What’s a Dynamic Disk?

Something you may have seen but probably don’t have.

It seems like a disk would be a simple thing. You put some data on it, add a little organization around it to find that data, and you’re done. Right?

Dynamic disks are a little more complex. Most folks don’t need to worry about them, but they are interesting. I’ll go into some detail on the different things they can do.

Dynamic disks

Dynamic disks differ from basic disks by handling features like merging, spanning, and mirroring volumes across multiple physical drives. Most home users have basic disks and don’t need dynamic disk support unless they require advanced functionality.

Related

What’s a Disk Partition?

Partitions allow a single physical disk drive to be treated as if it were multiple disks.

What makes a dynamic disk different?

Basic disks are the disks you’re familiar with already. A basic disk matches the relatively simple idea I described above. They typically contain one or more partitions, each partition has one or more volumes, and those volumes are formatted using a file system like FAT32 or NTFS.

While there are several layers to this scheme, the approach is pretty straightforward.

Dynamic disks are different. For example, rather than having a partition table to determine where all those partitions physically are on a disk, they have a partition database. That database enables you to do a few more things with dynamic disks than you can with a basic disk.

Merging space

Consider this common scenario. You have a basic disk with two partitions and some unused free space. It’s partition A next to partition B followed by that free space.

With a basic disk, if you want to use that free space to increase the size of partition A, you must rearrange things so that the free space is next to partition A. That typically means moving partition B so you end up with partition A next to that free space followed by partition B.

You could then merge the free space into partition A so partition A takes up the total free space. You’re left with a larger partition A followed by partition B and that’s it.

There are several partition-management tools out there that can do what I’ve just described. In later versions of Windows, the disk management tool itself can do some of this kind of thing.

“Merging” space

Dynamic disks are more flexible in terms of partitioning. You can merge without needing to shuffle data around.

Rather than move anything, you can create a partition of sorts out of the free space so that partition A is made up of the area in front of partition B and the area behind it. Your disk then looks like the first part of partition A, followed by partition B, followed by the second part of partition A.

To your computer, it looks like two pretty normal partitions. But on the disk, the partition is broken up. It lives in two different places.

The technical term for those parts of a partition is an extent. Extents don’t have to live next to each other to create a single volume. So partition A is now made up of two extents: one before and one after partition B.

Spanned volumes

Extents don’t even have to live on the same physical drive. The simplest example is taking two hard drives, setting them up as dynamic disks, and then creating a single partition that encompasses both disks. Your system now sees one partition. The fact that it’s physically implemented as two separate drives is hidden. This is what they call a spanned volume since it spans multiple physical disks.

The downside? You’ve doubled your risk of failure; if either disk fails, the entire partition and all the data within could be lost.

Striped volumes

Striped volumes are like spanned volumes except that instead of putting, say, the first part on one drive and the second part on another, the data is intermixed. Chunks of data alternate between one drive and the other. While this sounds confusing, there’s a good reason for it. It turns out that when you distribute your data evenly across drives like this, it’s faster to read and write the data to and from the combination of drives.

(“Sections” is arbitrary and undefined. How data is divided is an unspecified implementation detail.)

Once again, though, you’ve doubled your risk of failure; if either disk fails, the entire partition could be lost. This is often referred to as “RAID-0” (Redundant Array of Independent Disks).

Mirrored volumes

Mirrored volumes are conceptually the opposite of striped. Two drives are used, but the amount of data represented is equivalent to one drive. Why? Well, when you write data to the drive, it’s written to both. If one fails, its mirror can continue without data loss. Again, there are two drives but to the system, it looks like a single partition.

This is often referred to as RAID-1. The downside is that it might be slightly slower, and, of course, it takes twice as many disks to get the storage you need.

RAID-5

RAID-5 is a little bit of everything so far. A RAID-5 setup can be implemented using dynamic disks. Three drives are used: two for data and one for error checking. When configured properly, any of those three drives can fail without data loss.

RAID implementations can get more complex, but this touches the surface of what you can do with dynamic disks in Windows. In practice, most RAID implementations don’t rely on Windows or dynamic disks at all; they’re on dedicated hardware RAID controllers.

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