Re: ATARAID userspace configuration tool

On Tuesday February 10, thomas@horsten.com wrote:

Hi,

I'm writing a userspace utility to detect/configure Medley (and later
other) ataraid devices in 2.6.

...

On top of this it would be useful to make the underlying devices
inaccessible after the mapped device is created (to prevent people from
doing things like fdisk /dev/hda, when what they really wanted was
something like fdisk /dev/ataraid/disc).

The best way to avoid this sort of problem is to change "fdisk" (and
mkfs and fsck and ...) to (optionally) open the device with O_EXCL.

In 2.6, a device that is "claimed" by a kernel subsystem - ie is
mounted, or is part of an MD or DM array, or has a partition which is
claimed in one of these ways, cannot be opened O_EXCL.

So these tools that operate on block devices and expect exclusive
access should ask for it.
They probably should have a way to not use O_EXCL if the admin
promises they know what they are doing, as fsck does need to run on a
mounted partition some times, and fdisk can reasonably be used on
devices with mounted partitions.  But the default should be O_EXCL.

NeilBrown

Detecting the partition table in userspace would fix this, but it's not
planned before 2.7 and I don't think it is safe to leave the false
partitions exposed.

- Some RAID types will need (I think) to use the MD framework as well as
DM (e.g. RAID0+1), so the device the users would be the md device which
would be composed of two dm devices. Is there a way to hide the underlying
dm devices from the user so he they only see the ones they should use (or
prevent these from being used directly some other way)?

// Thomas

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