RAID on Linux

Linux and Unix-Like Linux
1

Dear All,

I have a linux Server having a single disk /dev/sda and two partitions
/dev/sda1 and /dev/sda2. Up on which a Logical Volume is created for root partition.

Now the task is I need to implement RAID on this machine , namely RAID 1. Kindly clarify whether i can directly go and create RAID for those two partitions sda1 and sda2 in mirrored partition sdb1 and sdb2

OR I need to create logical volume first in Sdb2 and then create mirror.

Also pl let me know if any links for mirroring under logical volumes in linux

Rgds
Rj

2

The "norm" if using software raid would be to use mdadm (RAID multi disk) to set up the mirrored device.... then place it under LVM as a PV.

However, LVM does provided simple mirroring... but most would not recommend doing it that way.

Even better would be to use some type of HW RAID or RAID subsystem... but obviously those things are pricey.

3

Having a RAID array consisting of two partitions in the same disk defeats the whole purpose of using RAID at all.

...Unless it's a lab environment or you're just practicing md commands.

4

HI verdepollo,

Can u pl explain how ...why you are saying so.

5

The main purpose of RAID is to provide some level of redundancy and/or performance gain (RAID 0) by using more than 1 disk.

If all your partitions reside on the same disk then there is no benefit of using RAID since in case of disk failure all your data will be gone regardless of RAID level. (E.g. RAID 5 won't be more "safe" than RAID 0).

In the case of RAID 0 where you're supposed to merge two or more disks together to achieve faster I/O operations, it's completely useless as your disk has a limited amount of mechanical heads and they cannot move faster than their assembly allows, so you'll see no I/O benefit either.

Software RAID also requires system resources (CPU, RAM, etc) as opposed to a *true* hardware RAID which provides its own electronic circuitry that handles the requirements.

Also, if your /boot partition is controlled by software RAID it's more difficult to recover in case of failure. Some systems won't recognise it as a valid partition and you'll need to perform some extra steps to work around the issue.

Software RAID is better than no RAID at all and may provide some benefits here and there, but a RAID array that uses partition that reside in the same disk is pointless.

6

"If all your partitions reside on the same disk then there is no benefit of using RAID since in case of disk failure all your data will be gone regardless of RAID level. (E.g. RAID 5 won't be more "safe" than RAID 0)."

In the above point , I wanted to know anyhow we aare going to mirror it right in case of RAID 1. So if one disk goes off we have another disk.

Kindly clarify.