[sclug] Raiding partitions

[Matt]

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), or is it best done using software RAID?
> 
> 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).

This probably rules out using software RAID 100% then, as Windows
wouldn't be able to access the software RAID that Linux created, and
(assuming Windows has software RAID) vice versa. They tend to be
OS-specific.

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. 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.

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.

> Is this the right approach, and how should I proceed? Any pitfalls to 
> look out for?

You could use software RAID for your Linux /home partition and a cron
job to backup your Windows partition. If you don't access/modify your
Windows data under Linux, then you can simply perform this once when
booting into Linux.

> BTW If you get the temptation to buy an IBM/Hitachi DeskStar drive, 
> don't. I've twice in succession had mine fail under warranty within 2 
> years. Must be the most unreliable drives I've ever come across.

These will be the ones that have adopted the moniker "DeathStar" then
;-)

HTH

Matt