rarara

Prime buffer cache for a file via readahead from the command line (linux only).

readahead() initiates readahead on a file so that subsequent reads from that file will be satisfied from the cache, and not block on disk I/O (assuming the readahead was initiated early enough and that other activity on the system did not in the meantime flush pages from the cache).

Usage

$ rarara FILENAME [OFFSET [COUNT]]

Try to cache the whole file starting at the beginning by default.

Alternatives

dd, which is POSIX. Example 2.5G file:

$ time rarara big.file 0 2459650481
real    0m13.803s

$ time dd if=big.file of=/dev/null bs=4096 count=600501 skip=0
real    0m14.394s

7.4 GB file:

$ time dd if=bigger.file of=/dev/null bs=64k
113123+1 records in
113123+1 records out
7413691873 bytes (7.4 GB) copied, 43.7198 s, 170 MB/s

real    0m43.778s
user    0m0.028s
sys     0m3.672s

$ time rarara bigger.file
readahead [0:7413691873]

real    0m23.753s
user    0m0.004s
sys     0m0.464s

See also

• http://unix.stackexchange.com/q/145034/376
• http://www.reddit.com/r/linux/comments/2axv21/tiny_tool_to_prime_buffer_cache_for_a_file/
• http://www.tldp.org/LDP/sag/html/buffer-cache.html
• http://man7.org/linux/man-pages/man2/readahead.2.html
• http://insights.oetiker.ch/linux/fadvise/
• https://lwn.net/Articles/155510/