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Old 10-15-2009, 12:49 PM   #1
ternarybit
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Implementing RAID1 mirror - hardware vs mdadm/LVM


I searched and did not find this covered, so I apologize if this is a dupe. I also hope this is the correct forum to post this.

Concerning RAID1 (simple mirroring):
  • Does mdadm/LVM offer any advantages over a simple integrated motherboard RAID controller?
  • If mdadm/LVM is better, can I implement it without formatting my existing HDD?
  • Also if mdadm/LVM is better, can you direct me to resources explaining how to set it up?

Thank you for your time!

Edited for clarity and brevity

Last edited by ternarybit; 10-15-2009 at 06:28 PM. Reason: Solved
 
Old 10-15-2009, 02:11 PM   #2
never say never
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Generally speaking Hardware Raid (any level) is better than Software Raid.

You don't say what motherboard you have, but most raid hardware controllers have a battery to back up data that has not been committed yet, in case of power failure . . . Among other things this means you don't lose sync in case of a power outage.

Software Raid is generally slower than the Hardware Raid, but may have some configuration advantages in certain situations.

If you are after quick and reliable, I think I would stay with Hardware Raid. If you want to test out Software raid well that is another story all together. ;^)

You can still use LVM and hardware raid by the way.
 
Old 10-15-2009, 02:19 PM   #3
mostlyharmless
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Unfortunately, motherboard "hardware" raid is not hardware raid, in general, but software raid setup by the bios. Using Raid 1, it has only the advantage of allowing dual booting with Windows on RAID and is referred to as "fakeraid" or firmware raid. Most people prefer mdadm over LVM or either over firmware raid.

Depending on your distro, instructions on mdadm vary. Most have instructions for setting it up on install.
 
Old 10-15-2009, 02:56 PM   #4
ternarybit
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Quote:
Originally Posted by mostlyharmless View Post
Unfortunately, motherboard "hardware" raid is not hardware raid, in general, but software raid setup by the bios. Using Raid 1, it has only the advantage of allowing dual booting with Windows on RAID and is referred to as "fakeraid" or firmware raid. Most people prefer mdadm over LVM or either over firmware raid.

Depending on your distro, instructions on mdadm vary. Most have instructions for setting it up on install.
Thank you for this information. I am running my system on an Asus P5W DH Deluxe w/ current BIOS.

My first priority by a huge margin is data availability. I want the easiest way to access my data in the event of a hard disk failure. Performance is a distant second priority, and further behind is cost.

I am leaning toward mdadm since I want to learn how to set it up anyway. Speaking generally, is it possible to set up mdadm without having to reinstall Linux? Also, what are the (dis)advantages of purchasing a true hardware RAID controller to accomplish this?

Thanks again for the information!
 
Old 10-15-2009, 03:30 PM   #5
never say never
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Good point! Being in the server room, guess what I was thinking about, servers. Ooops!

Yeah, if this is a desktop system I would definitely stay away from the firmware cruft.

Quote:
Speaking generally, is it possible to set up mdadm without having to reinstall Linux?
Sure. Raid1 should be easy to implement with mdadm without having to re-install. You don't say how you have your drive(s) partitioned now but here is one article on how to do this:http://www.excentral.org/archives/20...id-1-migration


Quote:
Also, what are the (dis)advantages of purchasing a true hardware RAID controller to accomplish this?
Other than cost none that I can think of (unless of course the hardware card dies). Hardware Raid with a good controller is faster and far more secure than software raid, normally including the battery backup for the cache that I spoke of previously.

Good Luck!
 
Old 10-15-2009, 03:49 PM   #6
ternarybit
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Quote:
Originally Posted by never say never View Post
Good point! Being in the server room, guess what I was thinking about, servers. Ooops!

Yeah, if this is a desktop system I would definitely stay away from the firmware cruft.

Sure. Raid1 should be easy to implement with mdadm without having to re-install. You don't say how you have your drive(s) partitioned now but here is one article on how to do this:http://www.excentral.org/archives/20...id-1-migration

Other than cost none that I can think of (unless of course the hardware card dies). Hardware Raid with a good controller is faster and far more secure than software raid, normally including the battery backup for the cache that I spoke of previously.

Good Luck!
Again, great information. Thank you!

I have 3 partitions on this disk. 1 is an XP pro install, 1 is Ubuntu 9.04 root, 1 is Ubuntu 9.04 /home. Am I correct in assuming that mdadm is incompatible with this dual-boot setup?

Also, if I were to purchase a true hardware RAID card (so I can keep dual-booting), in the event that it fails, would my data become unavailable until I find an identical replacement? Isn't this only true of RAID0?

I want the solution which provides, ideally, immediate access to my data in the event of hardware failure (controller or disk). If I can do this while dual-booting, that's even better. If I must ditch my XP partition, I'm ready to do that. If I can get a speed improvement over mdadm and the ability to dual-boot with a hardware RAID, without sacrificing data availability in the event the controller fails, then that is the best solution for me. Otherwise I will use mdadm as you suggest. Thank you very much for your time!
 
Old 10-15-2009, 04:57 PM   #7
mostlyharmless
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Dual booting, you need either a *real* hardware card or use the firmware RAID. mdadm's raid can't be used directly by Windows, so you could dual boot and have RAID 1 only in Ubuntu and no RAID in Windows with mdadm, else firmware or hardware.

Regardless of whether your hardware card failed, you could reuse your mirrored drives with any other controller. Even Raid 0 data could probably be recovered, though it might be more problematical.

So: speed and $: hardware

software speed, no additional cost but more complexity in setup: firmware raid (google "fakeraid ubuntu" for instructions; there's a ubuntu forum instruction page) [edit] here it is https://help.ubuntu.com/community/FakeRaidHowto

no need for Windows raid: mdadm

Last edited by mostlyharmless; 10-15-2009 at 05:00 PM.
 
Old 10-15-2009, 06:28 PM   #8
ternarybit
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Thank you very much. Between the two of you, you have answered my question perfectly. Cheers!
 
  


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