Getting started the hard way with RAID

[Kevin O'Gorman]

On Sat, Dec 3, 2011 at 4:08 PM, Marius Gedminas <marius at pov.lt> wrote:
> On Sat, Dec 03, 2011 at 11:26:44AM -0800, Kevin O'Gorman wrote:
>> On Sat, Dec 3, 2011 at 9:46 AM, Kevin O'Gorman <kogorman at gmail.com> wrote:
>> > I just worked up the courage to try dmraid.  So far, I've
>> > only tried discovery, but it's promising:
>> > root at treat:~# dmraid -r
>> > /dev/sdd: ddf1, ".ddf1_disks", GROUP, ok, 976494592 sectors, data@ 0
>> > /dev/sdb: ddf1, ".ddf1_disks", GROUP, ok, 976494592 sectors, data@ 0
>
> Neat!  But why isn't 3ware mentioned in dmraid's README then?  Google
> gives me the impression 3ware is a big name in the hardware RAID
> business.

It probably isn't supported.  See below.

>> > That's exactly the two disks that had the mirror halves, and the size
>> > is convincing.  I don't understand everything it's showing, and I
>> > haven't had the courage to actually turn it on yet....
>>
>> With a little more courage, I tried and failed.
>>
>>   root at treat:~/work# dmraid -r
>>   /dev/sdd: ddf1, ".ddf1_disks", GROUP, ok, 976494592 sectors, data@ 0
>>   /dev/sdb: ddf1, ".ddf1_disks", GROUP, ok, 976494592 sectors, data@ 0
>>   root at treat:~/work# dmraid -ay
>>   ERROR: ddf1: wrong # of devices in RAID set "ddf1_SimpleVol" [2/1] on /dev/sdd
>>   ERROR: ddf1: wrong # of devices in RAID set "ddf1_SimpleVol" [2/1] on /dev/sdb
>>   RAID set "ddf1_SimpleVol" was not activated
>
> Oh :(

Actually, the size does not stand up to scrutiny.  Apparently, 3ware
marks the drive as having raid info in a way that looks like one of
the standards, but isn't really.

Fdisk reports sdb has 500107862016 bytes, or 976,768,002 sectors. It's
geometry is set to 60801 cylinders, 255 heads and 63 sectors, or
976,768,065 sectors.  It has two non-RAID partitions totalling
16,771,860 sectors.  So dmraid is showing a size so large that it has
to overlap the non-RAID paritions, and is clearly wrong.  I take it
that the volume is marked RAID in a way that looks a little like a
supported format but isn't.

>
>> In fact, I find the man page for dmraid very confusing and have been
>> unable to get any other commands to even try...

[snip]

>
> The dmraid manual page is, I think, the worst manual page I've ever
> seen.

Agreed, but it no longer matters because I'm convinced dmraid won't
help me.  Since the mirror set comprises a complete volume that I
formatted with fdisk, I'm going to write some programs/scripts to scan
for a partition table.  If the controller didn't mess with the actual
volume image, I can copy it wholesale to a spare drive I have and
mount that drive in the normal way.  Maybe.

> It could be worse: at least it exists...

Grump grumble.  Damning with faint praise. Grumble.

-- 
Kevin O'Gorman, PhD