2014-12-02 8:50 GMT+01:00 Goffredo Baroncelli <kreij...@inwind.it>:
On 12/02/2014 01:15 AM, MegaBrutal wrote:
2014-12-02 0:24 GMT+01:00 Robert White <rwh...@pobox.com>:
On 12/01/2014 02:10 PM, MegaBrutal wrote:
Since having duplicate UUIDs on devices is not a problem for me since
I can tell them apart by LVM names, the discussion is of little
relevance to my use case. Of course it's interesting and I like to
read it along, it is not about the actual problem at hand.
Which is why you use the device= mount option, which would take LVM names
and which was repeatedly discussed as solving this very problem.
Once you decide to duplicate the UUIDs with LVM snapshots you take up the
burden of disambiguating your storage.
Which is part of why re-reading was suggested as this was covered in some
depth and _is_ _exactly_ about the problem at hand.
Nope.
root@reproduce-1391429:~# cat /proc/cmdline
BOOT_IMAGE=/vmlinuz-3.18.0-031800rc5-generic
root=/dev/mapper/vg-rootlv ro
rootflags=device=/dev/mapper/vg-rootlv,subvol=@
Observe, device= mount option is added.
device= options is needed only in a btrfs multi-volume scenario.
If you have only one disk, this is not needed
I know. I only did this as a demonstration for Robert. He insisted it
will certainly solve the problem. Well, it doesn't.
root@reproduce-1391429:~# ./reproduce-1391429.sh
#!/bin/sh -v
lvs
LV VG Attr LSize Pool Origin Data% Move Log Copy% Convert
rootlv vg -wi-ao--- 1.00g
swap0 vg -wi-ao--- 256.00m
grub-probe --target=device /
/dev/mapper/vg-rootlv
grep " / " /proc/mounts
rootfs / rootfs rw 0 0
/dev/dm-1 / btrfs rw,relatime,space_cache 0 0
lvcreate --snapshot --size=128M --name z vg/rootlv
Logical volume "z" created
lvs
LV VG Attr LSize Pool Origin Data% Move Log Copy% Convert
rootlv vg owi-aos-- 1.00g
swap0 vg -wi-ao--- 256.00m
z vg swi-a-s-- 128.00m rootlv 0.11
ls -l /dev/vg/
total 0
lrwxrwxrwx 1 root root 7 Dec 2 00:12 rootlv -> ../dm-1
lrwxrwxrwx 1 root root 7 Dec 2 00:12 swap0 -> ../dm-0
lrwxrwxrwx 1 root root 7 Dec 2 00:12 z -> ../dm-2
grub-probe --target=device /
/dev/mapper/vg-z
grep " / " /proc/mounts
rootfs / rootfs rw 0 0
/dev/dm-2 / btrfs rw,relatime,space_cache 0 0
What /proc/self/mountinfo contains ?
Before creating snapshot:
15 20 0:15 / /sys rw,nosuid,nodev,noexec,relatime - sysfs sysfs rw
16 20 0:3 / /proc rw,nosuid,nodev,noexec,relatime - proc proc rw
17 20 0:5 / /dev rw,relatime - devtmpfs udev
rw,size=241692k,nr_inodes=60423,mode=755
18 17 0:12 / /dev/pts rw,nosuid,noexec,relatime - devpts devpts
rw,gid=5,mode=620,ptmxmode=000
19 20 0:16 / /run rw,nosuid,noexec,relatime - tmpfs tmpfs
rw,size=50084k,mode=755
20 0 0:17 /@ / rw,relatime - btrfs /dev/dm-1 rw,space_cache
<----- THIS!
21 15 0:20 / /sys/fs/cgroup rw,relatime - tmpfs none rw,size=4k,mode=755
22 15 0:21 / /sys/fs/fuse/connections rw,relatime - fusectl none rw
23 15 0:6 / /sys/kernel/debug rw,relatime - debugfs none rw
24 15 0:10 / /sys/kernel/security rw,relatime - securityfs none rw
25 19 0:22 / /run/lock rw,nosuid,nodev,noexec,relatime - tmpfs none
rw,size=5120k
26 19 0:23 / /run/shm rw,nosuid,nodev,relatime - tmpfs none rw
27 19 0:24 / /run/user rw,nosuid,nodev,noexec,relatime - tmpfs none
rw,size=102400k,mode=755
28 15 0:25 / /sys/fs/pstore rw,relatime - pstore none rw
29 20 253:1 / /boot rw,relatime - ext2 /dev/vda1 rw
After creating snapshot:
15 20 0:15 / /sys rw,nosuid,nodev,noexec,relatime - sysfs sysfs rw
16 20 0:3 / /proc rw,nosuid,nodev,noexec,relatime - proc proc rw
17 20 0:5 / /dev rw,relatime - devtmpfs udev
rw,size=241692k,nr_inodes=60423,mode=755
18 17 0:12 / /dev/pts rw,nosuid,noexec,relatime - devpts devpts
rw,gid=5,mode=620,ptmxmode=000
19 20 0:16 / /run rw,nosuid,noexec,relatime - tmpfs tmpfs
rw,size=50084k,mode=755
20 0 0:17 /@ / rw,relatime - btrfs /dev/dm-2 rw,space_cache
<----- WTF?!
21 15 0:20 / /sys/fs/cgroup rw,relatime - tmpfs none rw,size=4k,mode=755
22 15 0:21 / /sys/fs/fuse/connections rw,relatime - fusectl none rw
23 15 0:6 / /sys/kernel/debug rw,relatime - debugfs none rw
24 15 0:10 / /sys/kernel/security rw,relatime - securityfs none rw
25 19 0:22 / /run/lock rw,nosuid,nodev,noexec,relatime - tmpfs none
rw,size=5120k
26 19 0:23 / /run/shm rw,nosuid,nodev,relatime - tmpfs none rw
27 19 0:24 / /run/user rw,nosuid,nodev,noexec,relatime - tmpfs none
rw,size=102400k,mode=755
28 15 0:25 / /sys/fs/pstore rw,relatime - pstore none rw
29 20 253:1 / /boot rw,relatime - ext2 /dev/vda1 rw
So it's consistent with what /proc/mounts reports.
And more important question: it is only the value
returned by /proc/mount wrongly or also the filesystem
content is affected ?
I quote my bug report on this:
"The information reported in /proc/mounts is certainly bogus, since
still the origin device is being written, the kernel does not actually
mix up the devices for write operations, and such, the phenomenon does
not cause data corruption. (I did an entire distro release upgrade
while the conditions were present, and I centainly would have suffered
severe data corruption otherwise. Fortunately, the origin device had
the new distro, and the snapshot device had the old one, so besides
the mixup in /proc/mounts, no actual damage happened.)"
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