1SD_READAHEAD(3)                  sd_readahead                  SD_READAHEAD(3)
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NAME

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

9       #include "sd-readahead.h"
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11       int sd_readahead(const char *action);
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DESCRIPTION

14       sd_readahead() may be called by programs involved with early boot-up to
15       control ongoing boot-time disk read-ahead operations. It may be used to
16       terminate read-ahead operations in case an uncommon disk access pattern
17       is to be expected and hence read-ahead replay or collection is unlikely
18       to have the desired speed-up effect on the current or future boot-ups.
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20       The action should be one of the following strings:
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22       cancel
23           Terminates read-ahead data collection, and drops all read-ahead
24           data collected during this boot-up.
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26       done
27           Terminates read-ahead data collection, but keeps all read-ahead
28           data collected during this boot-up around for use during subsequent
29           boot-ups.
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31       noreplay
32           Terminates read-ahead replay.
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RETURN VALUE

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

39       This function is provided by the reference implementation of APIs for
40       controlling boot-time read-ahead and distributed with the systemd
41       package. The algorithm it implements is simple, and can easily be
42       reimplemented in daemons if it is important to support this interface
43       without using the reference implementation.
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45       Internally, this function creates a file in /run/systemd/readahead/
46       which is then used as flag file to notify the read-ahead subsystem.
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48       For details about the algorithm check the liberally licensed reference
49       implementation sources:
50       http://cgit.freedesktop.org/systemd/systemd/plain/src/readahead/sd-readahead.c
51       and
52       http://cgit.freedesktop.org/systemd/systemd/plain/src/systemd/sd-readahead.h
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54       sd_readahead() is implemented in the reference implementation's drop-in
55       sd-readahead.c and sd-readahead.h files. It is recommended that
56       applications consuming this API copy the implementation into their
57       source tree. For more details about the reference implementation, see
58       sd-readahead(3)
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60       If -DDISABLE_SYSTEMD is set during compilation, this function will
61       always return 0 and otherwise become a NOP.
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EXAMPLES

64       Example 1. Cancelling all read-ahead operations
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66       During boots where SELinux has to relabel the file system hierarchy, it
67       will create a large amount of disk accesses that are not necessary
68       during normal boots. Hence it is a good idea to disable both read-ahead
69       replay and read-ahead collection.
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71           sd_readahead("cancel");
72           sd_readahead("noreplay");
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SEE ALSO

75       systemd(1), sd-readahead(3), daemon(7)
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79systemd 219                                                    SD_READAHEAD(3)
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