preallocate

Preallocate is the practice of reserving resources in advance of their use, with the aim of reducing runtime allocation overhead, fragmentation, or latency. It is used in software development, file systems, and database storage, as well as in server provisioning and virtualization. The approach involves estimating the needed amount of a resource and requesting that amount upfront, rather than acquiring it on demand.

In memory management, preallocation often means allocating a data structure's capacity ahead of time. For example, a vector or array may reserve space for N elements to avoid repeated reallocations as the structure grows. Languages provide different mechanisms, such as capacity settings or preallocation APIs. Tradeoffs include higher immediate memory usage and potential waste if the reserved space remains unused.

In storage systems, preallocation allocates disk blocks for a file or region in advance. This can reduce fragmentation and improve sequential I/O performance. Operating systems offer commands or system calls (for example, fallocate on Linux, SetFileSize on Windows) to allocate space without writing data. If the allocated space is later trimmed or not fully used, some systems may release unused blocks.

In databases and other large-scale data stores, preallocation may reserve extents or pages for future writes, minimize allocation overhead during peak loads, and improve write throughput. Prefetching and write-ahead strategies are related techniques that anticipate data needs.

Considerations for preallocation include workload predictability, memory or storage constraints, and the potential cost of wasted reservations. It is most effective when allocation costs are high or latency sensitivity is important; inappropriate preallocation can worsen resource contention or waste capacity.