Dell POWERVAULT MD3600I - Manual (Page 70)

Virtual capacity is capacity that is reported to the host, while physical capacity is the amount of actual physical disk
space allocated for data write operations. Generally, physical capacity is much smaller than virtual capacity.
Thin provisioning allows virtual disks to be created with a large virtual capacity but a relatively small physical capacity.
This is beneficial for storage utilization and efficiency because it allows you to increase capacity as application needs
change, without disrupting data throughput. You can also set a utilization warning threshold that causes MD Storage
Manager to generate an alert when a specified percentage of physical capacity is reached.
Changing Capacity On Existing Thin Virtual Disks
If the amount of space used by the host for read/write operations (sometimes called consumed capacity) exceeds the
amount of physical capacity allocated on a standard virtual disk, the storage array cannot accommodate additional
write requests until the physical capacity is increased. However, on a thin virtual disk, MD Storage Manager can
automatically expand physical capacity of a thin virtual disk. You can also do it manually using Storage → Virtual Disk
→
Increase Repository Capacity. If you select the automatic expansion option, you can also set a maximum expansion
capacity. The maximum expansion capacity enables you to limit the automatic growth of a virtual disk to an amount less
than the defined virtual capacity.
NOTE: Since less than full capacity is allocated when you create a thin virtual disk, insufficient free capacity may
exist when certain operations are performed, such as snapshot images and snapshot virtual disks. If this occurs,
an alert threshold warning is displayed.
Thin Virtual Disk Requirements And Limitations
The following table provides the minimum and maximum capacity requirements applicable to thin virtual disks.
Table 3. Minimum and Maximum Capacity Requirements
Capacity Types Size
Virtual capacity
Minimum 32 MB
Maximum 63 TB
Physical capacity
Minimum 4 GB
Maximum 64 TB
The following limitations apply to thin virtual disks:
• The segment size of a thin virtual disk cannot be changed.
• The pre-read consistency check for a thin virtual disk cannot be enabled.
• A thin virtual disk cannot serve as the target virtual disk in a Virtual Disk Copy.
• A thin virtual disk cannot be used in a Snapshot (Legacy) operation.
• A thin virtual disk cannot be used in a Remote Replication (Legacy) operation.
Thin Volume Attributes
When you create a thin virtual disk from free capacity in an existing disk pool, you can manually set disk attributes or
allow MD Storage Manager to assign default attributes. The following manual attributes are available:
• Preferred Capacity — Sets the initial physical capacity of the virtual disk (MB, GB or TB). Preferred capacity in a
disk pool is allocated in 4 GB increments. If you specify a capacity amount that is not a multiple of 4 GB, MD
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