The Podcast for the Home Server Enthusiast.

GPT Sermon

pcdoc • February 26, 2011 • Comments (1)

GPT/MBR Disk Partitioning

Master Boot Record

The MBR is used for booting your operating system and holding your disk’s primary partition table. Think of it as a tiny bit of space at the “front” of your disk that that tells the disk and your PC how to use the disk. The MBR stores this information in 32-bit sectors but because the sector size is only 512 bytes, you are limited to a disk space of 2.2 TBs. I have mentioned on BYOB before, take 512 bytes x 2³² (for 32 bit) and you get the magical 2.2TBs.

In addition, another limit to MBR is that you can have at most only four primary partitions.

It is for these limits that the GPT scheme becomes important.

GPT

A GPT formatted disk, or a GUID Partition Table disk, is the replacement layout for BIOS based MBR on hard disks. As MBR disks were limited to 2.2TB, GPT disks are limited to 9,444,732,965 TB or just 9.4 ZB to make it easier to say. Not going to be seeing drives hitting that maximum any time soon.

GPT has actually been around since the 90’s and has been supported by Microsoft since Windows Vista. Three requirements of note: 32-bit machines do not support GPT, GPT is only supported as a boot disk in a UEFI environment and Removable media must be MBR or “Superfloppy.

So I have established that GPT is bigger than MBR so what? Why do BYOB listeners care? Well, from 1956 up until recently, most disks used sectors of 512 bytes. Currently, the hard drive manufacturers are moving towards Advanced Formatting for their larger drives which use 4,096 byte sectors, or 4K sectors for short. This allows drives to write more data on the same space.

Western Digital has a great two-page PDF file that explains Advanced Format. The link is in the show notes at http://www.wdc.com/wdproducts/library/WhitePapers/ENG/2579-771430.pdf

GPT partitioned disks actually have a “protective MBR” but it is only there to tell older utilities that the GPT partitions exist and not to erase or write over those areas. If you were to format the drive in MBR the system will only recognize the first 2048GB and leave the rest as unusable.

For those of us that want 3TB or larger disks with 4K sectors, GPT is the only option. As a side note, the external 3TB drives you see in Best Buy and Fry’s translate the sectors so they can be used on any system. In addition, GPT is not just made for large PC disks. Apple has been using GPT disks for years. APM, or Apple Partition Map, introduced in 1987 was replaced by GPT in 2006 when Apple moved to Intel CPUs. Mac OS X and later mount on GPT disks.

http://developer.apple.com/library/mac/#technotes/tn2006/tn2166.html

As you can see below, once a disk has been formatted as MBR, any space past the 2048GB is not usable.

Windows Home Server and GPT

With Windows Home Server v1 and Windows Home Server 2011 there are a couple of issues. First, with v1, you cannot use GPT disks at all. Diehard and I got a 3TB to be recognized in Disk Management using Paragon software but the Console would always format and repartition a pool drive as MBR. The Console is designed to do that so we could only get a 3TB drive to be recognized as a backup drive. Even then we were concerned about data safety and have given up until we hear back from Paragon.

If you have a GPT client that backs up to a WHS v1 the backup will succeed but a standard Restore will fail.

With 2011 there are a few concerns too. GPT disks are supported in Windows Home Server 2011 as storage disks but the Dashboard will split drives larger than 2040GB into multiple disks. You still get the full space but in multiple partitions. The reason is due to the fact that “dynamic or difference VHDs cannot exceed 2040GB. The reason for the 2040G limit is the length of each Block Allocation Table entry is set to 4 Bytes…If you multiply that value by 512B sector size and then subtract the overhead of on disk meta-data structures, 2040GB will be the maximum size of dynamically or differencing VHDs.” The Dashboard will convert any disk larger than 2TB to more than one partition to prevent a folder share from exceeding the maximum Windows Home Server 2011 backup size.

If you have a GPT client it CANNOT backup to Windows Home Server 2011. There is a GPT bug files in Connect so if it is important to you then vote it up.

This means that if you have a GPT disk as your OS in a UEFI system neither Windows Home Server can back up or Restore your data. Bummer.

Anyway, enough of the short sermon. Check your Sandy Bridge systems and let us know in the forums where you stand on GPT.

http://en.wikipedia.org/wiki/Disk_partitioning