Logical Disk Manager
The Logical Disk Manager is an implementation of a logical volume manager for Microsoft Windows NT, developed by Microsoft and Veritas Software. It was introduced with the Windows 2000 operating system, and is supported in Windows XP, Windows Server 2003, Windows Vista, Windows 7, Windows 8, and Windows 10. The MMC-based Disk Management snap-in hosts the Logical Disk Manager. On Windows 8 and Windows Server 2012, Microsoft deprecated LDM in favor of Storage Spaces.
Logical Disk Manager enables disk volumes to be dynamic, in contrast to the standard basic volume format. Basic volumes and dynamic volumes differ in their ability to extend storage beyond one physical disk. Basic partitions are restricted to a fixed size on one physical disk. Dynamic volumes can be enlarged to include more free space - either from the same disk or another physical disk.
Overview
Basic storage involves dividing a disk into primary and extended partitions. This is the route that all versions of Windows that were reliant on DOS-handled storage took, and disks formatted in this manner are known as basic disks. Dynamic storage involves the use of a single partition that covers the entire disk, and the disk itself is divided into volumes or combined with other disks to form volumes that are greater in size than one disk itself. Volumes can use any supported file system.Basic disks can be upgraded to dynamic disks; however, when this is done the disk cannot easily be downgraded to a basic disk again. To perform a downgrade, data on the dynamic disk must first be backed up onto some other storage device. Second, the dynamic disk must be re-formatted as a basic disk. Finally, data from the backup must be copied back over to the newly re-formatted basic disk.
Dynamic disks provide the capability for software implementations of RAID. The main disadvantage of dynamic disks in Microsoft Windows is that they can only be recognized under certain operating systems, such as Windows 2000 or later, FreeBSD, or the Linux kernel starting with version 2.4.8.
Dynamic disks under Windows are provided with the use of databases stored on disk. The volumes are referred to as dynamic volumes. It is possible to have 2000 dynamic volumes per dynamic disk, but the maximum recommended by Microsoft is 32.
Partition table types
On a disk partitioned with the MBR Partition Table scheme, the Logical Disk Manager metadata are not stored in a partition, but are stored in a 1 MiB area at the end of the disk that is not assigned to any partition. The disc partitioning tools in Windows 2000 up to Windows 10 will not use that area for disk partitions, but the tools in other operating systems might. The aforementioned reservation of 1 MiB would only occur on a disk which is identified by Windows as a "Local Disk" as opposed to a "Removable Disk". Thus a CompactFlash card capable of true IDE mode connected to a PC running Windows through an IDE port or a USB to ATA bridge, a mobile HDD enclosure case for instance, would get this reservation in contrast to one being connected through a normal USB card reader or a passive 16-bit CF to PCMCIA adapter where it does not deserve this reservation.Basic and dynamic disks and volumes
Dynamic disk is a proprietary format of Microsoft developed together with Veritas.A basic volume is a volume stored on a basic disk, while a dynamic volume is a volume stored on a dynamic disk. Basic volumes and dynamic volumes differ in ability to extend storage beyond one physical disk. The basic partitions are confined to one disk and their size is fixed. Dynamic volumes allow to adjust size and to add more free space either from the same disk or another physical disk. Striped volumes and spanned volumes are dynamic volumes using space on different physical disks. Presently, spanned volume can use a maximum of 32 physical disks.
The main differences between basic and dynamic disks are:
- Dynamic disks support multi-partition volumes; basic disks do not.
- Windows stores basic disk partition information in the registry and dynamic disk partition information on the disk
Some space at the end of the disk is reserved by the setup program of Windows with a kernel version of 5.x lest the disk would be upgraded to a dynamic one. The amount that is reserved is one cylinder or 1 MiB whichever is greater. One cylinder depending on drive geometry and translation can be up to 8 MiB which account for a remaining 8 MiB free space once Windows setup is used to create a partition. Coincidentally, Solaris 11 leaves exactly this amount of space at the beginning of a disk. Furthermore, this is not true since Windows Vista which always leave 1 MiB that is aligned to 2048 sector boundaries at the end of a local disk for the purpose of dynamic disk.
Compatibility problems
Disk Management in Windows Vista creates partitions according to a 1-MiB alignment boundary, ignoring the previous conventions called "drive geometry" or "CHS". In other words, Vista's Disk Management acts like it is using a non-standard CHS geometry of 2048 sectors per track/head and 1 track/head per cylinder.Compatibility problems with using a 1-MiB alignment boundary
- Extended partition tables that are edited with Vista Disk Management should not be edited with Windows XP Disk Management. XP Disk Management may delete these extended partitions without warning. One way to correct these mixed partition tables is to delete all partitions and use only one partition editor on the drive.
- You may not be able to install XP or Windows Server 2003 to partitions created with Vista Disk Management.
- These 1 MiB alignments ignore prior CHS conventions such as the regular intervals at which partitions begin and end, and the distance between the extended boot records and their logical drives.
Advantages of using a 1-MiB alignment boundary
- Microsoft states that on large-sector drives, also known as advanced format, there is a "performance issue" when the starting offset of the first partition is an "odd" number. This partition alignment offset also affects newer generation drives not necessarily specified as advanced format such as SSD drives. Misalignment causes severe degradation not only with random write performance, but also with sequential write performance.
- Using a 1-MiB alignment boundary allows safer editing of the partition table with Vista Disk Management.
- A disk that is solely partitioned with Vista Disk Management can create partitions in exact MiB sizes and never leave leftover bits of space between partitions.