[systemd-devel] [PATCH] rules: add by-parttypeuuid rule for GPT labeled partitions

[Kay Sievers]

On Sat, May 10, 2014 at 12:34 AM, Sage Weil <sage at inktank.com> wrote:
> On Sat, 10 May 2014, Kay Sievers wrote:
>> On Sat, May 10, 2014 at 12:00 AM, Sage Weil <sage at inktank.com> wrote:
>> > On Fri, 9 May 2014, Kay Sievers wrote:
>> >> On Fri, May 9, 2014 at 11:31 PM, Sage Weil <sage at inktank.com> wrote:
>> >> > The Ceph OSD initialization relies on identifying GPT partitions by type
>> >> > in order to mount data volumes and start daemons.  Currently we ship this
>> >> > rule separately, but it is awkward to duplicate the conditional logic that
>> >> > precedes this block and it would be much simpler if it were simply included
>> >> > in the upstream rules.
>> >>
>> >> Types are by definition not unique. The symlinks in /dev/disk/by-*/
>> >> are *expected* to be unique.
>> >>
>> >> We handle duplicated labels, but they are specified by humans,
>> >> multiple partitions with the same GPT types are just normal expected
>> >> behavior; and they would have no order or priority, they just
>> >> overwrite each other depending on probing order.
>> >
>> > This is why the label has both the type (fixed, to identify this as a ceph
>> > partition) and the label (random):
>> >
>> >  /dev/disk/by-parttypeuuid/$env{ID_PART_ENTRY_TYPE}.$env{ID_PART_ENTRY_UUID}
>> >
>> >> We should not add such things, the logic to find these volumes at
>> >> bootup are better handled by a specific program like systemd's
>> >> systemd-gpt-auto-generator, without putting unreliable and
>> >> unpredictable content into /dev.
>> >
>> > I think this is what we're trying to accomplish with the ceph-disk tool,
>> > which relies on these (reliable and predictable) symlinks.  The labels
>> > alone (by-partuuid) aren't sufficient since we want to be able to scan for
>> > partitions of a given type without re-running blkid on every volume.
>>
>> /dev is an API which should by default not contain custom links which
>> are not generally useful, and these links are not useful for other
>> tools.
>
> FWIW I was surprised that there wasn't already a way to find partitions by
> type in /dev, but I assume you know better than I how other tools are
> using udev.  It seems at least as useful as by-partuuid to me.
>
>> These links are not even recognizable by type without doing readdir()
>> over it and string match operations to find the types, we really
>> should not add such stuff to the default rules set. We have to be
>> careful here, it seems like the wrong approach to put that in the
>> public visible /dev API.
>>
>> Tools can get all this information programatically out of the udev
>> database, there is no create symlinks or to run blkid.
>
> I just looked up libudev and it looks like there is even a pyudev wrapper,
> so that could indeed work better.  I take it that queries via
> udev_enumerate for (say) ID_PART_ENTRY_TYPE=x are efficient?

Sure, filter for "block" devices and this or other GPT properties. The
libudev API will just find the devices is /sys and read the database
files in tmpfs /run and, it will not talk to any devices, so it should
perform pretty well.

Kay