What is Difference Between GPT and MBR

What is Difference Between GPT and MBR

GPT brings with it many advantages, but MBR is still the most compatible and is still necessary in some cases. This isn’t a Windows-only standard — Mac OS X, Linux and other operating systems can also use GPT.

MBR

 It’s called Master Boot Record because the MBR is a special boot sector located at the beginning of a drive. This sector contains a boot loader for the installed operating system and information about the drive’s logical partitions. The boot loader is a small bit of code that generally loads the larger boot loader from another partition on a drive. If you have Windows installed, the initial bits of the Windows boot loader reside here — that’s why you may have torepair your MBR if it’s overwritten and Windows won’t boot. If you have Linux installed, the GRUB boot loader will typically be located in the MBR.

your primary partitions an “extended partition” and create logical partitions inside it. This is a silly little hack and shouldn’t be necessary.

The MBR holds the information on how the logical partitions, containing file systems, are organized on that medium. The MBR also contains executable code to function as a loader for the installed operating system—usually by passing control over to the loader's second stage, or in conjunction with each partition's volume boot record (VBR). This MBR code is usually referred to as a boot loader.

The organization of the partition table in the MBR limits the maximum addressable storage space of a disk to 2 TiB (232 × 512 bytes). Approaches to slightly raise this limit assuming 33-bit arithmetics or 4096-byte sectors are not officially supported, as they fatally break compatibility with existing boot loaders and most MBR-compliant operating systems and system tools, and can cause serious data corruption when used outside of narrowly controlled system environments. Therefore, the MBR-based partitioning scheme is in the process of being superseded by the GUID Partition Table (GPT) scheme in new computers. A GPT can coexist with an MBR in order to provide some limited form of backward compatibility for older systems.

MBRs are not present on non-partitioned media such as floppies, superfloppies or other storage devices configured to behave as such. On such a superfloppy, the entire media is treated as a single partition.

GPT

GPT stands for GUID Partition Table. It’s a new standard that’s gradually replacing MBR. It’s associated with UEFI — UEFI replaces the clunky old BIOS with something more modern, and GPT replaces the clunky old MBR partitioning system with something more modern. It’s called GUID Partition Table because every partition on your drive has a “globally unique identifier,” or GUID — a random string so long that every GPT partition on earth likely has its own unique identifier. 

This system doesn’t have MBR’s limits. Drives can be much, much larger and size limits will depend on the operating system and its file systems. GPT allows for a nearly unlimited amount of partitions, and the limit here will be your operating system — Windows allows up to 128 partitions on a GPT drive, and you don’t have to create an extended partition.

On an MBR disk, the partitioning and boot data is stored in one place. If this data is overwritten or corrupted, you’re in trouble. In contrast, GPT stores multiple copies of this data across the disk, so it’s much more robust and can recover if the data is corrupted. GPT also stores cyclic redundancy check (CRC) values to check that its data is intact — if the data is corrupted, GPT can notice the problem and attempt to recover the damaged data from another location on the disk. MBR had no way of knowing if its data was corrupted — you’d only see there was a problem when the boot process failed or your drive’s partitions vanished.

Like modern MBRs, GPTs use logical block addressing (LBA) in place of the historical cylinder-head-sector (CHS) addressing. The protective MBR is contained in LBA 0, the GPT header is in LBA 1, and the GPT header has a pointer to the partition table, or Partition Entry Array, typically LBA 2. The UEFI specification stipulates that a minimum of 16,384 bytes, regardless of sector size, be allocated for the Partition Entry Array. On a disk having 512-byte sectors, a partition entry array size of 16,384 bytes and the minimum size of 128 bytes for each partition entry, LBA 34 is the first usable sector on the disk.

Hard-disk manufacturers are transitioning to 4,096-byte sectors. The first such drives continued to present 512-byte physical sectors to the OS, so degraded performance could result when the drive's physical 4-KB sector boundaries did not coincide with the 4 KB logical blocks, clusters and virtual memory pages common in many operating systems and file systems. This was a particular problem on writes, when the drive is forced to perform two read-modify-write operations to satisfy a single misaligned 4 KB write operation.

For backward compatibility with most legacy operating systems such as DOS, OS/2, and versions of Windows before Vista, MBR partitions must always start on track boundariesaccording to the traditional CHS addressing scheme and end on a cylinder boundary. This is also true of partitions with emulated CHS geometries (as reflected by the BIOS and the CHS sectors entries in the MBR partition table) or partitions accessed only via LBA. Extended partitions must start on cylinder boundaries as well. This typically causes the first primary partition to start at LBA 63 on disks accessed via LBA, leaving a gap of 62 sectors with MBR-based disks, sometimes called "MBR gap", "boot track", or "embedding area". That otherwise unused disk space is commonly used by bootloaders such as GRUB for storing their second stages.

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