Thin LUN and Thick LUN [Lecture from Mr. V] Highlighted

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This post talks about the Thin LUN and Thick LUN, in the form of a lecture from Mr. V. Please study the materials below for a deeper understanding of the topic.

Definitions of LUN, thick LUN and thin LUN

LUN

The Logical Unit Number (LUN) is an independent storage unit of a storage device that can be recognized by application servers. In Huawei RAID 2.0+, the LUN space comes from the storage pool. The disks in disk domains provide space for the storage pool. From the perspective of application servers, an LUN can be used as a disk. For example, on UNIX, LUNs have device names in the /dev/rdsk and /dev/dsk directory. On Windows, new LUNs after being formatted correspond to drive letters such as D:, E:, and F:.

Thick LUN

As a traditional type of LUN, thick LUNs support virtual resource allocation. They can be created, expanded, and compressed easily. A thick LUN gets full storage capacity from the storage pool once being created, namely, the LUN size equals to the allocated space. Therefore, the performance of a thick LUN is relatively high and predictable.

Thin LUN

As the other type of LUNs, thin LUNs support virtual resource allocation. They can be created, expanded, and compressed easily. An initial capacity allocation policy is set during the creation of thin LUNs. After thin LUNs are created, the storage system allocates an initial capacity to each LUN and retains the remaining capacity in the storage pool. When the usage of the allocated storage capacity reaches the threshold, the storage system allocates a certain amount of capacity from the storage pool to the thin LUNs. This process repeats until the thin LUNs reach the preset full capacity. Therefore, thin LUNs have higher storage capacity utilization.

Differences between a thick LUN and a thin LUN

Capacity allocation

• Thick LUNs, once being created, get full storage capacity from the storage pool.

• Thin LUNs get storage capacity on demand. A thin LUN is allocated with an initial capacity when created and then allocated with more capacity dynamically.

The following figure shows the difference between a thick LUN and a thin LUN in terms of capacity allocation.

Capacity reclamation

Capacity reclamation here refers to releasing the capacity of some LUNs to the storage pool for the use of other LUNs.

• Cpacity reclamation does not apply to a thick LUN, as it gets full capacity from the storage pool when created. Though data in a thick LUN is deleted, the allocated capacity is occupied by the thick LUN and cannot be used by other LUNs. However, if a thick LUN is deleted manually, its capacity can be reclaimed.

• When data in a thin LUN is deleted, space in the thin LUN can be released. In this way, storage capacity can be used dynamically, improving the utilization rate. The following figure shows the capacity reclamation of a thin LUN.

Performance

• A thick LUN provides higher performance for sequential reads/writes as it gets full storage capacity from the beginning, but it has some storage capacity wasted.

• The performance of a thin LUN is hampered as background formatting is required every time when the thin LUN expands capacity. In addition, capacity allocations may cause discontinuous disk storage space, so it takes more time for sequential reads/writes to find storage locations.

Application scenarios

Thick LUNs are applicable to scenarios where:

• High performance is required.

• Storage space utilization is less concerned.

• Costs are insensitive.

Thin LUNs are applicable to scenarios where:

• Moderate performance is required.

• Storage space utilization is more concerned.

• Costs are sensitive.

• Required storage capacity is hard to predict.

Summary

Thick LUNs and thin LUNs have their respective merits and weaknesses. Thick LUNs provide higher performance but lower storage space utilization. Thin LUNs utilize storage space fully and flexibly but provide lower performance. Select a proper type of LUNs based on onsite requirements.