RAID

[Anders Karlsson]

On 12/31/05, Phillip Susi <psusi at cfl.rr.com> wrote:
> 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.

Just had a chat with a few friends dotted around the world (.no, .au
and .ca) and although LVM2 supposedly have raid1 capabilities, they
are far from implemented all the way. If you don't enable the mirror
at creation of the LV, you are not going to be able to create a mirror
for that LV. You can remove the mirror later, if you did enable it
when you created the LV that is.

Linux LVM has a loooong way to go before it comes even close to AIX
LVM in capabilities and functionality. The one handy thing you'd
expect from the LVM system, enable a mirror on the fly to migrate your
data for example, that is what it can not do.

You are correct though that LVM2 and device-mapper is the same thing.

> 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.

The hardware fakeraids are mostly a waste of time and effort. dmraid
may be able to manipulate them, but IMO (and I have such a card) you'd
be better off disabling the fakeraid and using MD.

> 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.

I'd hope so too, they are kind of handy... :)

--
Anders Karlsson <trudheim at gmail.com>