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Guide to overcome the 2TB limitation for CentOS 6.7 at Hetzner
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Installing CentOS 6.7 64 bit on Hetzner with partitions larger than 2TB.md

Installing CentOS 6.7 on Hetzner with partitions larger than 2TB

Install the CentOS 6.7 64 bit minimal

  • Start the server in Rescue mode [1],
  • Type
$ installimage
  • Select CentOS -> CentOS-67-64-minimal
  • Configure the installation by modifying the configuration file, here's an example:
## ======================================================
##  Hetzner Online GmbH - installimage - standard config
## ======================================================
DRIVE1 /dev/sda
DRIVE2 /dev/sdb
DRIVE3 /dev/sdc
DRIVE4 /dev/sdd

SWRAID 1
SWRAIDLEVEL 5

BOOTLOADER grub

HOSTNAME <hostname>

PART swap swap 16G
PART /boot ext3 512M
PART / ext4 100G
PART /data ext4 all

IMAGE /root/.oldroot/nfs/install/../images/CentOS-67-64-minimal.tar.gz

Note: The / partition here should not be larger than 2TiB.

  • Type F10 and Save,
  • The script will show the message:
CentOS only supports MS-DOS partition tables when using grub. We changed the space of your "all" partition to match the 2TiB limit.
  • Press Enter and go ahead,
  • The script will ask you to delete the data on the disks, click on 'Yes' and go ahead
  • After the installation, type:
$ reboot

Convert MBR to GPT

  • Access via SSH the server,
  • Umount the 'all' partition (here /data) and comment the /etc/fstab entry:
$ umount /data
$ grep data /etc/fstab
#/dev/md/3 /data ext4 defaults 0 0
  • Remove the RAID device:
$ mdadm --stop /dev/md3
$ mdadm --remove /dev/md3
  • Type the command:
$ gdisk /dev/sda
  • Follow the instructions, as shown below:
GPT fdisk (gdisk) version 0.8.10

Partition table scan:
  MBR: MBR only
  BSD: not present
  APM: not present
  GPT: not present


***************************************************************
Found invalid GPT and valid MBR; converting MBR to GPT format
in memory. THIS OPERATION IS POTENTIALLY DESTRUCTIVE! Exit by
typing 'q' if you don't want to convert your MBR partitions
to GPT format!
***************************************************************


Command (? for help): w

Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING
PARTITIONS!!

Do you want to proceed? (Y/N): Y
OK; writing new GUID partition table (GPT) to /dev/sda.
Warning: The kernel is still using the old partition table.
The new table will be used at the next reboot.
The operation has completed successfully.

  • Perform the previous command and instructions for all the disks,

  • Reinstall the boot loader:

$ grub-install /dev/sda
Installation finished. No error reported.
This is the contents of the device map /boot/grub/device.map.
Check if this is correct or not. If any of the lines is incorrect,
fix it and re-run the script `grub-install'.

(hd0) /dev/sda
(hd1) /dev/sdb
(hd2) /dev/sdc
(hd3) /dev/sdd

  • Perform the previous command for all the disks,

  • Reboot:

reboot

Prepare the new partitions

  • Type:
$ gdisk /dev/sda
  • Follow the instructions, as shown below:
GPT fdisk (gdisk) version 0.8.10

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): p
Disk /dev/sda: 11721045168 sectors, 5.5 TiB
Logical sector size: 512 bytes
Disk identifier (GUID): 6D41909E-8BB0-4FD1-8649-E87690A0A995
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 11721045134
Partitions will be aligned on 2048-sector boundaries
Total free space is 7323667561 sectors (3.4 TiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048        33556480   16.0 GiB    FD00  Linux RAID
   2        33558528        34607104   512.0 MiB   FD00  Linux RAID
   3        34609152       104513536   33.3 GiB    FD00  Linux RAID
   5       104517632      4397387776   2.0 TiB     FD00  Linux RAID

Command (? for help): d
Partition number (1-5): 5

Command (? for help): n
Partition number (4-128, default 4): 5
First sector (34-11721045134, default = 104515584) or {+-}size{KMGTP}: 104517632
Last sector (104517632-11721045134, default = 11721045134) or {+-}size{KMGTP}:
Current type is 'Linux filesystem'
Hex code or GUID (L to show codes, Enter = 8300): FD00
Changed type of partition to 'Linux RAID'

Command (? for help): w

Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING
PARTITIONS!!

Do you want to proceed? (Y/N): Y
OK; writing new GUID partition table (GPT) to /dev/sda.
Warning: The kernel is still using the old partition table.
The new table will be used at the next reboot.
The operation has completed successfully.
  • Perform the previous command and instructions for all the disks,
  • Reboot:
$ reboot

Create the RAID device and format the partition

  • Create the RAID device:
$ mdadm --create --verbose /dev/md3 --level=5 --raid-devices=4 /dev/sda5 /dev/sdb5 /dev/sdc5 /dev/sdd5

Note. The actual number of devices for the RAID will depend on the avaialable disks, as well as the RAID level.

  • Update accordingly the file /etc/mdadm.conf based on the output of the command:
$ mdadm --detail --scan
  • Generate the new initramfs:
mv /boot/initramfs-$(uname -r).img /boot/initramfs-$(uname -r).img.old
dracut --mdadmconf --force /boot/initramfs-$(uname -r).img $(uname -r)
  • Create the filesystem:
$ mkfs.ext4 /dev/md3
  • Comment out the /etc/fstab:
$ grep data /etc/fstab
/dev/md3 /data ext4 defaults 0 0
  • Et voilà!

Final notes

If you have a partition larger than 16TiB, the mkfs.ext4 provided by the CentOS distribution will not work, exiting with the message:

$ mkfs.ext4 /dev/md3
mke2fs 1.41.12 (17-May-2010)
mkfs.ext4: Size of device /dev/md3 too big to be expressed in 32 bits
	using a blocksize of 4096.

The solution is to compile and execute the latest version of e2fsprogs Ext2/3/4 file system utilities:

$ yum install gcc
$ curl -L -o e2fsprogs-v1.42.13.tar.gz https://github.com/tytso/e2fsprogs/archive/v1.42.13.tar.gz
$ tar xvzf e2fsprogs-v1.42.13.tar.gz
$ cd e2fsprogs-1.42.13/
$ ./configure --prefix=<your prefix>
$ make
$ make install
$ cd <your prefix>/sbin
$ ./mkfs.ext4 /dev/md3

[1] http://wiki.hetzner.de/index.php/Hetzner_Rescue-System/en

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