Booting from a large hard disk II

Four months later, I have again found the time to finish the research on the GPT. But first a little flashback to June outlining why the classic PC-style partition table has finally reached its end. Its 32 bit sector numbers preclude us from having partitions that reach beyond the 2 TB barrier.

The GUID partition table (GPT), has been invented to cope with disks that are larger than 2 TB. Unfortunately, legacy PCs (nearly every box sold today) cannot boot a “pure GPT”, since they lack the EFI firmware which can be used to boot from a pure GPT. EFI firmware has been developed to ultimately replace the PC BIOS and is already in use on Apple hardware. But most PCs sold today still have a “classic BIOS” and therefore need a workaround to boot from a disk partitioned with GPT.

On Linux, the tool to create a GPT and to partition the disk within the GPT is GNU parted. It is available with a text console front end which reminds of the classic Linux fdisk found in util-linux, and with a GTK and a QT front end. A curses-like front end as we know from util-linux’ cfdisk is unfortunately missing, which makes parted somewhat awkward to use at installation time when one doesn’t have a GUI yet (or did never plan to actually install a GUI on the machine). This article gives a nice introduction about how to use parted to create a GPT.

A disk having a GPT looks to a classic partitioning tool like this:

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1               1      267350  2147483647+  ee  GPT

this is a big primary partition of 2 TB size with partition ID 0xee. util-linux’ fdisk will also print prominent warnings that it won’t be able to partition that disk. Other partitioning tools probably won’t.

You need to cater for two caveats before the system can actually boot: grub 2 strongly suggests that you use a "BIOS Boot Partition " for grub 2’s later stages. That partition needs to be at last 32 KB large. Additionally, I wasn’t able to boot from a partition filling the entire 5.7 TB disk, so I suspect that the boot file system needs to be inside the first two TB. This is how my test disk was finally partitioned:

# parted /dev/sda print
Model: ATA VBOX HARDDISK (scsi)
Disk /dev/sda: 5243GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt

Number  Start   End     Size    File system  Name       Flags
 1      17.4kB  1000kB  983kB                bios_grub  bios_grub
 2      1000kB  1000GB  1000GB  ext3         boot
 3      1000GB  5243GB  4243GB  ext3         data

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It is kind of a challenge to coax parted into creating partitions that are directly adjacent to each other. I didn’t find a way to do this short of setting parted’s unit to sectors and manually typing the exact sector number.

After the disk was partitioned, I created the ext3 file systems on the two big partitions. mke2fs takes quite some memory (256 MB memory were not enough for the virtual machine, resulting into a not very helpful “killed” message on mke2fs) and time (like two hours for the larger of the two file systems), but generates the file systems just fine. They are mounted just as any other file system:

# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda2             917G  317M  870G   1% /mnt/sda2
/dev/sda3             3.8T  195M  3.7T   1% /mnt/sda3
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and grub 2 can be installed just fine by using grub-install /dev/sda after temporarily mounting /dev/sda2 as /boot.

This setup boots grml just fine, and I suspect that any Linux OS can be booted like this as well. I guess that it will be kind of a challenge do dual-boot a disk this large with any other OS.