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 OPTIONS COMMAND DIRECTORY | FILE 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 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. systemd-readahead understands the following options: -h --help Prints a short help text and exits. --version Prints a short version string and exits. --max-files= Maximum number of files to read ahead. Only valid for thes collect command. --file-size-max= Maximum size of files in bytes to read ahead. Only valid for the collect and replay commands. --timeout= Maximum time in microseconds to spend collecting data. Only valid for the collect command. The following commands are understood by systemd-readahead: collect [DIRECTORY] Collect read-ahead data on early boot. When terminating, it will write out a pack file to the indicated directory containing the read-ahead data. replay [DIRECTORY] Perform read-ahead on the specified directory tree. analyze [FILE] Dumps the content of the read-ahead pack file to the terminal. For each file, the output lists approximately how much will be read ahead by the replay command. systemd1