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)