RAID

[Phillip Susi]

Anders Karlsson wrote:
> I really want to read the documentation that details how to set up
> RAID5 with just LVM2, so please post links to that documentation. I
> can not find them at all, but am very interested in how they do it.
> Similarly, I can not find any documentation on how to do RAID1 with
> just LVM2, so if you have links for that as well, they would be
> appreciated.
> 

Hrm... I just checked and it looks like there is NOT yet a raid5 module 
for the device mapper, only linear, stripe, mirror, and snapshot.  I am 
not very familliar with the lvm utility programs, but I'm sure that it 
exports the functionality availible in the underlying device mapper.

> I did locate one interesting document about device mapper being
> "capable of mapping block devices in various ways (eg. linear,
> striped, mirrored)" by Heinz Mauelshagen. The tool dmraid is required
> for this, much like mdadm is required for managing MD devices.
> 

dmraid is a utility that scans physical drives for the parameter block 
that the bios for hardware fakeraids writes to the disk, then configures 
the device mapper to access the raid volume.  LVM uses metadata stored 
on the disk in a different way, but also configures the kernel device 
mapper to access the volumes.  You can directly manipulate the device 
mapper with the dmsetup command.

> It may be that device mapper will take over from MD at some point in
> the future, but that seems a long way off considering the disparity in
> capabilities.
> 

Right now it looks like the only features md offers that dm does not are 
raid5 and raid6.  My guess is those will be forthcoming soon.