From Windows 2000 onwards, Microsoft started introduce the concept of dynamic disks. The counterpart, basic disk, is used from the era of DOS onward. Both have their advantages and disadvantages.
In this article we will discuss about both disks concept. Our explanation would be as general as possible, in fact there is no instruction on specific platform.
Basic Disk uses a partition table to manage all partitions on the disk on first sector of disk. It is supported by DOS and all Windows versions. A disk with installed OS would be default initialized to a basic one. A basic disk contains basic volumes, such as primary partitions, extended partition, and all logical partitions are contained in extended partition.
Basic Disks provide a simple storage solution that can accommodate a useful array of changing storage requirement scenarios. Basic Disks also support clustered disks, IEEE 1394 disks, and USB removable drives.
Basic Disks also use the same Master Boot Record (MBR) partition style as the disks used by Microsoft MS-DOS operating system. It can also support GUID Partition Table (GPT) partitions on systems that support it.
The following operations can be performed only on Basic Disk:
- Create and delete primary and extended partitions.
- Create and delete logical drives within an extended partition.
- Format a partition and mark it as active.
Dynamic Disk is supported in Windows 2000 and later operating system. Dynamic disks do not use a partition table to track all partitions, but use a hidden database (LDM) to track information about dynamic volumes or dynamic partitions on the disk. With dynamic disks we can create volumes that span multiple disks such as spanned and striped volumes, and can also create fault-tolerant volumes such as mirrored volumes and RAID 5 volumes. Compared to a Basic Disk, Dynamic Disk offers greater flexibility.
What is LDM database? LDM or Logical Disk Manager, is a hidden database which size is 1 MB at the end of Dynamic Disk. This space records all the information of the volumes on a single disk, and also holds some related information on each dynamic disk. The information including Drive Letter, Volume Label, the begin sector of Volume, Volume size, the file system of volume.
All disks are interrelated and will hold information mentioned above if there are several dynamic disk on computer. The relevance of each dynamic disk let we will see a “Missing” disk which is shown in Windows Disk Management if we remove a dynamic disk from your system. All this is saved in LDM database, so LDM database is vary important the same as Partition Table of Basic Disk.
Clearly, we can illustrate the Dynamic disk as following:
The blue area at the beginning of Dynamic Disk is the MBR which sabes the information of the Partition Table on the disk. However, this partition table is not the same as one of Basic Disk. Its main function is to make Windows and Other Disk Manager can know the disk is a dynamic disk instead of empty disk. The red one at the end of disk is the LDM database.
If you are familiar with Linux, Dynamic Disks system is similar to Logical Volume Manager (LVM).
The following operations can be performed only on dynamic disks:
- Create and delete simple, spanned, striped, mirrored, and RAID-5 volumes.
- Extend a simple or spanned volume.
- Remove a mirror from a mirrored volume or break the mirrored volume into two volumes.
- Repair mirrored or RAID-5 volumes.
- Reactivate a missing or offline disk.
- Capacity – Once Basic Disk create a partition, its capacity cannot be change unless we modify the partition table (using gpart, fdisk, or similar software). Dynamic Disk however can expand the capacity of volumes without data loss.
- Disk space limitation – On Basic Disk, the maximum capacity of a partition (volume) can be limited to 2 TB. Dynamic Disks can well handle the large partition of more than 2 TB.
- Number of partitions – Basic Disk is using primitive MBR disk layout, so it support only 4 primary partition. At best, it use 3 primary partition and use the last primary partition as extended partition. In Dynamic disks, unlimited number of partition can be created.
- Volumes type – Basic Disks only can create any primary or logical partition. Dynamic disks can create simple volume, spanned volume, stripped volume, mirrored volume, and RAID-5 volume (software-based RAID).
- Supported file systems – Basic Disks and Dynamic Disks support FAT, FAT32, and NTFS file systems.
- Have a partition table – Dynamic Disks have a partition table too, however this partition table is different from Basic Disks Its main function is to let Windows and other partition manager know the disk is a dynamic disk instead of an empty disk.
- Label and Drive Letter – On both Basic Disks and Dynamic Disks, every partition (volume) can be assigned to a unique drive letter (in operating system perspective). such as “System C:”
- Disk Layout Supported – Both Basic Disks and Dynamic Disks support MBR and GPT partition styles.
Volume on Dynamic Disk
On Dynamic Disk, the volumes are divided into several categories: Simple Volume, Spanned Volume, Mirrored Volume, and RAID 5 Volume. They have drive letter and volume label to differentiate.
Simple volume only can be created on the single disk. This volume is similar as partition of Basic Disk, but its space can be inconsecutive.
It is created from free space that is linked together from multiple disks (up to 32 disks). The sequence of writing data for Spanned Volume is that the volume on the first disk is filled full and then turn to fill the next dynamic disk. Spanned Volume can allow the fragmentary free space of multiple disks is recomposed as one volume, so it can fully utilize the resources of multi-disk. However, it can not be fault-tolerant volume and can not improve performance of the disk.
It’s similar with Spanned Volume, and consists of two and more disks. However, the difference is that it can improve the efficiency and performance of disk, because when operating system writes data to Striped Volume, this data will be separated into many pieces of 64KB, and then concurrent writes a different data block to each disk. A striped volume cannot be mirrored or extended and is not fault-tolerant. The screenshot is below:
We can simply understand that Mirrored Volume is a duplicate of Simple Volume. It needs two disks; one stores the data which is being used, and another keep a copy of previous one. When a disk fails, the other one can be used immediately.
A RAID-5 requires three disks at least; it not only can enhance the efficiency of the disk but also provide the best fault-tolerant. You could simply consider RAID-5 is a combination of Striped and Mirrored Volume. A RAID-5 volume is a fault-tolerant volume whose data is striped across an array of three or more disks. Parity (a calculated value that can be used to reconstruct data after a failure) is also striped across the disk array. If a physical disk fails, the portion of the RAID-5 volume that was on that failed disk can be recreated from the remaining data and the parity.