systemd Developer Lennart Poettering lennart@poettering.net systemd-readahead-replay.service 8 systemd-readahead-replay.service systemd-readahead-collect.service systemd-readahead-done.service systemd-readahead-done.timer systemd-readahead Disk read ahead logic systemd-readahead-replay.service systemd-readahead-collect.service systemd-readahead-done.service systemd-readahead-done.timer /usr/lib/systemd/systemd-readahead systemd-readahead-collect.service is a service that collects disk usage patterns at boot time. systemd-readahead-replay.service is a service that replays this access data collected at the subsequent boot. Since disks tend to be magnitudes slower than RAM this is intended to improve boot speeds by pre-loading early at boot all data on disk that is known to be read for the complete boot process. systemd-readahead-done.service is executed a short while after boot completed and signals systemd-readahead-collect.service to end data collection. On this signal this service will then sort the collected disk accesses and store information about them disk in /.readahead. Normally, both systemd-readahead-collect.service and systemd-readahead-replay.service are activated at boot so that access patterns from the preceding boot are replayed and new data collected for the subsequent boot. However, on read-only media where the collected data cannot be stored it might be a good idea to disable systemd-readahead-collect.service. On rotating media, when replaying disk accesses at early boot systemd-readahead-replay.service will order read requests by their location on disk. On non-rotating media, they will be ordered by their original access timestamp. If the file system supports it systemd-readahead-collect.service will also defragment and rearrange files on disk to optimize subsequent boot times. systemd1