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systemd-readahead-collect.service(8) [centos man page]


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)

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SD_READAHEAD(3) 						   sd_readahead 						   SD_READAHEAD(3)

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: 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
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 208 SD_READAHEAD(3)
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