RAID

[Phillip Susi]

Anders Karlsson wrote:
> MD does that as well, I have such a setup. I would agree that if you
> want to gamble your data on a RAID0 setup, *then* the fakeraid may be
> your only option *if* you insist on striping across the whole disks
> and not keep your root/boot partition separate from the RAID0.
> 
> 
>>3) Can boot from volume types other than raid1 ( i.e. raid0 )
> 
> 
> See above.
> 

No, md definately does not support directly booting from the raid.  The 
bios and grub have no understanding of the md raid, they can only boot 
from a stand alone disk/partition.  If md is mirroring, then both disks 
are the same, so the bios+grub can boot from one as if it were a stand 
alone disk.  With raids other than raid1, this is not possible so you 
have to set up a seperate /boot that is either stand alone or mirrored.

> 
>>4) In the event that the primary drive in a mirror fails, the system
>>still boots using the second drive rather than hang trying to read the
>>first.
> 
> 
> That will hold true for MD as well. The whole point of mirroring is
> that both (all) copies are equal, and contain the same data.
> 

No, it won't because the bios only tries to boot the first disk as if it 
were not part of a mirror.  If that fails, the system doesn't boot.  The 
bios and grub can not fail over to the second disk on error 
automatically like the kernel md driver can, so in the event that 
something goes wrong on the first drive, you will have to unplug the 
first drive and use only the second drive in the mirror to boot.

> 
> Why oh why these fakeraid cards are incapable of 'hiding' the raid
> members they have attached, and present logical devices through int
> 13h instead like sane raid controllers is beyond me. I do regret not
> calling bull on the adapter I bought, and either spending less on a
> normal pic ide card, or spending more getting a proper raid card.
> Each one to their own I guess.
> 

They ARE capable of presenting a single disk via int 13h, that is their 
entire point.  That is what makes them hardware (fake)raid and is what 
allows them to be bootable.  In windows you load a driver that also 
presents a single disk to the OS, but in linux the driver only shows the 
individual disks, and leaves the raiding up to the device mapper.