As far as I can tell rsync doesn't support this. How hard would it be
to implement this? Is it trivial enough to just change the calls in the
code with sed? I think this can significantly reduce CPU usage and
increase I/O speed when dealing with fast storage solutions. It can make
a huge difference when say transferring 30TB of data.
Here are some tests I did. So far the only thing I know of that can use
direct I/O is dd which showed a huge improvement. CPU usage went from
very high down to under 15% and the speed almost doubled.
Is there anything else I can do to tune rsync to give me better speed
and/or less CPU usage as I seem to be hitting a CPU bottleneck in the
case of rsync.
In my tests I saw the highest CPU usage with rsync and lowest throughput:
*cp (no direct I/O):*
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
23356 root 20 0 10740 676 584 D 74 0.0 0:16.48 cp
23344 root 20 0 0 0 0 D 9 0.0 0:04.80 flush-8:64
639 root 20 0 0 0 0 R 9 0.0 0:12.28 kswapd0
root@dekabutsu: 05:54 AM :/data2# time cp -avf /data/winxp.img /data2/
`/data/winxp.img' -> `/data2/winxp.img'
real 1m10.767s
user 0m0.692s
sys 0m47.401s
*Rsync (no direct I/O):*
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
23397 root 20 0 11572 616 256 R 97 0.0 0:08.48 rsync
23395 root 20 0 11864 1292 848 R 84 0.0 0:07.35 rsync
root@dekabutsu: 05:56 AM :/data2# time rsync --progress -av
/data/winxp.img /data2/
building file list ...
1 file to consider
winxp.img
37589497344 100% 223.43MB/s 0:02:40 (xfer#1, to-check=0/1)
sent 37594086010 bytes received 42 bytes 234231065.74 bytes/sec
total size is 37589497344 speedup is 1.00
real 2m40.552s
user 3m36.020s
sys 1m8.181s
*dd (direct I/O)=*
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
23452 root 20 0 14960 1664 580 D 13 0.0 0:00.76 dd
root@dekabutsu: 06:00 AM :/data2# time dd bs=1M iflag=direct
oflag=direct if=/data/winxp.img of=/data2/winxp.img
35848+1 records in
35848+1 records out
37589497344 bytes (38 GB) copied, 46.8669 s, 802 MB/s
real 0m46.869s
user 0m0.031s
sys 0m6.043s
*dd (no direct I/O):*
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
23455 root 20 0 14960 1664 576 R 65 0.0 0:06.59 dd
23344 root 20 0 0 0 0 S 20 0.0 0:17.88 flush-8:64
root@dekabutsu: 06:01 AM :/data2# time dd bs=1M if=/data/winxp.img
of=/data2/winxp.img
35848+1 records in
35848+1 records out
37589497344 bytes (38 GB) copied, 83.5282 s, 450 MB/s
real 1m23.530s
user 0m0.048s
sys 0m54.304s
(so pretty close to the speed of cp).
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