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 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
