[plug] Good RAID Howto

[Kai Jones]

Adam Davin wrote:
> Hello Kai,
> 
> On Wed, 14 Sep 2005 09:12:28 +0800
> Kai Jones <kai.jones at broome.wa.gov.au> wrote:
> 
> 
>>Running "dmesg | grep sd" gives the following:
>>
>>Attached scsi disk sda at scsi0, channel 1, id 0, lun 0
>>SCSI device sda: 71127040 512-byte hdwr sectors (36417 MB)
>>  sda: sda1 sda2 < sda5 sda6 sda7 >
>>
>>I'm guessing this means sda, sda1 and sda2 are the active RAID1 and 
>>sda5, 6 and 7 are spares because it doesn't yet know the 3 x 73 gig 
>>disks are plugged in and waiting ?
> 
> <snip>
> 
> <disclaimer> 
> I haven't really been following this thread but, 
> </disclaimer>
> 
> As my understanding of the partitions in linux goes, sda is a single
> drive (The first scsi drive <s>csi <d>isk <a>, followed by sdb sdc
> etc) and sda1, sda2 are primary partitions 1 and 2 on the drive sda.
> sd5, sd6 ans sd7 are logical partitions 5, 6 and 7 inside the extended
> partition (usually the next available primary partition number?) 
> denoted by "< ... >".
>  If sda1 and sda2 are raided (mirrored) together, you are
> weakening the strength of having a raid system as the partitions are
> residing on the same drive. If you end up with bad sectors on one of the
> partitions you might be able to save your data, however, if your drive
> dies completely, you will loose everything. 
> 
> If you already knew all this then my apologies. I don't like to see
> people having false warm fuzzy feelings when their data might not be as
> save as they thought it was. 
> 
> Hope this helps, 
> 
> Regards

Ummm, lemme see if I can clear something up.

The current file system is not using three logical drives, on one 
physical disk.. It's using three physical drives, each drive mirrored to 
reflect the first drive which has three logical drives. So if one drive 
dies for whatever reason we still have two other, seperate, physical 
drives, that are identical and can keep working.

Does that make sense ? :)
Cheers

Kai