Storage / Stage 4 of 11 · step 6 of 7

Move the data to a new disk

~25 min

Not running Starting usually ~20 s

Situation

The application keeps its orders in /srv/data, and right now that directory sits on the root partition. A new empty disk has been attached to the server for the data. The server also has a disk with last year's archive.

Task

Make sure that:

the new disk has a GPT partition table with a partition on it;
the partition holds an ext4 filesystem labelled data;
/etc/fstab has a line for /srv/data by this partition's UUID with the nofail option;
the partition is mounted at /srv/data and comes back there after sudo umount /srv/data and sudo mount -a;
the application's files are in /srv/data on the new disk, the same as they are now.

Constraints

Leave the archive disk alone: its partitions, its filesystem and its files.

The tools you need are already installed. The machine has no Internet access.

Lab goals

0 of 6

The new disk has a GPT table with a partition — not checked yet
The partition holds ext4 labelled data — not checked yet
/etc/fstab has a line for /srv/data by the partition's UUID, with nofail — not checked yet
The partition is mounted at /srv/data and comes back after umount and mount -a — not checked yet
The application's files are in /srv/data on the new disk — not checked yet
The archive disk is untouched — not checked yet

Hints

0 of 3 open · they open one at a time

Hint 1

Show the solution after 3 failed checks

Storage and file systems · picked for this lab

fdisk and parted — partition a disk

Cheat sheet

$_ — insert into the terminal

lsblk which disks exist
sudo fdisk -l /dev/vdb the disk's partition table
sudo parted /dev/vdb print the same with parted
sudo parted -s /dev/vdb mklabel gpt new table (wipes the old one)
sudo parted -s /dev/vdb mkpart data ext4 1MiB 100%
sudo parted /dev/vdb unit MiB print free partitions and free space
sudo fdisk /dev/vdb step by step: g, n, w
sudo partprobe /dev/vdb kernel re-reads the table

parted has few options; the work is done by commands: mklabel, mkpart, print, rm, resizepart, name. Inside fdisk you type one-letter commands; m lists them. The full lists are in man parted and man fdisk.

Partition an empty disk in one command

bash

lsblk /dev/vdb                       # right disk? empty?
sudo parted -s /dev/vdb mklabel gpt mkpart data ext4 1MiB 100%
lsblk /dev/vdb                       # vdb1 has appeared

The partition starts at 1 MiB: the table sits at the start of the disk, and this offset keeps the partition aligned to the disk's blocks. 100% means up to the end. This is a 512 MiB practice disk after the command:

text

student@ubuntu:~$ sudo parted /dev/loop0 unit MiB print free
Model: Loopback device (loopback)
Disk /dev/loop0: 512MiB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:

Number  Start    End      Size     File system  Name  Flags
        0.02MiB  1.00MiB  0.98MiB  Free Space
 1      1.00MiB  511MiB   510MiB                data
        511MiB   512MiB   0.98MiB  Free Space

The small free pieces at both ends hold the GPT table and its backup copy.

Two partitions on one disk

bash

sudo parted -s /dev/vdb mklabel gpt
sudo parted -s /dev/vdb mkpart data ext4 1MiB 50%
sudo parted -s /dev/vdb mkpart logs ext4 50% 100%

Add a partition in free space

bash

sudo parted /dev/vdb unit MiB print free    # where the Free Space is
sudo parted -s /dev/vdb mkpart logs ext4 2048MiB 100%

No mklabel here: it would replace the table and drop the existing partitions.

The same in fdisk, step by step

text

sudo fdisk /dev/vdb
  g        new empty GPT table (o — MBR)
  n        new partition: number, start, end — Enter
  p        preview the result
  w        write and quit (q — quit without writing)

For the end you can type a size such as +10G instead of Enter.

An MBR table for an older system

bash

sudo parted -s /dev/vdb mklabel msdos mkpart primary ext4 1MiB 100%

On MBR the first word after mkpart is the partition type (primary), not a name.

The kernel does not see the new partitions

bash

sudo partprobe /dev/vdb      # re-read the table
lsblk /dev/vdb

Error: /dev/vdb: unrecognised disk label

The disk has no partition table yet — the normal state of a new disk: print shows Partition Table: unknown. Create a table: sudo parted -s /dev/vdb mklabel gpt.

mount: wrong fs type, bad option, bad superblock

You are mounting a disk or partition that has no filesystem. Partitioning creates only partitions; mkfs creates the filesystem. More in mount.

The partition was created but there is no /dev/vdb1

The kernel has not re-read the table: the disk is busy, or the change was made behind its back. Run sudo partprobe /dev/vdb. On a loop device, partitions appear when it is attached with losetup -P — see loop device.

The kernel still uses the old table

fdisk wrote the table, but the kernel keeps the old one because a partition of this disk is mounted. Unmount the disk's partitions and run partprobe; the system disk picks up the change after a reboot.

Warning: The resulting partition is not properly aligned

The start is given in sectors or bytes that do not match the disk's blocks. Give boundaries in MiB or percent: 1MiB, 50%.

mklabel on a partitioned disk

mklabel writes a new empty table. The old partitions vanish from it, and with them access to the data. With -s, parted does not even ask. Before partitioning, check the disk name and its partitions with lsblk and findmnt.

The wrong disk

/dev/vdb on one machine and /dev/vdb on another (or after a reboot) can be different disks. Look at the size and contents in lsblk -f right before the command.

Move the data to a new disk

Situation

Task

Constraints

Signing in is needed to start the machine

Partition an empty disk in one command

Two partitions on one disk

Add a partition in free space

The same in fdisk, step by step

An MBR table for an older system

The kernel does not see the new partitions

Error: /dev/vdb: unrecognised disk label

mount: wrong fs type, bad option, bad superblock

The partition was created but there is no /dev/vdb1

The kernel still uses the old table

Warning: The resulting partition is not properly aligned

mklabel on a partitioned disk

The wrong disk