Libtorrent - Features - Disk Caching

Disk Caching

All disk I/O in libtorrent is done asynchronously to the network thread, by the disk io thread. When a block is read, the disk io thread reads all subsequent blocks from that piece into the read cache, assuming that the peer requesting the block will also request more blocks from the same piece. This decreases the number of syscalls for reading data. It also decreases delay from seeking.

Similarly, for write requests, blocks are cached and flushed to disk once one full piece is complete or the piece is the least recently updated one when more cache space is needed. The cache dynamically allocates space between the write and read cache. The write cache is strictly prioritized over the read cache.

The cache blocks that are in use, are locked into physical memory to avoid it being paged out to disk. Allowing the disk cache to be paged out to disk means that it would become extremely inefficient to flush it, since it would have to be read back into physical memory only to be flushed back out to disk again.

In order to conserve memory, and system calls, iovec file operations are used to flush multiple cache blocks in a single call.

On low-memory systems, the disk cache can be disabled altogether or set to smaller limit, to save memory.