I'm chasing issue with btrfs mounts under systemd
(https://github.com/systemd/systemd/issues/5781) - to summarize, systemd
waits for the final device that makes btrfs complete and mounts it using
this device name. But in /proc/self/mountinfo we actually see another
device name. Due to peculiarities of systemd implementation this device
"does not exist" from systemd PoV.
Looking at btrfs code I start to suspect that we actually do not know
what device was used to mount it at all. I.e.
static int btrfs_show_devname(struct seq_file *m, struct dentry *root)
{
...
while (cur_devices) {
head = &cur_devices->devices;
list_for_each_entry(dev, head, dev_list) {
if (dev->missing)
continue;
if (!dev->name)
continue;
if (!first_dev || dev->devid < first_dev->devid)
first_dev = dev;
}
cur_devices = cur_devices->seed;
}
if (first_dev) {
rcu_read_lock();
name = rcu_dereference(first_dev->name);
seq_escape(m, name->str, " \t\n\\");
rcu_read_unlock();
...
So we always show device with the smallest devid, irrespectively of what
device was actually used to mount it.
Am I correct? What I have here is
localhost:~ # ll /dev/disk/by-label/
total 0
lrwxrwxrwx 1 root root 10 Apr 30 08:03 Storage -> ../../dm-1
localhost:~ # systemctl --no-pager status thin.mount
● thin.mount - /thin
Loaded: loaded (/etc/fstab; generated; vendor preset: disabled)
Active: active (mounted) since Sun 2017-04-30 08:03:07 MSK; 6min ago
Where: /thin
What: /dev/dm-0
Docs: man:fstab(5)
man:systemd-fstab-generator(8)
Process: 982 ExecMount=/usr/bin/mount /dev/disk/by-label/Storage /thin
-t btrfs (code=exited, status=0/SUCCESS)
Tasks: 0 (limit: 4915)
CGroup: /system.slice/thin.mount
Apr 30 08:03:07 localhost systemd[1]: Mounting /thin...
Apr 30 08:03:07 localhost systemd[1]: Mounted /thin.
bur mountinfo shows
localhost:~ # grep /thin /proc/self/mountinfo
96 59 0:63 / /thin rw,relatime shared:47 - btrfs /dev/dm-0
rw,space_cache,subvolid=5,subvol=/
which matches the above algorithm
localhost:~ # btrfs fi show /thin
Label: 'Storage' uuid: a6f9dd05-460c-418b-83ab-ebdf81f2931a
Total devices 2 FS bytes used 640.00KiB
devid 1 size 20.00GiB used 2.01GiB path /dev/mapper/vg01-storage1
devid 2 size 10.00GiB used 2.01GiB path /dev/mapper/vg01-storage2
localhost:~ # ll /dev/mapper/
total 0
crw------- 1 root root 10, 236 Apr 30 08:03 control
lrwxrwxrwx 1 root root 7 Apr 30 08:03 vg01-storage1 -> ../dm-0
lrwxrwxrwx 1 root root 7 Apr 30 08:03 vg01-storage2 -> ../dm-1
The original device is presumably stored in kernel somewhere but I do
not know how can I query it?
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