Virtual Disk Service (Vds) - Aberdeen AberNAS 120 Manual

Shadow Copy Transport
All of the functionality discussed above is provided with the in-box shadow copy technology on the
operating system. The exception to this is shadow copy transport among systems—whether for
backup, data mining, testing or fast restores—which requires a hardware provider on the SAN. In a
direct attached storage configuration, this data transport between servers is physical. In a NAS-SAN
configuration with pooled storage, data on the SAN can be accessed (through masking and
unmasking) between servers. Although access to the storage pool is shared, each server can only
access the specific LUNs (Logical Unit Number) assigned to it (since two servers cannot both write to
the same volume without potentially causing data corruption). But using the hardware provider on the
SAN, a point-in-time shadow copy can be virtually "transported" to another server for use, through the
process of masking and unmasking.

Virtual Disk Service (VDS)

In order for a server to use new storage disks, they must first be made accessible to the server and
then formatted for use. Virtual Disk Service (VDS) controls the process of making storage accessible
to systems that need it. While it is irrelevant to application (or the user) how the data is stored—
whether it is on a single physical disk or spanned across several disks (a logical unit), in terms of data
protection and performance, the impact of how the data is stored is significant. Thus VDS can either
present a physical disk or a logical disk to a server. Physical disks do not require the first two steps.
1. Create logical units, assign number ids (these are now referred to as LUNs)
2. Unmask LUNs to server
3. Create partitions and volumes
4. Format the file system
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Basic Disks. VDS is used to partition
be mapped to drive letters for use. These volumes are known as "simple volumes" and do
not span multiple disks. Basic disks are the legacy disks, predating Windows 2000
capabilities. They do not offer the same performance and data protection that dynamic disks
offer.
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Dynamic Disks. VDS can be employed to create dynamic disks which can consist of either
simple volumes or multi-partition volumes
than a single disk but nevertheless are logically considered a single volume. Dynamic disks
can be spanned, striped (RAID-0), mirrored (RAID-1) or stripped with parity (RAID-5),
depending on the level of performance and data protection desired. VDS can be used to
expand dynamic disks to make more space available to a volume.
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Physical disks are divided into sectors; contiguous sectors are partitions. In the case of basic disks, the volumes are
created within partitions, and are thus restricted to a single disk.
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Volumes can span one or more partitions on the same or multiple disks. Dynamic disk types include RAID
configurations, and can offer better performance and reliability than basic disks
Microsoft® Windows Storage Server 2003 White Paper
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each physical disk and to create the volumes that can
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. Multi-partition volumes physically span more
.