[dm-devel] What is the deal with the partition separator?

[Hannes Reinecke]

On 02/16/2011 04:21 AM, Phillip Susi wrote:
> It used to be that partitions device names just had a digit added on to
> the base disk device name.  It seems that this became problematic at
> some point with device mapper and oddly named disks, and there have been
> several responses to it:
> 
> 1)  dmraid and (lib)parted now always add a 'p' between the base name
> and the partition number
> 
> 2)  kpartx from multipath-tools adds the 'p' only if the base name ends
> in a digit
> 
> 3)  Debian and Ubuntu's udev and init scripts tell kpartx to use 'part'
> instead of just 'p'.
> 
> 4)  gparted now explicitly tells dmraid to not use any character so that
> it behaves like older versions and is therefore compatible with the
> kpartx method that gparted has adopted, at least when the base name does
> not end with a digit.
> 
> Each of these components needs to agree on what the correct name is or
> chaos ensues.  I would like to discuss the merits of each and try to
> decide on a standard.
> 
> Having thought about it for a moment, it seems to me that deciding on
> always adding the 'p' is the way to go, since the 'art' just makes
> things longer for no good reason, and if you only sometimes add the 'p'
> then you can't tell if a device name that ends in a digit that does not
> follow a 'p' is a whole disk, or a partition.
> 
No.

The linux scheme since the dawn of time is to
a) Add the partition number to the device node name
b) If last letter of the device node name is a number, insert a 'p'
between device node name and partition number

with the advent of persistent device names (via udev) the partition
separator (for persistent links only!) is '-part'.

So you have

/dev/sda
/dev/sda1
/dev/disk/by-id/scsi-0WhatAStupidName
/dev/disk/by-id/scsi-0WhatAStupidName-part1

persistent device names are longish anyway, so we can as well use
something readable for partitions.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare at suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
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