[sclug] Raiding partitions

[Alex Butcher]

On Sat, 9 Apr 2005, Matt wrote:

[lots of good stuff]

> On Sat, 2005-04-09 at 13:47, Neil Haughton wrote:
>> Can anyone help me with some advice?
>>
>> Following a yet another hard drive disaster (head crash) I'm doing
>> something to prevent the anguish  from re-occurring. Can anyone advise
>> me what I need to do to set up partition-partition mirroring?
>>
>> I have two drives now, an 80G drive ADA-133 (/dev/hda) which has a 40G
>> partition for my home partition, and a 40G ATA-100 drive (/dev/hdc),
>> which I want to use as a mirror for the home partition on hda. I'm not
>> bothered about mirroring the other 40Gb 'system and apps' partition.
>>
>> Can this done with an IDE PCI RAID card (I've seen a cheap PCI IFDE RAID
>> card in Maplins)

The Promise RAID solutions only work on a whole-drive basis. I expect that
this will be the same for competing manufacturers' cards.

>, or is it best done using software RAID?

Well, as Matt said, the cheap RAID cards are effectively software RAID
anyway (the RAIDing is done in the OS driver). Again, as Matt said, Linux's
software RAID can't be accessed by Windows, and I don't think Windows' RAID
is readable by Linux (though this will probably be what happens first out of
the two!). Furthermore, Linux's software RAID requires you to encapsulate
your partitions inside a special RAID partition type, so you'd need to
backup your data, re-partition both drives, RAID them, then restore the
data. If you don't want to do that...

>> BTW I'm using Mandrake 10.0 and WinXP Pro (dual boot), so my 40G 'home'
>> partitions are each really two partitions, a 20G Linux /home and a 20G
>> Windows 'drive D:'. I was hoping to get both pairs of partitions to be
>> synchronised when I run Linux (which is most of the time).

...then I'd suggest having a script which unmounts the partitions and uses
dd to copy one to the other. You'll need to make sure that your 40G drive is
actually slightly larger than the two 20G partitions.

[snip]

> Cheap IDE PCI RAID cards are not real RAID cards, as a proper one
> handles all of the RAID functionality and just presents the
> mirrored/striped/... volume to the system which to all intents looks
> like a regular disk. These cheaper ones require the OS to do some of the
> RAID function which means you need both of your OS' to be able to drive
> your card as a RAID card. Linux tends to just use these as extra IDE
> controllers, although I think it can work with certain models.

Promise have drivers to allow their RAID format to work with Linux, but
IMHO, it's not worth the hassle - it forces you to stick to certain kernel
binaries, is less portable, and offers no performance gains. The only reason
to use it, IMHO, is if you have a pre-existing Promise RAID setup which you
wish to continue to use, or migrate data from.

> Also, I don't know of (m)any RAID controllers that can mirror just a
> single partition, they tend to deal only with whole disks.

Ditto.

> Alternatively, you can do your own pretend mirroring under Linux using a
> cron-driven script to use something like rsync to periodically copy the
> data, using a frequency of your own choosing. Downsides are running this
> too frequently will create extra load on your machine, and too long a
> period and you have the potential to lose data.

Probably best to unmount whilst you're doing this.

>> Is this the right approach, and how should I proceed?

IMHO, the best approach at this sort of budget is to buy two equally sized
discs and use each OS's native software RAID. The downside is the money
required to do that. :-/

> Matt

Best Regards,
Alex.
-- 
Alex Butcher      Brainbench MVP for Internet Security: www.brainbench.com
Bristol, UK                      Need reliable and secure network systems?
PGP/GnuPG ID:0x271fd950                         <http://www.assursys.com/>