Windows

Understand the use of basic and dynamic disks in Windows

The new option to format disks as dynamic volumes has made Windows 2000 storage more flexible and robust. Here are some special considerations for using this option, as well as information on how to convert disks from basic to dynamic.

If you’ve ever wondered how you can capitalize on Windows 2000’s improved high-level disk storage technologies, then you’ve come to the right place. I’m going to spill the beans on the differences between basic and dynamic disks and show how you can best utilize these technologies.

Basic partitions vs. dynamic volumes
Basic disks are limited to four partitions. This is due to the 64-byte partition table on the first sector of any basic disk. This table is a description of how the disk is split up into partitions. Each description takes 16 bytes, so that’s where the 4-partition limit comes in. Dynamic disks are part of a collective "disk group" under Windows 2000. Dynamic disks in Windows 2000 work as volumes. Each dynamic disk writes a 1-MB database of information that it stores at the end of its volume. When you have more than one volume on a Windows 2000 machine, then all the 1-MB databases work together to form the disk group. However, dynamic disks and the data on them can only be read by Windows 2000 (or later) operating systems.

Basic partitions and dynamic volumes differ in one crucial respect. The basic partitions (consisting of logical drives and primary partitions) are confined to one disk, and once you set their size, it's fixed. Dynamic volumes overcome this limitation because you can adjust their size and you can add more free space to them, either from the same disk or another disk. Dynamic volumes made up of space on different physical disks are called spanned volumes. You can create a spanned volume across a maximum of 32 disks.

This makes it extremely easy to store large files—especially ones that you know will increase in size. For example, your company may have a contacts database that you know will grow from its current 10 MB to 35 MB in the next year. By putting the file on a dynamic volume, you retain the option of adding free space to it later, as needed. This flexibility means you can use your disk space much more efficiently. In addition, dynamic disks offer advanced fault-tolerance options.

However, a dynamic volume cannot be shrunk without deleting it. If you need to reduce a volume’s size, make sure you move the data it contains elsewhere first, otherwise you’ll lose it all.

Upgrading basic disks to dynamic disks
The Win2K default setup program creates basic disks with an active partition from which the OS boots. You can partition such a disk first (if you want logical drives) and then upgrade it to a dynamic disk, to take advantage of the features. This is the most common method of upgrading. Before you rush to upgrade your basic disks to their dynamic cousins, you should consider a few issues that may come back and haunt you if you get them wrong.
  • If you want to reinstall your OS onto a dynamic disk, then you’ll only be able to do so if the disk was upgraded from a basic disk. Windows 2000 setup needs to use the partition table information from the original basic disk, and the install will fail if it can’t find it. This is because during early boot, Windows 2000 doesn’t load the driver that permits the OS to read the 1-MB database stored on the last cylinder of the dynamic disk. It only loads the driver capable of reading partition tables (on basic disks).
  • If you work in a dual-boot environment or if there are various operating systems in use across the network, then any pre-Windows 2000 machines will be unable to read the dynamic disks. This, of course, includes the ability to boot from those disks.
  • Although system partitions can be upgraded, you’ll need to perform a reboot to finish the job because all open files need to be closed for the process to complete without errors.
  • A basic disk with pre-Windows 2000 RAID partitions on it needs to be upgraded if you want to reap the benefits of Windows 2000 fault tolerance.
  • Volumes initially created on basic disks cannot be extended to include free space on the same or other disks. This can only be done if the volume was first created on a dynamic disk.
  • Dynamic disks can be converted back to basic disks, but all volumes must first be deleted and the data backed up, otherwise it will be overwritten.

Some disks can’t join the party
Upgrading basic disks is not always possible. Here are the main exceptions to the upgrade process:
  • Removable drives such as ZIP and Jazz drives cannot be converted to dynamic disks because they are not fixed, and they use different partition information mechanisms.
  • Laptop hard drives cannot be dynamic because there is usually only one of them. Further, if a laptop has access to extra hard drives when it is docked, then the dynamic disk would face synchronization challenges.

BIOS considerations
I have seen reports stating that when boot disks are upgraded to dynamic volumes, some machines may produce a STOP error pointing to an inaccessible boot device. A possible fix to this is to ensure that you’re running the latest version of BIOS on the machine in question.

Making the conversion
Now it's time to get your hands dirty and take a closer look at the conversion process itself. I'm going to show you how to complete two tasks in this section. The first will be to convert a basic disk to a dynamic disk. The second will be to convert a dynamic disk back to a basic disk.

Basic to dynamic
First, make sure you’ve got administrative rights or you won’t get very far. You’ll need to open up the Disk Management MMC to do the conversion. Right-click the gray area (with the disk information) just to the left of the disk you’re dealing with and then select Upgrade to Dynamic Disk from the menu.

If you see something similar to Figure A, where there is no option for Upgrade To Dynamic Disk, then you have a disk that doesn't support dynamic disks. In this case, the reason is quite simple: I’m writing this on a laptop and, from the discussion above, you’ll remember that laptops don’t support dynamic disks. If you have other types of volumes that don't support dynamic disks, then you'll run into this problem with them as well.

Figure A


Of course, on a typical server you’ll see the Upgrade To Dynamic Disk option on the menu. The next thing you need to stipulate is which of the available basic disks you wish to upgrade to dynamic status. You’ll then receive confirmation of your choice(s) and two warnings: the first with regard to pre-Win2K operating systems not being able to access dynamic disks (you already knew that), and the second telling you that any mounted file paths will be dismounted. After you click OK, the hard drive(s) will whir away for a few minutes, after which your disk(s) will be flagged as Dynamic. At this point, all partitions and logical drives will have become a simple volume. If you happened to have any files open during the conversion, then you need to reboot the system to finish the job. Then you’re done.

Dynamic to basic
The procedure to convert from dynamic to basic is identical to the one above, except that you’ll go the other way. One difference is that if the drive you wish to convert back to basic status contains volumes, and then when you right-click that disk, you’ll see that the Revert To Basic Disk option is grayed out. A reversion to basic disk status is impossible while volumes exist on the disk in question. This is a welcome safety feature, and it basically prompts you to ensure that all data on those volumes is backed up before you proceed in knocking them out. Once they’ve been deleted, the Revert To Basic Disk option becomes available on the menu. Select it and follow through the prompts until the process completes.

Once you have upgraded disks to dynamic status, you can take advantage of Win2K’s fault tolerance capabilities. You can also extend simple volumes to become spanned volumes (or even extend spanned volumes) but remember, you can only extend volumes that were originally created on dynamic disks. This means that any partitions that existed on the basic disk before conversion cannot be extended. To extend volumes, you need to right-click the volume in question and choose Extend Volume from the shortcut menu. This will start the self-explanatory Extend Volume Wizard, which you need to follow through to finish the job.

Adding, moving, and replacing disks
In the real world, it may be necessary to add disks to an existing computer, move disks between computers, or replace them entirely. If you add a disk to an existing Win2K system, then it will automatically be marked as a dynamic disk. Moving and replacing disks are similar operations because they both involve the physical removal of hard drives.

A word of caution: Moving and replacing disks is reasonably straightforward in general, but you’ll want to ensure that any spanned and/or fault tolerant volumes are moved at the same time. If you aren’t using either of these features, then you can safely move or replace drives as long as they’re still marked as "Healthy" in Windows 2000's Disk Management console.

Once you've moved them, to get the disks accepted and working again, open Disk Management and select Rescan Disks from the Action menu, as shown in Figure B. Make sure you power down all computers involved in disk changes, before you carry them out.

Figure B


If you move a dynamic disk to a different machine that also has dynamic disks in a disk group, then the imported disk will be marked as foreign and will need to be imported to that machine’s disk group by right-clicking on the disk marked as Foreign and selecting Import Foreign Disks. This will cause the newly added disk to be added to the local disk group. All simple volumes are then marked as active and healthy.

If you’re considering a move operation involving spanned and/or fault tolerant volumes, there are a few tips you’ll need to understand and remember so that you don’t lose any data:
  • Spanned volumes, which until now existed across more than one physical disk, should be moved concurrently. If they are not, then the volume on both the source and destination machines will be disabled. You can still move the remaining disks and recover the volume to its original state as long as the volume information exists on both machines, so deleting it should be done last.
  • If you move both drives in a mirror set, then the mirror will still function normally in the destination machine. However, if you moved one drive and then decided later to move it back to the original machine, then the original mirror would no longer function. You’d need to break the original mirror and then recreate it to achieve a working mirror.
  • RAID 5 volumes that need a drive replacement should continue to function online as long as all the parity information is correct. If a system crashes during a parity-write operation, then this will likely compromise the parity blocks. In that case, the rest of the disks in the RAID 5 array will go offline until the broken disk can be replaced and the array recovered from parity.

Summary
This article has looked at the differences between basic disks and dynamic disks. The principal advantages of dynamic disks are that they are more flexible in terms of sizing, and they offer advanced options for fault tolerance (software RAID). It is important to remember that there is no perfect backward-compatibility insofar as accessing dynamic disks from pre-Windows 2000 operating systems is concerned. Equally, users who are thinking about upgrading basic disks need to consider the potential pitfalls involved in the procedure. I examined the actual disk-conversion processes, and I also talked about the implications of adding, moving, or replacing disks, with emphasis on the need to move spanned and fault tolerant volumes at the same time to avoid the pitfalls of data loss.

 
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