Gentoo Archives: gentoo-dev

From: Duncan <1i5t5.duncan@×××.net>
To: gentoo-dev@l.g.o
Subject: [gentoo-dev] Re: newsitem: unmasking udev-181
Date: Mon, 12 Mar 2012 09:03:19
Message-Id: pan.2012.03.12.09.02.05@cox.net
In Reply to: Re: [gentoo-dev] Re: newsitem: unmasking udev-181 by "Robin H. Johnson"
1 Robin H. Johnson posted on Sun, 11 Mar 2012 23:14:46 +0000 as excerpted:
2
3 > On Sun, Mar 11, 2012 at 11:03:50PM +0000, Duncan wrote:
4 >> Meanwhile, also note that there's PARTLABEL, PARTUUID and ID, that the
5 >> mount manpage promises to honor. I've not used these myself, but there
6 >> was a thread on the btrfs list discussing GPT format and users of its
7 >> partition-labels (as opposed to filesystem labels), that pointed out
8 >> that mount honors these, since it internally uses the udev symlinks
9 >> mechanism to support (fs) labels, etc, so they get support for
10 >> gpt-partition- labels, etc, essentially "for free".
11
12 > What manpage are you reading?
13 > # man 8 mount |grep PART # man 2 mount |grep PART Nada.
14 >
15 > When the blkid tool can read PARTUUID/PARTLABEL, then it will just work
16 > with genkernel, as we use blkid for doing that.
17
18 mount (8) under device indication:
19
20 >>>>>
21
22 Most devices are indicated by a file name (of a block special device),
23 like /dev/sda1, but there are other possibilities. [...] It is possible
24 to indicate a block special device using its volume LABEL or UUID (see
25 the -L and -U options below).
26
27 The recommended setup is to use LABEL=<label> or UUID=<uuid> tags rather
28 than /dev/disk/by-{label,uuid} udev symlinks in the /etc/fstab file. The
29 tags are more readable, robust and portable. The mount(8) command
30 internally uses udev symlinks, so use the symlinks in /etc/fstab has no
31 advantage over LABEL=/UUID=. For more details see libblkid(3).
32
33 <<<<<
34
35 As I said, it wasn't apparent to me until someone pointed it out to me on
36 the btrfs list, but apparently, mount understands SOMETHING= as
37 referencing /dev/disk/by-something, using those symlinks internally, so
38 while the manpage doesn't specifically mention PARTLABEL, etc, according
39 to that person, it "just works". Upon seeing that claim, I reread the
40 manpage, and sure enough, that meaning can be seen "between the lines" if
41 you already know to look for it.
42
43 I had intended to try it, since I use gptfdisk and gpt partitions pretty
44 much universally now, and referencing the PARTLABEL would have meant that
45 I could for instance do a mkfs and redo my backup partitions without
46 having to update fstab's labels because I could use the partlabels
47 instead. Unfortunately, when I actually checked to see what symlinks
48 udev was putting in /dev/disk/by-partlabel, while indeed the gpt
49 partlabels for the physical disks were there, the partlabels for the
50 gpt-partitioned md/raid devices were NOT, and that's what I actually
51 needed, so unfortunately I couldn't try using partlabels after all.
52 That's why I've yet to actually verify the claim.
53
54 At some point I'll probably verify it with a USB attached external drive,
55 as it's my last-resort backup, and/or on my netbook, with only one drive
56 so no raid, but I've not gotten that far, yet.
57
58
59 FWIW, the thread started with someone complaining that a btrfs label on a
60 multi-device filesystem (since btrfs can do that) was attached to the
61 filesystem, NOT the device/partition. Various people pointed out that
62 it's a filesystem label and that btrfs thus had it correct. Meanwhile,
63 on one subthread I pointed out gpt partition labels as an alternative,
64 but said I didn't think Linux could actually do much with them yet.
65 That's when someone else replied that it could do more than I thought,
66 mount and fstab handled partlabel, and he thought the kernel root=
67 parameter could take it as well.
68
69 Here's his post on gmane:
70
71 http://permalink.gmane.org/gmane.comp.file-systems.btrfs/16023
72
73 As I said, after reading that, rereading the mount (8) manpage, it /did/
74 seem to hint that it should do so even if it doesn't outright say it,
75 since it specifically mentions using udev's symlinks internally.
76
77 But as I've not tried it yet I have only his post and my reparsing of
78 that manpage based on it, to go on. Is it incorrect?
79
80 --
81 Duncan - List replies preferred. No HTML msgs.
82 "Every nonfree program has a lord, a master --
83 and if you use the program, he is your master." Richard Stallman