Extent-based file systems use a strategy called "allocate-on-flush" to resist fragmentation primarily because it allows the system to make more informed decisions about how to allocate space. Here's how it works and why it's beneficial:

1. Delayed Allocation: Allocate-on-flush is a form of delayed allocation. Instead of immediately assigning disk blocks when data is written, the file system waits until the data is flushed from memory to disk. This delay allows the system to gather more information about the size and nature of the data being written.

2. Reduced Fragmentation: By delaying allocation, the file system can allocate larger contiguous blocks (extents) of disk space at once. This reduces fragmentation because the system can better match the size of the extent to the actual size of the data being written. If allocations were made immediately, smaller or misaligned extents might be used, leading to fragmentation over time.

3. Optimization Opportunities: With more information about the data, the file system can optimize the layout on disk. For example, it can place frequently accessed files closer together or align extents with underlying storage characteristics (like RAID stripe sizes).

4. Improved Performance: Reducing fragmentation and optimizing data layout can lead to better performance. Fewer seeks and more efficient read/write operations are possible when data is stored in larger, contiguous extents.

5. Adaptive Allocation: Allocate-on-flush enables the file system to adapt to changes in workload or file size patterns. It can dynamically adjust allocation strategies based on current conditions and usage patterns.

Overall, allocate-on-flush helps extent-based file systems maintain efficiency and performance by minimizing fragmentation and optimizing the use of disk space.