Re: Moving from RAID 0 to LVM RAID?

["Ben Holland" <user=122@nabble.blu.org>]

 The attraction to raid 5 and 6 is that you don't loose nearly as much HDD 
space over raid 1. Say you have a 4u rack with 16 drives, you want to split 
them, you could get 8 drives and set them up with RAID 1, thus giving you a 
50% HDD storage capability. If you want 2 LUNs on a raid 5 you have 14 
drives at your disposal and if one drive dies you don't have a problem. With 
raid 6 you have 12 drives at your disposal and a failure of 2 per LUN, much 
much better then RAID 1 where if you loose 2 drives and they happen to be 
the same LUN  you loose your data. ~Ben 

On Thu, Mar 6, 2008 at 8:54 PM, Matthew Gillen <[hidden email]> wrote: 

> Scott R. Ehrlich wrote: 
> > When I get a replacement disk and build the system from the ground up 
> > again, I could, conceivably, use hardware RAID 1 for the OS on two 
> > disks, and CentOS 5 64-bit's LVM for software RAID 5 (or maybe 1+0 if 
> > available) on the remaining for 4 disks, maybe 3 disks as active and the 
> > 4th as a hot spare? 
> > 
> > I've never had much faith in software raid, since it is not 
> > hardware-based, and there would be a performance hit, but in this case, 
> > it could be an option. 
> 
>  From a purely logical standpoint (I don't have the inclination to do 
> actual 
> performance tests myself, since I don't have any RAID hardware, and the 
> software works fine for me), there's not much that hardware will gain you 
> over software when doing RAID-1.  RAID-1 is all about bandwidth to the 
> drives, so as long as you have separate buses that can write to more than 
> one disk in parallel, that probably dwarfs any other issues. 
> 
> With RAID-5 the story is a little different, since the parity calculation 
> would actually chew up some CPU cycles.  There, hardware-assist would 
> probably make a noticeable difference.  Personally, I don't see the 
> attraction to RAID-5; it's more complexity for very marginal cost benefit 
> (which is probably negated completely by the cost of the RAID 
> controller...) 
> 
> I'd be curious to see some performance tests of a hardware RAID controller 
> vs. a couple SATA drives using SW RAID, for both the 1 and 5 flavors. 
> 
> Finally, one last point in favor of SW RAID: a bug in the implementation 
> can 
>  be fixed easily.  If your RAID-controller has a bug, you're fsck'd. 
> 
> Matt 
> 
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