Re: Moving from RAID 0 to LVM RAID?

["Jarod Wilson" <user=74@nabble.blu.org>]

 On Fri, 2008-03-07 at 11:40 -0500, Ben Holland wrote: 
> Ahhh I am not sure now, it very well could have been psudo raid but 
> there was a built in raid controller that once the bios booted up you 
> could configure your settings. It is quite possible that this was in 
> fact done with bios driven stuff which I don't think I would ever wish 
> to use. 

I find bios RAID to be okay for RAID-1 sometimes, but I don't really 
trust it beyond that, and I believe with some bios RAID setups, the only 
way to do any sort of recovery/rebuild is via their tools that load in 
the bios. I definitely prefer something where I can hot-swap drives and 
have utilities that run within the operating system while booted to prod 
the array. 

> I do know though that every time I've had a SCSI controller it's had 
> RAID built in to the hardware 

Dell boxes with on-board SCSI controllers tend to ship just a plain old 
SCSI controller by default. Getting RAID functionality requires adding a 
RAID personality card, which enhances the on-board controller with RAID 
functionality, or a replacement/add-on controller. 

> SATA most of the comps didn't but they were also really low end and 
> the newer ones said they had some form of RAID 

Just about guaranteed to be crap bios RAID. 

> the few that had >4 drives generally came with raid 5, in this day and 
> age I wouldn't buy a computer that could take >=4 drives without 
> getting hardware raid 5 built in to hardware, but that may mean i'm a 
> picky bastard, which I freely admit to being. 

Not as picky as I am. :) Any hardware RAID solution I bought today would 
need to support RAID6. I've been the unfortunate recipient of a 
double-disk failure a time or two, in both the form of two disks in an 
array hitting errors and dropping out of the array, and in the form of a 
second disk failing while rebuilding the array with a hot spare or 
replacement. 

> Just a quick question, how do you know if what you are getting has 
> real raid vs psudo raid if they both look the same? 

Not sure exactly, but I don't think I've actually seen anything but a 
Dell server with an add-on personality card that could actually do 
hardware RAID with an on-board SATA controller. I suppose one way to 
know is to boot a modern Linux live CD and see what controllers it sees, 
and what drives it sees attached to those controllers. If its true 
hardware RAID, Linux will see a single "drive" attached to the 
controller, like so: 

scsi 4:1:0:0: Direct-Access     MegaRAID LD 0 RAID5  915G 713N PQ: 0 
ANSI: 2 
sd 4:1:0:0: [sdc] 1875400704 512-byte hardware sectors (960205 MB) 

With bios RAID, Linux will actually see all the individual drives 
connected to the controller, it just knows to handle them as a single 
drive via the dmraid layer if the drives have appropriate bios RAID 
metadata scribbled on them. (dmraid == device mapper raid == bios 
pseudo-raid support, mdraid is linux kernel software raid). 



-- 
Jarod Wilson 
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