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
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:
Maximum number of files to read ahead. Only valid for thes
Maximum size of files in bytes to read ahead. Only valid for the
collect and replay commands.
Maximum time in microseconds to spend collecting data. Only valid
for the collect command.
Print a short help text and exit.
Print a short version string and exit.
The following commands are understood by systemd-readahead:
Collect read-ahead data on early boot. When terminating, it will
write out a pack file to the indicated directory containing the
Perform read-ahead on the specified directory tree.
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.
This page is part of the systemd (systemd system and service manager)
project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a bug
report for this manual page, see
page was obtained from the project's upstream Git repository
(git://anongit.freedesktop.org/systemd/systemd) on 2015-12-05. If
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systemd 216 SYSTEMD-READAHEAD-REPLAY.SERVICE(8)