Readdir-precache is a helper for serializing HDD access. It may help reduce noise and increase speeds when copying a large number of small files from HDDs.

Usually, copying a large number of files is performed in order, one file at a time. Even if file_3 can be adjacent to file_1 on disk, a copying software will try to copy file_2 before it even considers accesing file_3. That causes a lot of HDD head movements and a lot of access latency, which translate to lower speeds and higher noise levels.

Readdir-precache intercepts libc functions and tries to guess when a software copies files in bulk. It then peeks into a list of next to be copied files, determines where they are located on disk and reads them in sorted order. As Linux keeps just read data in its cache, when a software does read files, actuall data is served from the already populated cache in RAM.

Implemented as a library that needs to preloaded (LD_PRELOAD) into a process. Targeting primarily Midnight Commander and its way of accessing files. Should work for files on ext4 filesystems, but may work with other filesystems which have FIEMAP ioctl implemented.