> setup (hd0)

grub> device (hd1) /dev/sdb
grub> root (hd1,0)
grub> setup (hd1)

grub> quit

This is show on /boot/grub/device.map

grub> find /grub/stage1 (Find out which are the hard disks on which you can install grub)

This file show in /boot/grub/device.map

grub> device (hd0) /dev/sda
grub> root (hd0,0)
grub> setup (hd0)

How to Check or Monitoring Software RAID in Linux

# mdadm

# cat /proc/mdstat

# mdadm –detail /dev/md0 OR /dev/md1

# mdadm -E /dev/md0 (Spare rebuilding means sync & recovery data to each drive to other)

# mdadm -R /dev/md1 (Recovery & transfer in speed 20-40 Mbps depend on CPU & Hard-drive)

# mdadm /dev/md0 -add /dev/sdb1 [Add partition (RAID Device) /dev/sdb1 into /dev/md0]

How to Create Software RAID-1 Mirroring

# yum install mdadm

# fdisk /dev/sdb     (Create Software RAID Type)

# fdisk /dev/sdc     (Create Software RAID Type)

# mdadm –zero-superblock /dev/sdb /dev/sdc     (If device contains a valid md superblock, the block is overwritten with zeroes)

# mdadm –create /dev/md0 –level=1 –raid–devices=2 /dev/sdb1 /dev/sdc1     (Create RAID-1 using /dev/sdb1 & /dev/sdc1)

# mkfs.ext3 /dev/md0     (format /dev/md0 as ext3 file system)

# mkdir /raid1

# mount /dev/md0 /raid1

# df -hT

# vim /etc/fstab     (For automatically mount after reboot)

/dev/md0    /raid1    ext3    noatime,rw    0    0

# cat /proc/mdstat OR

# watch -n 2 cat /proc/mdstat (For RAID Status)

Difference between RAID and LVM

S.No.	RAID	LVM
1.	RAID is used for redundancy.	LVM is a way in which you partition the hard disk logically and it contains its own advantages.
2.	A RAID device is a physical grouping of disk devices in order to create a logical presentation of one device to an Operating System for redundancy or performance or a combination of the two.	LVM is a logical layer that that can be anipulated in order to create and, or expand a logical presentation of a disk device to an Operating System.
3.	RAID is a way to create a redundant or striped block device with redundancy using other physical block devices.	LVM usually sits on top of RAID blocks or even standard block devices to accomplish the same result as a partitioning, however it is much more flexible than partitions. You can create multiple volumes crossing multiple physical devices, remove physical devices without loosing data, resize the volumes, create snapshots, etc
4.	RAID is either a software or a hardware technique to create data storage redundancy across multiple block devices based on required RAID levels.	LVM is a software tool to manage large pool of storage devices making them appear as a single manageable pool of storage resource. LVM can be used to manage a large pool of what we call Just-a-bunch-of-Disk (JBOD) presenting them as a single logical volume and thereby create various partitions for software RAID.
5.	RAID is NOT any kind of Data backup solution. Its a solution to prevent one of the SPOFs (Single Point of Failure) i.e. DISK failure. By configuring RAID you are just providing an emergency substitute for the Primary disk. It NEVER means that you have configured DATA backup.	LVM is a disk management approach that allows us to create, extend, reduce, delete or resize the volume groups or logical volumes.

RAID Levels

  

RAID Disk Space Calculator

Raid 0 (Stripe)

Raid 1 (Mirror) — 2 Drives

Raid 5 (Drives with Parity) — Minimum 3 Drives

Raid 6 (Drives with Double Parity) — Minimum 4 Drives

Raid 10 (Mirror+Stripe) or 0+1 (Stripe+Mirror) — Minimum 4 Drives

Raid 50 (Parity+Stripe) — Minimum 6 Drives

Raid 60 (Double Parity+Stripe) — Minimum 8 Drives

RAID Types – 0, 1, 5, 6, 0+1, 10, 50, 60

RAID stands for Redundant Array of Independent Disks. RAID is a method of combining several hard drives into one unit. It offers fault tolerance and higher throughput levels than a single hard drive or group of independent hard drives. RAID levels 0, 1, 10 and 5 are the most popular.

The acronym RAID, originally coined at UC-Berkeley in 1987, stood for Redundant Array of Inexpensive Disks.

RAID Configurations

RAID 0 splits data across drives, resulting in higher data throughput. The performance of this configuration is extremely high, but a loss of any drive in the array will result in data loss. This level is commonly referred to as striping.Minimum number of drives required: 2Performance: High Redundancy: Low Efficiency: High	Advantages: High performance Easy to implement Highly efficient (no parity overhead) Disadvantages: No redundancy Limited business use cases due to no fault tolerance
RAID 1 writes all data to two or more drives for 100% redundancy: if either drive fails, no data is lost. Compared to a single drive, RAID 1 tends to be faster on reads, slower on writes. This is a good entry-level redundant configuration. However, since an entire drive is a duplicate, the cost per megabyte is high. This is commonly referred to as mirroring.Minimum number of drives required: 2Performance: Average Redundancy: High Efficiency: Low	Advantages: Fault tolerant Easy to recover data in case of drive failure Easy to implement Disadvantages: Highly inefficient (100% parity overhead) Not scalable (becomes very costly as number of disks increase)
RAID 5 stripes data at a block level across several drives, with parity equality distributed among the drives. The parity information allows recovery from the failure of any single drive. Write performance is rather quick, but because parity data must be skipped on each drive during reads, reads are slower. The low ratio of parity to data means low redundancy overhead.Minimum number of drives required: 3Performance: Average Redundancy: High Efficiency: High	Advantages: Fault tolerant High efficiency Best choice in multi-user environments which are not write performance sensitive Disadvantages: Disk failure has a medium impact on throughput Complex controller design
RAID 6 is an upgrade from RAID 5: data is striped at a block level across several drives with double parity distributed among the drives. As in RAID 5, parity information allows recovery from the failure of any single drive. The double parity gives RAID 6 additional redundancy at the cost of lower write performance (read performance is the same), and redundancy overhead remains low.Minimum number of drives required: 4Performance: Average Redundancy: High Efficiency: High	Advantages: Fault tolerant – increased redundancy over RAID 5 High efficiency Remains a great option in multi-user environments which are not write performance sensitive Disadvantages: Write performance penalty over RAID 5 More expensive than RAID 5 Disk failure has a medium impact on throughput Complex controller design
RAID 0+1 is a mirror (RAID 1) array whose segments are striped (RAID 0) arrays. This configuration combines the security of RAID 1 with an extra performance boost from the RAID 0 striping.Minimum number of drives required: 4Performance: Very High Redundancy: High Efficiency: Low	Advantages: Fault tolerant Very high performance Disadvantages: Expensive High Overhead Very limited scalability
RAID 10 is a striped (RAID 0) array whose segments are mirrored (RAID 1). RAID 10 is a popular configuration for environments where high performance and security are required. In terms of performance it is similar to RAID 0+1. However, it has superior fault tolerance and rebuild performance.Minimum number of drives required: 4Performance: Very High Redundancy: Very High Efficiency: Low	Advantages: Extremely high fault tolerance – cnder certain circumstances, RAID 10 array can sustain multiple simultaneous drive failures Very high performance Faster rebuild performance than 0+1 Disadvantages: Very expensive High overhead Limited scalability
RAID 50 combines RAID 5 parity and stripes it as in a RAID 0 configuration. Although high in cost and complexity, performance and fault tolerance are superior to RAID 5.Minimum number of drives required: 6Performance: High Redundancy: High Efficiency: Average	Advantages: Higher fault tolerance than RAID 5 Higher performance than RAID 5 Higher efficiency than RAID 5 Disadvantages: Very expensive Very complex / difficult to implement
RAID 60 combines RAID 6 double parity and stripes it as in a RAID 0 configuration. Although high in cost and complexity, performance and fault tolerance are superior to RAID 6.Minimum number of drives required: 8Performance: High Redundancy: High Efficiency: Average	Advantages: Higher fault tolerance than RAID 6 Higher performance than RAID 6 Higher efficiency than RAID 6 Disadvantages: Very expensive Very complex / difficult to implement