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On OE version 4.1, file systems can be configured with a minimum allocation size. This enables the storage administrator to control the guaranteed space allocated to a file system. When the minimum allocation size is configured during the creation of a new file system, the size specified is allocated immediately at creation. For example, you could create a 500 GB file system with a minimum allocation size of 100 GB. The host sees a 500 GB file system, but only 100 GB has been allocated to this file system from the pool. Since the remaining capacity is thinly provisioned, additional capacity is allocated as clients consume more than 100 GB of the file system. The minimum allocation size can be configured between 3 GB (default) or up to the size of the file system. This setting can be configured during file system creation and it can be changed at any time.
The minimum allocation size is also used to prevent the automatic shrink feature from reclaiming too much capacity from the file system. For example, if a file system only has 50 GB used but has 100 GB allocated due to the minimum allocation size, automatic shrink would normally be initiated. However, since the minimum allocation size is designed to guarantee allocation to the file system, automatic shrink is not initiated in this scenario. On the other hand, if the minimum allocation size is set to 30 GB but the file system still has 100 GB allocated, automatic shrink is initiated as normal.
Minimum allocation size is compatible with features such as replication and snapshots. When used on a replicated file system, both the source and destination file system have the same minimum allocation size. This is because this setting is propagated to the destination as part of the synchronizations. If a snapshot has a different minimum allocation size setting compared to the file system, restoring the snapshot also restores the minimum allocation size setting.
Starting with OE version 4.2, newly created file systems do not have the option to configure the minimum allocation size. This is due to architectural changes made on the file system to enable support for data reduction. However, Unisphere provides the ability to create thick file systems if space reservation is required. There is no way to convert from thin to thick or thick to thin after a file system is created.