Microsoft Reserved Partition
{{Short description|Data storage portion for Microsoft Windows}}
A Microsoft Reserved Partition (MSR) is a partition of a data storage device that uses the GUID Partition Table (GPT) layout. The Windows operating system uses this partition for compatibility purposes. No meaningful data is stored within the MSR. Rather, when compatibility needs arise, Windows shrinks this partition to make way for other special-purpose partitions, which may contain data. The GPT label for this partition type is E3C9E316-0B5C-4DB8-817D-F92DF00215AE.
Purpose
Formerly, on disks formatted using the master boot record (MBR) partition layout, certain software components used hidden sectors of the disk for data storage purposes. For example, the Logical Disk Manager (LDM), on dynamic disks, stores metadata in a 1 MB area at the end of the disk which is not allocated to any partition.{{cite web|url=https://learn.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2003/cc758035(v=ws.10) |title=How Dynamic Disks and Volumes Work |website=Windows Server 2003 Technical Reference |via=Microsoft Learn's Archive |publisher=Microsoft |date=8 October 2009}}
The UEFI specification does not allow hidden sectors on GPT-formatted disks. Microsoft reserves a chunk of disk space using this MSR partition type, to provide an alternative data storage space for such software components which previously may have used hidden sectors on MBR formatted disks. Small, special-purpose partitions can be allocated from a portion of the space reserved in the MSR partition.{{Cite web|url=https://learn.microsoft.com/en-us/windows-hardware/manufacture/desktop/windows-and-gpt-faq |title=Windows and GPT FAQ |work=Windows Hardware Developer |via=Microsoft Learn |publisher=Microsoft |date=16 Jul 2024}}
Specifications
The MSR should be located after the EFI System Partition (ESP) and any OEM service partitions, but it must be located before Windows partition. Microsoft expects an MSR to be present on every GPT disk, and recommends it to be created as the disk is initially partitioned.{{cite web |url=https://learn.microsoft.com/en-us/previous-versions/windows/it-pro/windows-7/dd744301(v=ws.10) |title=Recommended UEFI-Based Disk-Partition Configurations |website=Windows 7 Technical Library |via=Microsoft Learn's archive |publisher=Microsoft |date=30 October 2012}}
The GPT label for this partition type is E3C9E316-0B5C-4DB8-817D-F92DF00215AE.{{cite web |url=https://learn.microsoft.com/en-us/windows/win32/api/winioctl/ns-winioctl-partition_information_gpt |title=PARTITION_INFORMATION_GPT structure (winioctl.h) |website= Windows App Development |publisher=Microsoft |via=Microsoft Learn |date=24 Feb 2024}}
The Microsoft-recommended size of MSR (which Windows Setup uses by default) is different for each version of Windows:
See also
- Basic data partition (BDP)
- EFI System Partition (ESP)