The Open Source Journal

Now that the era of 1 TB drives is upon us, we're rapidly closing in on a limitation imposed by the legacy x86 MBR which contains the OS loader and partition tables. Due to the 32-bit nature of the MBR, the maximum partition size is 2 TB; a limit that must have seemed impossibly large 20 years ago, when hard disks were measured in MB and only the largest hard drives were more than one hundred megabytes in size!

To circumvent this problem, a new, more modern system has been created to replace the MBR: GPT, or GUID Partition Table. GPT is an extension to the legacy MBR, keeping the MBR intact in the first sector of the hard drive. More importantly to us in this article, is that GPT uses 64 bits to address LBAs, extending partition size limits to about 16 exabytes!

Now, because GPT is still fairly unused, there are may be some caveats with getting programs like grub to properly install and recognize GPT partitions. The best way to avoid difficulties is to simply keep your OS separate from your data; that is, leave your OS partitions on a separate disk or array using a legacy MBR for booting, and write a GPT to the disk/array that is larger than 2TB.

On many server systems which provide BIOS level virtual disks, such as the Dell PowerEdge 2950 with it's PERC 5/i controller, this is cake - simply create a RAID5 array using (all) your disks, create a small, virtual disk for your OS (maybe 20-120 GB, depending upon your needs), and then create another virtual disk for the remaining data (if you're using 6x1TB disks, you should have something around 5 TB remaining).

Once you've installed the operating system and booted into it, Linux will happily see the two devices as /dev/sda and /dev/sdb - with your OS on /dev/sda and your unpartitioned /dev/sdb ready for GPT. From here, getting your system up and running is only a few commands away. Let's launch parted and take a look at the disk first:

Very empty. Note the 4945GB of available space (yum!). We'll need to use this number later when creating the partition. First however, we need to create a GPT label on the disk (as opposed to the legacy MBR, called msdos in parted):

Ah, much better. parted now recognizes it as a GPT partition table, but no partitions have been created yet so we'll need to go back in and create one with the mkpart command (or two, or three, or however you want to break it up...):

The Partition name is arbitrary, so call it whatever you want. Don't worry about what you put for the filesystem, as you can just mkfs over whatever is created after the program is complete; however if you know what you want before hand (and more importantly, if parted will let you) parted will mkfs for you. Now that we've created our partition, we can quit and should discover /dev/sdb? partitions (if you don't, try rebooting to force the computer to rescan the device's partition table).

You can now mkfs your filesystem of choice (I prefer XFS). Note that due to numbering, df will only see something like 4.5T free (don't panic!).

As you can see from this article, creating partitions larger than 2TB is quite easy using the mklabel and mkpart parted commands. If you have problems, make sure that GUID partition tables are supported by your kernel. Sometimes a reboot is required to force the kernel to rescan the partition table, so don't freak out if /dev/sdb? doesn't exist. As always, please post comments and questions below!