[wellylug] My new linux server

[Brent Wood]

You are right & wrong :-)

If your motherboard is FULLY HARDWARE RAID, then yes, it will work as you
describe. All hardware RAID is totally transparent to the OS.

Most motherboards & some low end RAID controllers are technically software
RAID, they require OS drivers to recognise & support the RAID controller, and
require cpu to help manage it. CPU is generally a plentiful resource, esp on a
file srver which doesn't use it for much else.

So the hardware RAID you are used to works as you describe. Don't expect quite
the same flexibility out of software RAID. It is cheaper, can even be faster in
some circumstances, but does require the OS to know about & support the RAID
capability.

Companies like Accusys & Arena make Linux compatible genuine hardware RAID
devices for high spec server RAID, which work very well, but are well into the
$1000s range. They offer the usual hot spare, hot swap, auto rebuild & are
genuinely OS transparent. Or 3ware & others with PCI hardware RAID controllers.
But 'tis cheaper to set up your SATA array at day one :-)

You may find by manually getting into the way Linux manages software RAID that
you can add drives/partitions, I haven't tried to get too deeply into it, just
played with some RAID setups to tweak Postgres performance.

Also have a look at Linux LVM capabilities. I haven't looked into this much-
others here have prob done more. 


Brent




> 
> Mmmm I am very confuscious.  I thought the OS was talking to the RAID
> controller and not actually aware of the disk subsystem below.  The
> physical arrangement of disks is abstracted from the OS and all it
> sees is a metadevice or a volume (or whatever you want it to be
> called).
> 
> Here's how my simple brain thinks of it:
> 
> I build the box with one drive.  I subsequently want to add another
> drive and want to mirror it.  I shut the machine down and physically
> insert the second drive and then via the BIOS utils I can select to
> mirror data from original drive to the second and voila I have a
> mirrored pair of disks.
>  
> OS comes up still talking to the RAID controller and sees the same old
> metadevice.
> 
> Am I living in fairy-tale land?
> 
> I've worked with some pretty high end RAID controllers before and this
> is how they have worked - but they have hot swap capabilities - dunno
> if I am expecting too much from my new motheboards RAID controller...