[lug] software raid

[Harris, James]

The personalities line does indicate that RAID is setup, but, as Sean had
said, if you have proc/mdstat, you have software RAID support in the kernel.

Software RAID is composed of two major pieces, the low level RAID support in
the kernel (can not be built as a module), and then each level of RAID or,
"personalities".  These personalities are the RAID levels 0,1,4,5, and
hybrids.  Each personality can be compiled into the kernel directly, or it
can be built as a module.

For example, if you want to boot your system off of a mirror, then you'll
need to compile the raid1 personality directly into the kernel (this may
vary by distro and version of RAID, i.e. you can do it using an image and a
module, but in my opinion it's still not truly a redundant disk system when
you do it that way.)  This allows you to boot the root off the mirror set
directly without having to go through the hoops of old.  Anyway, then if you
wanted to have a large raid5 for data, you could compile the raid5
personality as a module instead of directly into the kernel, allowing you to
load the module before mounting the array without having to take up kernel
space.

If you're looking for more information the Software RAID How-TO is very
cool.  I find it very concise and easy to grasp onto.  I'd highly recommend
checking it out.  Unfortunately either linuxdoc.org is down, or our link is
cranky, so I can't give you a link directly to the doc, but you know where
to find it.  ;)

Jim

-----Original Message-----
From: Hugh Brown [mailto:hugh at vecna.com]
Sent: Sunday, August 19, 2001 19:59
To: lug at lug.boulder.co.us
Subject: Re: [lug] software raid


So does the Personalities line just mean that the kernel is capable of
dealing with raid if I wanted to set it up?

Hugh

"Sean Reifschneider"
> 
> On Sat, Aug 18, 2001 at 12:16:10PM -0400, Hugh Brown wrote:
> >cat /proc/mdstat gives
> 
> If software RAID is active, /proc/mdstat will show the drives you have set
> up:
> 
>    Personalities : [raid1] 
>    read_ahead 1024 sectors
>    md0 : active raid1 hdc6[1] hda6[0] 80192 blocks [2/2] [UU]
>    md1 : active raid1 hdc1[1] hda1[0] 2048192 blocks [2/2] [UU]
>    md2 : active raid1 hdc5[1] hda5[0] 264960 blocks [2/2] [UU]
>    md3 : active raid1 hdc7[1] hda7[0] 17647296 blocks [2/2] [UU]
>    unused devices: <none>
> 
> In this case there are 4 RAID1 partitions set up.
> 
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