Windows GPT vs MBR Disk Comparison

by Bruce Mackenzie-Low - April 28, 2011

The Microsoft Windows operating system offers two architectures for partitioning disk drives into usable areas to store data. The two approaches differ with how they track the mapping of physical disk sectors to logical block numbers.

The original method of partitioning disks is referred to as Master Boot Record (MBR), which was developed during the 1980’s. Although widely accepted, this scheme has many shortcomings including partitions being limited to 2 terabytes in size.

As disk capacity has increased to over a terabyte, a new partition architecture developed in the late 1990’s called GUID Partition Table (GPT) was created to accommodate the larger partition sizes.

Comparing GPT vs MBR

In addition to size, GPT disks also offer more partitions and greater resilience to corruption. This article examines MBR and GPT-based Windows disks discussing, their benefits and disadvantages.

GPT-based Disks

Microsoft adopted the GPT disk partitioning scheme in 2001 from Intel’s Extensible Firmware Interface (EFI) specifications. It accommodates disk partition sizes up to 18 exabytes (EBs) or 1 million terabytes (TBs). Each GUID partition contains a 36-character field to allow a readable name to be associated with each partition.

Another shortcoming of MBR-based disk partitions is the limitation of three primary partitions and one extended partition. The extended partition can be further divided into multiple logical partitions. By limiting the number of partitions on a disk, you effectively reduce the number of partitions to organize or manage the data.

With GPT-based disks, Microsoft supports up to 128 partitions per disk. There are several partitions reserved for dynamic disks and EFI-boot support so only 124 partitions are available for data.

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MBR-Based Disk Layout

Perhaps one of the biggest pitfalls of MBR-based disks is their potential for corruption of the partition table, a region on the disk that maps sectors to logical block numbers. MBR disks only have one partition table to keep track of all the blocks in the partition.

MBR (Master Boot Record) Disk Layout

MBR Disk Layout (Source: Microsoft TechNet Library)

If the table becomes corrupt, the entire disk must be recovered from backup. Windows GPT-based disks have multiple, redundant partition tables so that if one is detected as being corrupt, it can self-heal itself from a redundant copy of the table.

For compatibility purposes, the Master Boot Record is kept at LBA 0 in GPT-based drives, and the GPT header begins at LBA 1.  The partition type of a GPT disk is marked as 0xEE, which prevents MBR-based disk utilities from recognizing the partition type and potentially corrupting the data. It is possible to convert an MBR disk to a GPT-based disk and vice versa, but any data must first be backed up and all the partitions deleted.

GPT-Based Disk Layout

Despite the many advantages of GPT-based disks over MBR drives, many vendors still utilize the MBR technology because it is predominantly used in the real world. GPT disks are gaining popularity with their benefits in terms of partition size, number of partitions, and resilience.

Windows Failover Clusters now support GPT-based disks, which will broaden their use in enterprise data centers. See related article on Failover Cluster Setup. Although bigger isn’t always better, the many advantages of GPT-based drives make them an attractive alternative. Be sure to see the Petri IT Knowledgebase article, "Top Storage Pitfalls to Avoid with Windows Failover Clusters."

GPT (GUID Partition Table) Disk Layout

GPT-based Disk Layout (Source: Microsoft TechNet Library)

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