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
       and 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(3)

       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(3), daemon(7)



systemd 215                                                                       SD_READAHEAD(3)