netdev - Re: [patch net-next rfc 3/7] net: rtnetlink: add commands to add and delete alternative ifnames

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Date:   Sat, 10 Aug 2019 17:50:42 +0200
From:   Michal Kubecek <mkubecek@...e.cz>
To:     netdev@...r.kernel.org
Cc:     Roopa Prabhu <roopa@...ulusnetworks.com>,
        Jiri Pirko <jiri@...nulli.us>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        David Ahern <dsahern@...il.com>, dcbw@...hat.com,
        Andrew Lunn <andrew@...n.ch>, parav@...lanox.com,
        Saeed Mahameed <saeedm@...lanox.com>,
        mlxsw <mlxsw@...lanox.com>
Subject: Re: [patch net-next rfc 3/7] net: rtnetlink: add commands to add and
 delete alternative ifnames

On Sat, Aug 10, 2019 at 06:46:57AM -0700, Roopa Prabhu wrote:
> On Fri, Aug 9, 2019 at 8:46 AM Michal Kubecek <mkubecek@...e.cz> wrote:
> >
> > On Fri, Aug 09, 2019 at 08:40:25AM -0700, Roopa Prabhu wrote:
> > > to that point, I am also not sure why we have a new API For multiple
> > > names. I mean why support more than two names  (existing old name and
> > > a new name to remove the length limitation) ?
> >
> > One use case is to allow "predictable names" from udev/systemd to work
> > the way do for e.g. block devices, see
> >
> >   http://lkml.kernel.org/r/20190628162716.GF29149@unicorn.suse.cz
> >
> 
> thanks for the link. don't know the details about alternate block
> device names. Does user-space generate multiple and assign them to a
> kernel object as proposed in this series ?. is there a limit to number
> of names ?. my understanding of 'predictable names' was still a single
> name but predictable structure to the name.

It is a single name but IMHO mostly because we can only have one name.
For block devices, udev uses symlinks to create multiple aliases based
on different naming schemes, e.g.

mike@...n:~> find -L /dev/disk/ -samefile /dev/sda2 -exec ls -l {} +
lrwxrwxrwx 1 root root 10 srp  5 21:47 /dev/disk/by-id/ata-WDC_WD30EFRX-68AX9N0_WD-WMC1T3114933-part2 -> ../../sda2
lrwxrwxrwx 1 root root 10 srp  5 21:47 /dev/disk/by-id/scsi-SATA_WDC_WD30EFRX-68A_WD-WMC1T3114933-part2 -> ../../sda2
lrwxrwxrwx 1 root root 10 srp  5 21:47 /dev/disk/by-id/scsi-SATA_WDC_WD30EFRX-68_WD-WMC1T3114933-part2 -> ../../sda2
lrwxrwxrwx 1 root root 10 srp  5 21:47 /dev/disk/by-id/scsi-0ATA_WDC_WD30EFRX-68A_WD-WMC1T3114933-part2 -> ../../sda2
lrwxrwxrwx 1 root root 10 srp  5 21:47 /dev/disk/by-id/scsi-1ATA_WDC_WD30EFRX-68AX9N0_WD-WMC1T3114933-part2 -> ../../sda2
lrwxrwxrwx 1 root root 10 srp  5 21:47 /dev/disk/by-id/scsi-350014ee6589cfea0-part2 -> ../../sda2
lrwxrwxrwx 1 root root 10 srp  5 21:47 /dev/disk/by-id/wwn-0x50014ee6589cfea0-part2 -> ../../sda2
lrwxrwxrwx 1 root root 10 srp  5 21:47 /dev/disk/by-partlabel/root2 -> ../../sda2
lrwxrwxrwx 1 root root 10 srp  5 21:47 /dev/disk/by-partuuid/71affb47-a93b-40fd-8986-d2e227e1b39d -> ../../sda2
lrwxrwxrwx 1 root root 10 srp  5 21:47 /dev/disk/by-path/pci-0000:00:11.0-ata-1-part2 -> ../../sda2
lrwxrwxrwx 1 root root 10 srp  5 21:47 /dev/disk/by-path/pci-0000:00:11.0-scsi-0:0:0:0-part2 -> ../../sda2

Few years ago, udev even dropped support for renaming block and
character devices (NAME="...") so that it now keeps kernel name and only
creates symlinks to it. Recent versions only allow NAME="..." for
network devices.

Michal