Dell Unity: Thin File System Automatic Shrink And Extension Policy

Automatic Shrink

Under normal operation, Dell Unity file systems automatically adjust the allocated space to optimize storage pool usage. An automatic shrink is this automatic adjustment depending on the ratio of used-to-allocated space. This is because a low used-to-allocated ratio does not represent ideal space utilization, since the allocated but unused space is wasted, and could potentially be used by other pool resources if it were reclaimed to the pool. On Dell Unity, file systems become eligible to be automatically shrunk (have their allocated space reduced) after maintaining an unacceptably low used-to-allocated space ratio for a predefined period. 

To illustrate this, consider this file system: size = 3 TB, allocated = 1 TB, used = 500 GB. The used space is very low relative to the allocated space, using only half of the space reserved from the storage pool. Unless clients begin using additional space from the file system, the Dell Unity system will eventually deallocate a portion of the 1 TB allocated space back to the underlying storage pool to potentially be used by other resources requiring this space. Automatic shrink only affects the allocated size, so the Size of 3 TB does not change.

In UnityOS 4.4 and earlier, a file system is eligible for automatic shrink if the used-to-allocated ratio remains under the low watermark, which is 70%. For example, a file system with 400 GB allocated requires 120 GB to be freed before automatic shrink is triggered. Since automatic shrink is based on the used-to-allocated ratio as a percentage, large file systems require more data to be deleted to be eligible for automatic shrink. Due to this, file systems that have 400 GB or more allocated use a different mechanism for automatic shrink. For these large file systems, if the gap between the used and allocated space is 20 GB or more, then the file system is eligible for automatic shrink. However, this was not appropriate for very large file systems with greater capacity fluctuations.

In UnityOS 4.5, the system dynamically adjusts the auto-shrink low watermark depending on the allocated capacity on the file system. This provides a better balance between prematurely shrinking and reclaiming space when appropriate. The chart below shows how the low watermark scales with the file system size. 

FS Auto Shrink Policy

As the Used space in a file system increases due to more data being written to the file system, more space must be reserved from the storage pool to accommodate this new data. As a result, the file system reserves additional space from the pool, increasing the allocated space in the file system. This happens without user intervention and will continue up to the advertised size of the file system. It is important to monitor the amount of free capacity on the pool when overprovisioning thin file systems. If the pool runs out of space, no additional capacity can be allocated to the file system and the file system becomes read-only, until this condition is cleared.

In UnityOS 4.4 and earlier, the automatic extension is triggered once the used-to-allocated space ratio reaches the high watermark, which is 75%. However, this does not scale well since bigger file systems have substantial amounts of capacity allocated to it that will not be immediately used. For example, a file system with 100 TB used has an extra 25 TB allocated from the automatic extend operation. In OE version 4.5, the system dynamically adjusts the auto-extend high watermark depending on the used capacity on the file system. 

Using this method, there is less wasted capacity since the available capacity is used more efficiently. 

Automatic extension only affects the allocated size, so the size does not change. When the allocated space reaches the file system's size, no new allocations are made unless an administrator first manually extends the file system's size. 

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