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How do I configure RAID 1 on Red hat Enterprise Linux 5.3 OS. Is it necessary that if you configure RAID 1, the motherboard should support. If you have commands please submit

Posted on 2009-07-12
Medium Priority
Last Modified: 2012-05-07
To configure RAID 1 on red hat enterprise linux 5.3 server. I have installed the OS but now the customer wants to configure as RAID 1. There are two HDD's installed with 250G each.

Please advise

with kind regards
Question by:stevebungay
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Expert Comment

ID: 24836637
If you do it during the install phase, the installer includes a RAID wizard that walks you through, and its quite easy.

It it possible to re-install at this point? Any customizations yet? If not, I recommend reinstalling.
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Expert Comment

by:Michael Worsham
ID: 24836655
For hardware RAID-1, you need to do it through the motherboard's configuration (sometimes found in the BIOS).

For software RAID-1, try this link:

Author Comment

ID: 24836888

Hi. The OS is already installed. Although there is no data as such. But before going through reinstallation can we configure now RAID 1. I want to go for software RAID 1. How to go for Hardware configuration in BIOS

Please advise


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Expert Comment

ID: 24836913
The motherboard should have support with an integrated RAID solution, or you should use an addon hardware RAID card.  Whichever you use, make sure it's not a hostraid or fakeraid controller,  even software RAID is better than that.

With a Redhat-supported hardware controller,  whether integrated on the MB, or addon,  while the system is booting up, there will generally be a display of the controller manufacturer's name, and a prompt  to type a special keystroke such as F8, Control+S, or ^R.   e.g.

"Press  <F8> to enter array configuration utility"
You enter the utility,  setup a RAID1 logical disk using the installed drives, and then install the OS   on  top of the logical drive.

If the install is critical enough that RAID is a must, then generally speaking, software RAID is unacceptable.

If you choose RAID during installation of the OS (and there will be an option),  software RAID is definitely what you will get.

You should understand the drawbacks before you proceed in that direction; mainly, software RAID has additional failure modes, complexity, and a performance penalty that make it generally undesirable,  especially for server workloads.

RAID1  is simple mirroring, however, and it is more acceptable to implement this in software than RAID5,  if the server is fast enough, or the workload light enough to deal with the big  I/O  write speed, performance degradation and CPU penalty.

For a workstation install, software RAID is alright.    You should be aware, that  hot swapping is not necessarily seamless with Linux software RAID.

Unlike in a hardware RAID setup,  only one drive will be the bootdisk.  There are failure modes where  an issue with one drive can make the system unable to boot without manual intervention  (even though the other drive is still ok),   because the  software RAID drivers don't get loaded until the system is booting.

The Linux kernel is complex enough that,  some sort of bug or memory corruption is more likely to occur in there than on a hardware controller.
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Expert Comment

ID: 24837001
If there is a hardware RAID capability, you could reconfigure the system without the need to reinstall.
When going through the RAID setup, DO NOT initialize the array.  Make sure you know which drive has the data and which drive is empty.  Many RAID controllers provide for the COPY option.
I.e. group two drives into a RAID 1 with one selected as having data that needs to be synchronized to the other.

If there is no HW raid, your option is to use the currently unused drive and set it up as a single member of a RAID 1.
See if the following helps:

Since there is nothing on the system, it might be simpler/quicker to reinstall while configuring Software RAID from the begining.

Make sure you create the needed /boot partitions and RAID them versus creating a single RAID by combining the whole drive into a single RAIDed partition.

Author Comment

ID: 24837114
Hi Thanks

Which one is better Hardware RAID 1 or Software RAID 1

2. going through the steps above. The hardware RAID 1 requires separate Adaptec or any make controller card and install the driver of the card. Is there any settings provided on the Card to configure RAID 1 or it will go automatically

3. Presently there is no data on the server and is just new sevrer. How can we  know whether the motherboard supports RAID 1

4. Just advise the best method and i shall configure the RAID 1.



Accepted Solution

NotLogical earned 2000 total points
ID: 24837145
Hi Steve,

There is no single/simple answer for your question of "which is best." Why? Well, it really depends on what you are trying to achieve.

For simple data redundancy on a small server, software RAID is sufficient. This will work very reliably for a lot of configurations and provides good performance. This also does not increase the cost of your system.

For a more robust solution (please keep in mind that this varies with the motherboard manufacturer!), you can definitely use the on-board RAID of your system - if available. Please do a little investigation on the motherboard controller, as some on-board controllers have been known to be low-performance, or require assistance from the software driver. In the case of the latter, you are almost better off to use software RAID.

If you are looking for a most robust solution, which supports things like caching, disk hot-swapping, and is a "high performance" solution, you will want to add an extra RAID controller.

Please remember: any RAID solution does not ensure disaster recovery! It only allows you to mitigate a disk failure. In the case of other RAID levels (RAID-5, RAID-6, RAID-10), you can gain performance and reliability at the cost of using more disks.

Always backup your data... :^)

If you need more help or information, do not hesitate to ask!


LVL 23

Expert Comment

ID: 24839129
An additional RAID controller is used if the MB itself doesn't have a good solution, or to add support, if it doesn't provide RAID support at all,  in the case of poor RAID support (e.g. hostraid / fakeraid), there's a need to turn off the MB's integrated RAID support before installing the add-on controller (in the BIOS).

To know the quality of your MB RAID, or of an integrated RAID card you purchase, you need to lookup the model and read the spec sheet.

Software RAID solutions result in a 40 - 60%  performance hit compared to solid HW RAID solutions,   for disk heavy high CPU workloads, especially with RAID5; the latency required for a write is higher, and the number of  disk-related commands that must cross your system's  memory I/O bus  to reach the IDE controllers are doubled or tripled.    

Fakeraid/hostraid solutions have the same disadvantage, even though they use a dedicated controller for offload of some operations.

If your application is I/O bound normally,  SW RAID will make things a lot worse than a good HW RAID solution does.

Hardware RAID is more expensive, and has the disadvantage, that should the controller itself fail,  you can't just plug you hard drives in another computer.

Non-portability is one of the biggest disadvantages of HW RAID,  and a good reason not to use it for desktops,  or other setups, where you can't get a spare controller.

If you want HW RAID, you will need to manually setup the logical disks.  Most RAID controllers will not setup anything automatically, except JBOD mode which has no redundancy.  A true hardware RAID  implementation requires either a separate controller or a mid to high-end motherboard that has a full-blown on-board RAID solution.

Most server vendor's systems have on-board RAID solutions, e.g. basically all  IBM, HP, and Dell server products  have integrated RAID.  It's generally critical to have a hardware RAID solution for servers that need high uptime and have a heavy production workload.  

HW RAID1  accelerates disk read speeds  (the controller balances load across the disk),  helps stop a drive failure from impacting uptime, although, often a server will still  have to reboot after drive failure,  because software is potentially impacted by  speed-degradation first  (if the disk surface has been scratched, the drive may not fail gracefully,  the RAID controller has to eventually note the abnormal  command latency, and then take action),  the HW RAID controller helps maintain availability.

If you don't have a hardware RAID solution available integrated on your MB, it's almost certain  that you have desktop hardware, or a fairly low-end  (read: inexpensive) server solution.

You will need to assess your workload to determine if HW RAID's   applicable to the workload.

LVL 80

Expert Comment

ID: 24840028
Based on the drive size specified, you likely have SATA drives but IDE is a possibility as well.
Going through the BIOS, you can see what the operating modes of the sata Controller is i.e. whether RAID functionality is available.

Mysidia addressed the issues that could pose a problem if using hostraid/fakeraid.  Some have been known to have an issue with the Centos Install.

NotLogical, addressed the pitfalls.

Addressing some of your question, depending on the vendor of the server/system, you have have a manufacturer system setup disk which provides an option for the configuration of RAID as well as provides the requisite drivers to the OS installation.

A reasonable solution depends on what the purpose for the server is.
A database server will benefit from a Hardware RAID because of the high Disk I/O inherent in active databases.
Your zones, reference a Network Router.  If this is the purpose for this server and if you have two of them, you might be better off setting up a cluster without the need for RAID on either.

Expert Comment

ID: 24843278
Generally, the performance of RAID built in to motherboards is not better than using Linux's software RAID.  There are some disadvantages:  it may not be possible to read the drives on a different system which uses a different motherboard.  SW RAID is more flexible than the RAID implemented in a BIOS, since you can change a number of parameters, such as stride.  

Adding a RAID controller card does offer improved performance, if that is needed.  I'm not sure where Mysidia comes up with 40-60% performance hit for SW RAID.   Linux software RAID doesn't use more than a very small amount of CPU on the systems on which I've installed it.

A comparison of HW vs SW RAID can be found here: http://linux.yyz.us/why-software-raid.html

Setting up software RAID on Linux is pretty easy.  The GUI approach in RHEL 5 can be used or the command line approach that I've always taken.  See http://www.linuxhomenetworking.com/wiki/index.php/Quick_HOWTO_:_Ch26_:_Linux_Software_RAID
LVL 23

Expert Comment

ID: 24845093
Most software RAID operations are done in kernel space, and Linux does not provide good facilities to measure that CPU usage,  it still reduces application speed.     But the _main_  concern is the added latency of  synchronous writes and memory bus / PCI bus  bandwidth usage.

The 40-60% figure comes from testing using IOMETER, on  Redhat Enterprise Linux 5.2 software RAID on  servers with with  2 dual-core  2.4Ghz CPUs and

Software RAID using 4 on-board SATA ports VS a LSI  MegaRAID SATA 300-4XLP hardware RAID controller in a  4-drive  RAID1+0  array  with a  battery backup unit installed and -- full writeback caching enabled.

* LSI is my prefered choice for controller vendors,  for the most part, the majority of their controllers are supported by major OSes without needing driver disks,   the PERC/5i  integrated option cards on certain Dells are re-badged LSI SAS 1086Es,  which is no different than adding your own controller  (it is not as if there is a performance penalty for  "being included with the system"), and the SmartArray  E200is  shipped with certain HP servers had excellent performance compared to Linux SW RAID also,  but lest I digress....

On a 25% random / 25 write / 75% read workload, with a maximum of 100 queued commands, the hardware controller yielded a response of approximately  10,000 IOPs   (I/O operations per second).

The same server, and same drives, reconfigured with 4 drives in software RAID,  yielded a response of approximately  4,000 IOPs.

You may be able to get better performance out of SW RAID,  if you do, then good for you.    HW RAID   won't  give  stellar performance for all workloads, either.

If your server does not perform massive numbers of random I/Os, then the added latency for your seek commands and writes won't matter.    But SW RAID can seriously hurt you if you are trying to run a database app on it that has heavy load.

Make sure your server config  gives you acceptable performance prior to deployment.


Expert Comment

ID: 24846315
You are comparing apples and oranges.  

Yes, a system with a hardware RAID board will provide better I/O performance than a system without one.  But the 40-60% is not a performance "hit" as you described it.  It is a 40-60% performance improvement, resulting from adding an additional processor and memory dedicated to handling disk I/O.

The performance "hit" of SW RAID, comparing RAID disks to non-RAID disks, is small.  

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