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systemd-readahead(8) [centos man page]

SYSTEMD-READAHEAD-REPLAY.SERVICE(8)			 systemd-readahead-replay.service		       SYSTEMD-READAHEAD-REPLAY.SERVICE(8)

NAME

       systemd-readahead-replay.service, systemd-readahead-collect.service, systemd-readahead-done.service, systemd-readahead-done.timer, systemd-
       readahead - Disk read ahead logic

SYNOPSIS

       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]

DESCRIPTION

       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.

OPTIONS

       systemd-readahead understands the following options:

       -h, --help
	   Prints a short help text 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.

COMMANDS

       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.

SEE ALSO

       systemd(1)

systemd 208												       SYSTEMD-READAHEAD-REPLAY.SERVICE(8)

Check Out this Related Man Page

SD_READAHEAD(3) 						   sd_readahead 						   SD_READAHEAD(3)

NAME

       sd_readahead - Control ongoing disk boot-time read-ahead operations

SYNOPSIS

       #include "sd-readahead.h"

       int sd_readahead(const char *action);

DESCRIPTION

       sd_readahead() may be called by programs involved with early boot-up to control ongoing boot-time disk read-ahead operations. It may be
       used to terminate read-ahead operations in case an uncommon disk access pattern is to be expected and hence read-ahead replay or collection
       is unlikely to have the desired speed-up effect on the current or future boot-ups.

       The action should be one of the following strings:

       cancel
	   Terminates read-ahead data collection, and drops all read-ahead data collected during this boot-up.

       done
	   Terminates read-ahead data collection, but keeps all read-ahead data collected during this boot-up around for use during subsequent
	   boot-ups.

       noreplay
	   Terminates read-ahead replay.

RETURN VALUE

       On failure, these calls return a negative errno-style error code. It is generally recommended to ignore the return value of this call.

NOTES

       This function is provided by the reference implementation of APIs for controlling boot-time read-ahead and distributed with the systemd
       package. The algorithm it implements is simple, and can easily be reimplemented in daemons if it is important to support this interface
       without using the reference implementation.

       Internally, this function creates a file in /run/systemd/readahead/ which is then used as flag file to notify the read-ahead subsystem.

       For details about the algorithm check the liberally licensed reference implementation sources:
       http://cgit.freedesktop.org/systemd/systemd/plain/src/readahead/sd-readahead.c resp.
       http://cgit.freedesktop.org/systemd/systemd/plain/src/systemd/sd-readahead.h

       sd_readahead() is implemented in the reference implementation's drop-in sd-readahead.c and sd-readahead.h files. It is recommended that
       applications consuming this API copy the implementation into their source tree. For more details about the reference implementation see sd-
       readahead(7)

       If -DDISABLE_SYSTEMD is set during compilation this function will always return 0 and otherwise become a NOP.

EXAMPLES

       Example 1. Cancelling all read-ahead operations

       During boots where SELinux has to relabel the file system hierarchy, it will create a large amount of disk accesses that are not necessary
       during normal boots. Hence it is a good idea to disable both read-ahead replay and read-ahead collection.

	   sd_readahead("cancel");
	   sd_readahead("noreplay");

SEE ALSO

       systemd(1), sd-readahead(7), daemon(7)

AUTHOR

       Lennart Poettering <lennart@poettering.net>
	   Developer

systemd 							    10/07/2013							   SD_READAHEAD(3)
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