FreeBSD and Linux SSD Write Speeds

Valeri Galtsev galtsev at
Wed Jun 21 16:28:01 UTC 2017

On Wed, June 21, 2017 11:04 am, Tim Daneliuk wrote:
> Disclaimer: Tests below run on lightly loaded systems, but results are ...
> surprising:
> Test Case: dd if=/dev/zero of=foo bs=8M count=512

First, I would modify the test and include sync command inside timing,
something along these lines:

sync; date; dd if=/dev/zero of=foo bs=8M count=512; sync; date

( and your measure is the difference of output of two "date" commands, not
what dd reports )

Second, I would make the size dd sends really big, say 10 GB.

( your bs probably _is_ multiple of fs block size, but that is something
to watch for too).

Next, I would exclude filesystem at all, cut small ( larger than 10 GB)
partition off the SSD, and make dd destination that partition, and see
what difference does that make.

Incidentally, what filesystem is it on FreeBSD side, and how many open
files (owned by other processes) do you have on the fiesystems your dd
destination file is on?


> Linux 4.4.0-21-generic on a 2.66GHz Core2 Duo w/8GB memory, older OCZ
> SSD/ext4:  310MB/sec writes
> FreeBSD 10-STABLE on an 3.2 GHz Quad Core i5 w/8GB memory, newer Kingston
> SSD/ufs:  210MB/sec writes
> Results are repeatable.
> So, what is the likely culprit making FreeBSD 1/3 slower?  The FreeBSD
> system does does / nfs exported (which I don't quite yet understand since
> all the nfs mount points are below it) at the moment, but there is little
> or no nfs traffic.
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Valeri Galtsev
Sr System Administrator
Department of Astronomy and Astrophysics
Kavli Institute for Cosmological Physics
University of Chicago
Phone: 773-702-4247

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