Ways in which you can invoke ‘dd’ to test the write speed:
dd bs=1M count=256 if=/dev/zero of=test
dd bs=1M count=256 if=/dev/zero of=test; sync
dd bs=1M count=256 if=/dev/zero of=test conv=fdatasync
dd bs=1M count=256 if=/dev/zero of=test oflag=dsync
What is the difference between those?
dd bs=1M count=256 if=/dev/zero of=test
The default behaviour of dd is to not “sync” (i.e. not ask the OS to completely write the data to disk before dd exiting). The above command will just commit your 128 MB of data into a RAM buffer (write cache) – this will be really fast and it will show you the hugely inflated benchmark result right away. However, the server in the background is still busy, continuing to write out data from the RAM cache to disk.
dd bs=1M count=256 if=/dev/zero of=test; sync
Absolutely identical to the previous case, as anyone who understands how *nix shell works should surely know that adding a ; sync does not affect the operation of previous command in any way, because it is executed independently, after the first command completes. So your (wrong) MB/sec value is already printed on screen while that sync is only preparing to be executed.
dd bs=1M count=256 if=/dev/zero of=test conv=fdatasync
This tells dd to require a complete “sync” once, right before it exits. So it commits the whole 128 MB of data, then tells the operating system: “OK, now ensure this is completely on disk”, only then measures the total time it took to do all that and calculates the benchmark result.
dd bs=1M count=256 if=/dev/zero of=test oflag=dsync
Here dd will ask for completely synchronous output to disk, i.e. ensure that its write requests don’t even return until the submitted data is on disk. In the above example, this will mean sync’ing once per megabyte, or 128 times in total. It would probably be the slowest mode, as the write cache is basically unused at all in this case.
Which one do you recommend to use?
This behaviour is perhaps the closest to the way real-world tasks behave:
dd bs=1M count=256 if=/dev/zero of=test conv=fdatasync
If your server or VPS is really fast and the above test completes in a second or less, try increasing the count= number to 1024 or so, to get a more accurate averaged result.