sd_readahead

systemd
Developer Lennart Poettering lennart@poettering.net
sd_readahead 3
sd_readahead Control ongoing disk boot-time read-ahead operations

    #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: and

`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

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