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Backup & Storage

Windows GPT Disks - Is Bigger Really Better?

The Microsoft Windows operating system offers 2 architectures for partitioning disk drives into usable areas to store data.  The 2 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 MBR (Master Boot Record) which was developed during the 1980’s.  Although widely accepted, this scheme has many shortcomings including partitions being limited to 2TB (terabytes) in size.

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

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GPT-based Disks

Microsoft adopted the GPT disk partitioning scheme in 2001 from Intel’s EFI (Extensible Firmware Interface) specifications.  It accommodates disk partition sizes up to 18 EB (Exabyte’s) or 1 million terabytes.  Each GUID (globally unique ID) 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 3 primary partitions and 1 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.

MBR Disk Layout

The following diagram from the Microsoft TechNet Library provides an example of a typical MBR disk layout.

MBR (Master Boot Record) Disk Layout

Figure 1: MBR 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 1 partition table to keep track of all the blocks in the partition.  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

The following diagram from the Microsoft TechNet Library provides an example of the disk layout for a GPT-based disk.

GPT (GUID Partition Table) Disk Layout

Figure 2: GPT-based Disk Layout

Summary

Despite the many advantages of GPT-based disks over MBR drives, many vendors still utilize the MBR technology since it is still 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. So while bigger isn’t always better, the many advantages of GPT-based drives make them an attractive alternative.

Watch for future articles on Windows storage technologies such as “Disk Fragmentation and Why You Should be Concerned”.  This article will examine how files are placed on disk and what causes them to become fragmented.  It will explore how fragmented files affect performance and what can be done to avoid it.

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Comments (14)

14 responses to “Windows GPT Disks – Is Bigger Really Better?”

  1. Failover Cluster Setup

    [...] Fortunately, most storage subsystems allow storage LUNs (logical unit numbers) to be dynamically expanded.   The Windows operating system also allows partitions to be dynamically expanded with tools such as Diskpart.  Unfortunately, the procedure to dynamically expand disk drives in a cluster environment is non-trivial and typically requires downtime.  Furthermore, if more than 2TB (terabytes) of disk space is needed on a single partition, GPT disk formats must be used instead of the typical MBR-based disks.  To convert from MBR to GPT-based disks, all partitions must be deleted before the drive can be converted. So making the correct decision about the partition size can have a direct bearing on the cluster’s availability. Further information can be found here in article on GPT vs MBR Disk Comparison. [...]
  2. Top Storage Pitfalls to Avoid with Windows Failover Clusters » www.vinilrichard.com/mylife/ – My Internet Diary

    [...] Fortunately, most storage subsystems allow storage LUNs (logical unit numbers) to be dynamically expanded.   The Windows operating system also allows partitions to be dynamically expanded with tools such as Diskpart.  Unfortunately, the procedure to dynamically expand disk drives in a cluster environment is non-trivial and typically requires downtime.  Furthermore, if more than 2TB (terabytes) of disk space is needed on a single partition, GPT disk formats must be used instead of the typical MBR-based disks.  To convert from MBR to GPT-based disks, all partitions must be deleted before the drive can be converted. So making the correct decision about the partition size can have a direct bearing on the cluster’s availability. Further information can be found here in article on GPT vs MBR Disk Comparison. [...]
  3. Strucne stvari by djmitar - Pearltrees

    [...] GPT vs MBR Disk Comparison Figure 2: GPT-based Disk Layout Summary Despite the many advantages of GPT-based disks over MBR drives, many vendors still utilize the MBR technology since it is still predominantly used in the real world. GPT disks are gaining popularity with their benefits in terms of partition size, number of partitions, and resilience. [...]
  4. Not enough disk space - Windows 8 Forums

    [...] around with disks though -- decent DISK imaging program like acronis or macrium should be fine. GPT vs MBR Disk Comparison Cheers jimbo   My System Specs You need to have JavaScript enabled so that you [...]
  5. Recreating VMFS Partitions using Hexdump - VirtuallyHyper

    [...] The first thing is to understand the different types of partition tables. ESX/ESXi pre 5.x uses Master Boot Record (MBR/MSDOS) partitions, where as ESXi 5.0 introduced support for GUID Partition Tables (GPT) partitions. A good article about the difference between MBR and GPT partitions can be found here. [...]
  6. w2k8R2 Partition Size Limit!! | Resellernews.com.au

    [...] If you are not sure change the disk to GPT. Be careful, there are some disadvantages: https://petri.com/gpt-vs-mbr-based-disks.htm [...]
  7. Best Partition Layout

    [...] Win7 recovery partition if you are committed to Windows 8. Do you have a partition map preference--GPT or MBR? More detailed GPT info here. I have found conflicting info on this point, but I think you need [...]
  8. Using cloud services as glorified drive: virtual iSCSI targets (part VI) | Random Oracle

    [...] disk” from the context menu to initialize the disk. (For this application, choosing GUID partition table over MBR is fine, since the disk can only be accessed by recent vintage Windows instances that support [...]
  9. Vikas

    How to check that a computer is running on mbr or gpt.
  10. Разлики между GPT и MBR | Viraldim's Blog

    [...] https://petri.com/gpt-vs-mbr-based-disks.htm [...]
  11. Software Nieuwe pc geeft maar 2tb van de 4 weer.

    […] - Storage probeer dit eens,weet wel niet of je w8 of w7 gebruikt Om aan te sluiten bij dit ^^ GPT vs MBR Disk Comparison Met citaat […]
  12. Failover Cluster Setup

    […] Fortunately, most storage subsystems allow storage LUNs (logical unit numbers) to be dynamically expanded.   The Windows operating system also allows partitions to be dynamically expanded with tools such as Diskpart.  Unfortunately, the procedure to dynamically expand disk drives in a cluster environment is non-trivial and typically requires downtime.  Furthermore, if more than 2TB (terabytes) of disk space is needed on a single partition, GPT disk formats must be used instead of the typical MBR-based disks.  To convert from MBR to GPT-based disks, all partitions must be deleted before the drive can be converted. So making the correct decision about the partition size can have a direct bearing on the cluster’s availability. Further information can be found here in article on GPT vs MBR Disk Comparison. […]

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