Re: [PATCH 0/3] sysfs representation of stacked devices (dm/md)

[Alasdair G Kergon]

On Fri, Feb 17, 2006 at 01:00:17PM -0500, Jun'ichi Nomura wrote:
> These patches provide common representation of dependencies
> between stacked devices (dm and md) in sysfs.

I'm neutral on this change so long as it can be done without 
introducing problems for device-mapper.

> Though md0, dm-0, dm-1 and sd[a-d] contain same LVM2 meta data,
> LVM2 should pick up md0 as PV, not dm-0, dm-1 and sdXs.
> mdadm should build md0 from dm-0 and dm-1, not from sdXs.
> Similar things will happen on 'mount' and 'fsck' if we use
> file system labels instead of LVM2.
 
I can't speak for the 'mount' code base, but I don't think it'll
make any significant difference to LVM2 - we'd still have to do 
all the same device scanning as we do now because we have to be
aware of md devices defined in on-disk metadata regardless of 
whether or not the kernel knows about them at the time the 
command is run.

> Currently, these relationships are determined by each tool
> combining information like the existence of md metadata
> and dm dependency ioctl.
 
And attempts to open a device exclusively.  That's one check LVM2 
does before running 'pvcreate' on a device.

> thus we only need to check "holders" directory of the device
> to decide whether the device is used by dm/md.
> Also we can walk down the "slaves" directories to collect
> the devices conposing the given dm/md device.

For device-mapper devices, 'dmsetup deps' and ls --tree already
gives you this information reasonably efficiently.

Would others find the proposal useful for non-dm devices?

And rather than adding code just to dm and md, would it be better 
to implement it by enhancing bd_claim()?
 
Alasdair
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