Volumes - Overview

Filesystems and Containers

In theory, the filesystems and the containers are independent of one another. In practice there are some limitations. Windows cannot resize FAT partitions on-the-fly. A dynamic disk can contain any pair of filesystem and container

Filesystems:

• NTFS
• FAT

Containers:

• Simple
• Spanned
• Stripe (RAID 0)
• Mirror (RAID 1)
• Stripe with parity (RAID5)
• Mirrored Stripe (RAID 1+0)

Container Types

Simple

This is the basic unit. One container that fills one partition.
Redundancy: None.
Efficiency: Poor.

Spanned

Spanning is achieved by concatenating two partitions together. It is sometimes know as RAID Linear. This allows the user to extend a volume using any free space on any disk.
Redundancy: None.
Efficiency: Poor.

Stripe (RAID 0)

By striping the data across two, or more disks, there is a performance gain for reading and a small hit for writing.
Redundancy: None.
Efficiency: Medium.

Mirror (RAID 1)

This is the simplest redundant RAID type. A disk's data is copied exactly to one or more disks. It can survive all disk failures
Redundancy: Medium.
Efficiency: Medium.

Stripe with parity (RAID 5)

With a minium of three disks, you can add parity information to a stripe and get RAID5. This can survive a disk failure, but there is a large hit to rebuild the data on a new disk.
Redundancy: Good.
Efficiency: Good.

Mirrored Stripe (RAID 1+0)

This requires a minimum of four disks. Take a striped pair of disks and mirror them completely onto two more disks.
Redundancy: Medium.
Efficiency: Good.

Extending Containers

Starting with this example configuration of disks, we can see how each container type can be extended, some in several ways. We will deal with breaking up partitions in the next section.

Simple

Extend - By using the free space after R:, the container will still be simple. If some space on another disk is used, the container type will be promoted to spanned.

Add mirror - Adding a copy of R: will improve access speed and provide fault tolerance.

Spanned

Extend - N: is already spanned. This will add another partition.

Add mirror - Adding a copy of N: will improve access speed and provide fault tolerance. The mirror could be split across disks 2 and 3, or put entirely on disk 4.

Stripe

Extend - Adding to the end of stripe W: will require at least two disks. The partition on disk 3 could be extended, then a similar amount of space could be used from disks 2 or 4.

Add mirror - This too will require two disks. They cannot be the two that the stripe already resides on. This will give the volume fault tolerance.

Mirror

Extend - This will require space on the two original disks or more others.

Add another mirror - Volume P: already has one mirror. This action will add a third copy of the volume. It cannot be on the same disk as an existing mirror. This will increase the volume's fault tolerance.

Stripe with parity

Extend - This will require at least three disks. Each part of M: can be extended on the same disk, or one or more others.

Mirrored Stripe (not shown)

Extend - This needs space on at least four disks. Like the stripe with partity, each section can be extended on the same disk, or one or more others.

Add another mirror - This is adding a mirror of the underlying stripe and will need only two disks. They may not be the four already in use. This will improve the volume's fault tolerance.

Breaking Containers

In this section we find out the results of breaking volumes and removing disks.

Simple

Delete - Remove the partition containing the volume R:.

Spanned

Delete - Remove all the partitions belonging to the volume N:.

Stripe

Delete - Remove all the partitions belonging to the volume W:.

Mirror

Delete - Remove one copy of the underlying volume, P:.

Break - Split the mirror into two identical volumes. For P: this would result in two volumes with no fault tolerance. For volumes with more mirrors, the result would be a smaller mirror and a simple, spanned, or striped volume.

Remove - The same as breaking a mirror, except the selected part is deleted after the split.

Stripe with parity

Delete - Remove all the partitions belonging to the volume M:.

Mirrored Stripe (not shown)

Delete - Remove all the partitions belonging to the volume

Break - As with breaking a mirror, this will leave two volumes. One will be a stripe and the other will be a stripe or a mirrored stripe.

Remove - The same as breaking a mirrored stripe, except the selected part is deleted after the split.

Disk Failures

If a disk fails, then only mirrored and RAID5 volumes will survive. Simple, spanned and striped volumes do not have any redundancy. Losing a part of one of these is losing part of the filesystem. If a mirror loses a disk, then the fault tolerance is lost, but it can continue. Likewise, RAID5 can lose one disk. If a new disk is added, then the volume may be repaired. For mirrors, this means copying the whole volume to a new partition. For RAID5, all the parity checks must be recalculated, which is very time-consuming.

Similar effects will be seen if only some of the disks of a group are moved to another machine. For RAID5, one machine will have enough information to continue, but the other will not. By splitting a mirror, each computer can use its volume, but they will not be fault tolerant and they will become out of sync. Once this happens, they cannot be recombined into one mirror.