The size of a LUN (Logical Unit Number) is variable and depends on several factors, including the storage system's capabilities, the operating system's limitations, and the initial allocation. There's no single answer.
Factors Affecting LUN Size
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Storage System Limits: The maximum size a storage array can create is a primary factor. Some systems support LUNs up to 64TB, while others may have smaller limits. For example, one source mentions a 16TB maximum limit for LUNs in a specific NetApp configuration. ^1 Another source states that the maximum LUN size for VMFS datastores and physical raw devices in ESXi 5 is approximately 64TB. ^5 The size limit also depends on the ONTAP version used. ^9
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Operating System Compatibility: Windows and other operating systems may have different limitations on the size of LUNs they can address. Windows LUNs can often be resized to larger sizes due to their larger cylinder sizes. [^0] LUNs less than or equal to 50GB can sometimes be resized to approximately 500GB. [^0]
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Initial Allocation: When a LUN is created, it's given an initial size. However, this size can often be expanded later, depending on system capabilities. For instance, LUNs smaller than 6GB might be allowed to grow to 64GB due to their geometry. [^0] Conversely, reducing a LUN's size might require an offline process, remapping, and shrinking the volume. ^2
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Application Requirements: The application using the LUN dictates the minimum space required. A virtual machine needing 100GB of storage would need a LUN at least that large, or larger to accommodate growth. ^3
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Snapshots: Creating snapshots of a LUN doesn't directly change the LUN size, but it does consume additional storage space. This can indirectly influence how much additional space is available for the LUN's growth. ^10
Practical Examples
- Example 1: A virtual machine requires a 100GB disk. The datastore and LUN should be at least 100GB, perhaps larger to accommodate future expansion. ^3
- Example 2: A LUN initially created at 50GB could potentially be resized to approximately 500GB under specific circumstances. [^0]
- Example 3: A NetApp system might have a 16TB maximum LUN size limit, impacting how large a LUN can be created. ^1
[^0]: Any LUN created smaller than 6 GB will be allowed to grow to 64 GB due to its geometry, referring back to the 1 MB cylinder size with 65535 cylinders. Note: Because Windows LUNs have much larger cylinder sizes, they can be resized to large sizes. LUNs that are <= 50GB can all be resized to approximately 500GB.12-Feb-2024
