Software Raid combined with hardware Raid  - raid 11?

Posted on 2014-04-29
Last Modified: 2016-12-08
I have a Dell server with a perc controller.

I has 8 drives, with 4 Raid1 configurations.

In Windows Server 2008R2, it views each hardware raid1 as a single volume.
(drives C,E,F,G)

Can I use Windows software to create a software Raid1 with 2 of the drives (let's say F and G) of the hardware Raid 1 volumes?
Neither would hold an OS, just data files.

If no, why not?
If so, are there cautions, or is it generally a bad idea?

I know that some view Windows software raid to be 'not ideal' but aside from that opinion, what are your thoughts?

this is only for redundancy, and not performance...and just wondered if it can/should be done.
Question by:SeaSenor
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Expert Comment

by:Dan Craciun
ID: 40029855
So you have a mirror and want to create a mirror of that mirror?

You're better off keeping a spare hdd for each mirror. In that case you're only losing a hdd, not 2.

LVL 87

Expert Comment

ID: 40029875
Of course that's possible. Also, OS Software RAID isn't inferior to real hardware RAID. But you'll have to be aware that you loose a lot of disk capacity with a double RAID 1 array (all you get is the capacity of 1 physical HD, you loose 3 of them). Do you really need that much redundancy? Wouldn't a standard RAID 1 with hotspare suite you fine?

Besides, even with 3 redundant disk, you wouldn't be spared from creating regular and good backups, as lots of problems with data loss isn't caused by hardware disk-failures, which is what RAID protects against.

Author Comment

ID: 40029877
recently I had both drives fail in a hardware raid 1 config.

this dell server has some extra drives I won't be using, so I wondered if I could use software raid to mirror the hardware mirrors.

the drive usage isn't a concern as they are just sitting there for now.

is that what you mean by 'losing a hdd, not 2' ?

and yes, I keep backups religiously. Just wondering if it's been done, and if there are any issues.
LVL 34

Assisted Solution

by:Dan Craciun
Dan Craciun earned 250 total points
ID: 40029918
is that what you mean by 'losing a hdd, not 2' ?
On standard RAID1 + spare, you're making a mirror out of 2 drives and keep another drive ready as hotspare so the moment one of the mirror drives fails the spare hdd takes it's place.
Total drives used: 3

On your solution, you want to use 4 drives to mirror the information on a single drive. This will minimize the possibility that all 4 drives die at the same time, but will increase the possibility of data corruption.
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LVL 87

Accepted Solution

rindi earned 250 total points
ID: 40029924
I'm sure it has been done. You loose the capacity of 3 out of 4 HD's. As I mentioned, I don't see much point though.

If both drives failed on a previous system, chances are that there was no hot-spare, and you waited too long to replace the first failed disk. Bad disks should always be replaced asap (which means the system should constantly be monitored). You could also use RAID 6 (provided your PERC controller supports it), which allows for two simultaneous disk failures. This is pretty widely used.

Author Comment

ID: 40029929
increase possibility of data corruption due to introducing the software raid component?

if so, (or if not), please explain
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Expert Comment

by:Dan Craciun
ID: 40029937
You have 2 controllers (one HW, one SW), so 2 points that can introduce corruption.

Author Comment

ID: 40029982
ok, that's what I assumed was meant.

thanks Gentlemen.

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