Google Why Mount /dev/root AND /dev/sda1 on / in Ubuntu 18.04 LTS Minimal Image

google-cloud-platformgoogle-compute-enginemountubuntu-18.04

Google is mounting two different disks on / in Ubuntu 18.04 LTS Minimal image. This uglifies VM monitoring since the disk labels are inconsistent from machine to machine depending on what type of Ubuntu image is used.

Normal Ubuntu 18.04 LTS image does NOT have this problem. /dev/sda1 shows up correctly as the ONLY disk mounted on /

BUT, on minimal image Ubuntu 18.04 LTS, Google is doing something wrong with the disks. It is mounting two different disks on the same mount path. /dev/root and /dev/sda1 are both mounted on /

df shows /dev/root mounted on /

Filesystem     1K-blocks   Used Available Use% Mounted on  
/dev/root        9983232 829708   9137140   9% /  
devtmpfs         1888736      0   1888736   0% /dev  
tmpfs            1890960      0   1890960   0% /dev/shm  
tmpfs            1890960    856   1890104   1% /run  
tmpfs               5120      0      5120   0% /run/lock  
tmpfs            1890960      0   1890960   0% /sys/fs/cgroup  
/dev/sda15        106858   3682    103177   4% /boot/efi  
/dev/loop0         90880  90880         0 100% /snap/core/7396  
/dev/loop1         67200  67200         0 100% /snap/google-cloud-sdk/99  
tmpfs             378192      0    378192   0% /run/user/1001

lsblk shows sda1 is ALSO mounted on /

NAME    MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT  
loop0     7:0    0 88.7M  1 loop /snap/core/7396  
loop1     7:1    0 65.6M  1 loop /snap/google-cloud-sdk/99  
sda       8:0    0   10G  0 disk   
├─sda1    8:1    0  9.9G  0 part  /  
├─sda14   8:14   0    4M  0 part   
└─sda15   8:15   0  106M  0 part /boot/efi

more info: df /dev/sda1 gives the wrong result:

Filesystem     1K-blocks  Used Available Use% Mounted on  
devtmpfs         1888736     0   1888736   0% /dev

I'd like to use /dev/sda1 as the disk because that's what all other Ubuntu images do on GCP. The minimal Ubuntu images are REALLY fast to boot up so it would s** to have to go back to full Ubuntu image.

Best Answer

/dev/root is not a device. It is a symbolic link to the device mounted as root.

Your system does not have a problem with mounting two disks as /.

readlink -f /dev/root will display the real device mounted as /, which will usually be /dev/sda1.

Related Solutions

Linux – Differences between /dev/sda and /dev/sda1

On a modern system, a partition device will only appear if the partition actually exists.

On a disk with an MBR partition table, partition numbers 1 through 4 correspond to the four slots in the partition table, called "primary" partitions. They don't have to be filled sequentially, so it's possible, for example, to have an sda2 but no sda1. Partition numbers 5 and up correspond to "logical drives" in an extended partition, and those are always numbered sequentially, so you can't have an sda6 without having an sda5 too.

On a disk with a GPT partition table, there can be many more (typically up to 128) partitions, and all are "primary". So you could have a disk whose only partition is sda9, for example.

If the disk has no partition table, then it'll have no partition devices, of course.

Older systems — those using a static /dev rather than one managed by udev — will typically have device nodes for all the possible partition numbers, regardless of whether the partitions actually exist. (Trying to open the device file for a nonexistent partition will fail, of course.)

It's possible to forego partitioning and put a filesystem directly on a disk. When you mount a block device, the filesystem driver typically looks for a superblock at a predetermined offset from the beginning of the device, and since the beginning of a partition is not the beginning of the disk itself, the superblock for a filesystem in a partition is located at a different place on the disk than the superblock for a filesystem created on the "whole-disk" device.

So if the disk used to just have a filesystem, and then it was partitioned and a filesystem was created in a partition, the old superblock might still be there, e.g. in the small gap before the beginning of the first partition. So the disk still appears to have a filesystem on both the raw disk device and on the partition device, because whichever one you try to mount, when the filesystem driver goes looking for the superblock it'll find one.

It's not actually safe to mount and use both filesystems, though, since they overlap on the disk. One may have important bookkeeping data in what the other thinks is free space. That's why it's a good idea to zero the beginning of a block device, to remove any unwanted superblocks, when you want to change a raw disk to a partitioned one, or vice versa, or change the type of filesystem used on a partition, etc.

Ubuntu 12.04 glusterfs volume failed to mount at boot time

A potential solution can be adding more nobootwait and direct-io-mode to fstab, try something like this:

     serverip:/vol  mountpoint  glusterfs  defaults,nobootwait,_netdev,direct-io-mode=disable  0       0

Also, check your /etc/init/mounting-glusterfs.conf and add:

     exec start wait-for-state WAIT_FOR=networking WAITER=mounting-glusterfs-$MOUNTPOINT

I hope this helps as I experienced similar issue in the past and solved using above combinations/configurations.

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