Welcome to the ITFreeTraining video on MBR and GPT partition tables. A partition table
forms the low level data structure of a drive. By the end of this video you will understand
what partition tables are available and what features each offers.
Before a drive can be used by an operating system, whether it be physical or virtual,
a partition table needs to be created on the drive. The partition table forms the low level
data structure on the drive. This structure allows the drive to be divided up into partitions.
If I have a physical drive that has a partition table created on it, the drive will contain
only unused space. Before the unused space can be used, the drive needs to be divided
up into one or more partitions. In this case, the drive will be divided up into 3 partitions.
It is common for a drive to be divided into a large partition for the operating system
and its data, and some smaller partitions for booting, recovery and tools.
Once a partition table is created, it can be formatted for the operating system to store
data on. There are two partition tables that are available. These are, MBR and GPT. MBR
is older and thus has more compatibility with older operating systems and hardware. GPT
is a newer standard and has more features, but requires newer operating systems and,
depending on what you are trying to achieve with it, it may also require newer hardware.
To start with, I will look at MBR.
The MBR or Master Boot Record, was first introduced on PC’s back in 1983. Since then it has
become the de facto standard on PC’s working on DOS based, Windows based, and alternative
based operating systems. If the MBR partition table is used, this allows the drive to be
divided up into a maximum for 4 primary partitions.
The operating system allows each partition to be formatted to use a file system. So with
MBR the administrator could create 4 primary partitions and format them so the operating
systems can see 4 drives. In most cases this should be enough. However, in some rare cases
more than 4 partitions are required. When this is the case, one of the primary partitions
can be changed to an extended partition. In the DOS days, this extended partition could
hold 23 partitions. This was a limitation of DOS since it could only support 26 drive
letters.
On modern Windows operating systems the administrator can create as many extended partitions as
they wish. However, extended partitions are linked to each other so if you use a lot of
them they could have an impact on performance. Also, the BIOS in the computer may not support
booting from an extended partition; however, a workaround would be to place a boot loader
on a primary partition that would boot the operating system stored in the extended partition.
The MBR partition table offers the fundamental features required for an operating system
and offers compatibility with older operating systems and hardware. The only limitation
is that it is limited to 2 Terabytes of usable space. If the administrator uses MBR on a
drive larger than 2 Terabytes, all the space after 2 Terabytes will be unavailable to the
operating system and thus unusable. There is some new technology that has been developed
to extend the life of MBR which I will look at later on in this video. Before this, I
will have a look at MBR’s replacement GPT.
The replacement for MBR is GPT or GUID Partition table. Using Windows, GPT supports 128 partitions.
To be specific, Windows uses the minimum default settings when using GPT. The standard for
GPT allows an unlimited number of partitions. So using an alternative operating system the
administrator could potentially have access to more than 128 partitions. Why you would
need more than that, I can’t really say.
The biggest difference with GPT is that it supports drives up to a Zettabyte. It is not
limited to 2 Terabytes like MBR. The actual amount may vary a little depending on how
large the drive manufacturer makes the sectors. Currently, with Terabyte hard disks on the
market, first a Petabyte and Exabyte drives would need to be made before we get close
to a Zettabyte hard disk.
In order to boot an operating system using GPT, a few requirements need to be met. First
the computer needs to support Unified Extensible Firmware Interface or UEFI. UEFI is designed
to be a replacement to Bios; however, UEFI generally does require 64bit hardware. It
is possible to run UEFI on 32bit hardware but manufacturers generally don’t create
hardware that use UEFI on 32bit systems.
To boot Windows using a drive with a GPT partition table, you will need Windows XP professional
64bit or Windows Server 2003 64bit or above. If you are using Linux, most modern 32bit
and 64bit Linux systems will support booting from GPT hard disks or the GPT hard drive
being used as a data drive. You will need to check the details of your particular distribution
to see if it is supported.
If your computer does not meet these requirements, it may still be possible to use a GPT drive
as a data drive. For Windows, this requires Windows Vista or Windows Server 2003 with
Service Pack 1 or above. Data drives using GPT do not require a 64bit version of Windows
or 64bit hardware. Many Linux distributions support GPT drives as a data drive, however
you need to check the individual distribution to confirm if it is supported.
You can see GPT addresses the issue of MBR only being able to be used on drives up to
2 Terabytes in size. However, in order to do this, it requires hardware and software
support. So what do you do if you need to use more than 2 Terabytes on MBR?
To improve support for larger drives with older hardware, Advanced Format was developed
and is available for use with all hard drives manufactured since 2011. This potentially
allows hard disks larger than 2 Terabytes to be used on older hardware and software
that does not support GPT.
To better understand how this advanced format works, I will first look at how it works without
advanced format. When an MBR partition table is used, it uses 32 bits to hold its data.
The largest sector size that can be used is 512 bytes. This gives us a total limit of
2 Terabytes in size.
You can see this is where the 2 Terabyte limit comes from. A limit imposed by MBR of 32 bits
and a sector size of 512 bytes. So how does the advanced format bypass this limit while
still working with old hardware?
When using a drive with advanced format, you still have a limit of 32 bits and 512 bytes,
this is what the older hardware expects and supports. However, the firmware on the drive
allocates each block of data as eight 512 byte sectors. Because of this the hardware
thinks it is accessing a 512 byte sector but in reality it is accessing eight 512 byte
sectors making the sector 4 kilobytes in size.
The end result is that the old hardware can fully access up to 16 Terabytes of data on
the drive. This allows larger drives to be used with old hardware but it is not without
some problems.
On the older operating systems, the drive will need to be partitioned with Hybrid MBR
in order to use more than 2 Terabytes.
Unfortunately, Windows does not have the ability to format a drive in this way so you will
need to format the drive using an alternative operating system. It may be possible, for
example, to format the drive in Linux and use it in Windows as a data drive. To use
the drive for booting is difficult, but not impossible to implement.
Other operating systems may support this format, but you would need to perform your own checks
to see if it does and how well it is supported.
Many disk utilities will expect a 512 byte sector and will not understand what is happening
when they encounter the larger 4 kilobyte sector. Even attempting to *use* a disk utility
that doesn't support this format may damage the data on the drive as the disk utility
tries to correct a problem that is not there. This applies to Linux and Windows utilities.
In the real world, if you have a drive less than 2 Terabytes, MBR should meet all your
needs. There is no need to change to GPT. If, however, you do find that you need to
convert between the two, it is possible in Windows; however, this will require all the
existing partitions to be deleted which will erase all the data on the drive.
If you are using Linux it is possible to convert the partition table without losing data; however,
you should always backup your data beforehand just in case. Lastly, there is 3rd party software
available that will perform the conversion if you are willing to pay for it. Paragaon
offers free software that will do the conversion between MBR and GPT for home use. You may
also look at some of the free Linux distributions to achieve the same result.
In a next video I will look at how disk management with MBR and GPT works in Windows. I hope
you have found this video useful and hope to see you in the next video. Bye for now.