On 03/09/2012 11:00, Josef Bacik wrote:
On Fri, Mar 9, 2012 at 10:11 AM, David Quigley
<seli...@davequigley.com> wrote:
On 03/09/2012 08:42, Przemek Klosowski wrote:
On 03/09/2012 01:43 AM, Adam Williamson wrote:
On Thu, 2012-03-08 at 22:19 -0700, Chris Murphy wrote:
I'm not sure how useful 'time' is as a benchmark for file copies.
Don't file transfers get cached and return to a console as
'complete'
long before the data is ever written, sometimes?
I'm pretty sure you sometimes hit the case where you copy 200MB to
a USB
stick, it returns to the console pretty fast, but the light on the
stick
is still flashing, and if you run 'sync', it sits there for quite
a
while before returning to the console, indicating the transfer
wasn't
really complete. So I'm not sure 'time'ing a 'cp' is an accurate
test of
actual final-write-to-device.
That is true---but in that case, we could flush the disks. and then
time the operation followed by another flush, i.e.:
sync; time (cp ...; sync)
I assume that the old-time Unix superstition of calling sync three
times no longer applies :)
Perhaps a dedicated disk benchmark like bonnie++ would be a better
test, though.
If you want to look seriously into file-system benchmarking I would
suggest
looking into the work done by the fsbench people at Stony Brook
University's
Filesystem and Storage Lab (FSL). There is a survey paper there for
the last
decade of FS benchmarks and their short commings and what should be
addressed.
http://www.fsl.cs.sunysb.edu/project-fsbench.html
fsbench is amazing, I also use fio and fs_mark to test various
things.
But these are artificial workloads! These numbers don't mean a
damned thing to anybody, the only way you know if a fs is going to
work for you is if you run your application on a couple of fses and
figure out which one is faster for you! For example if you mostly
compile kernels, btrfs is fastest. However if you mostly use a fs
for
your virt images, don't use btrfs! It's all a matter of workloads
and
no amount of benchmarking is going to be able to tell you if your pet
workload is going to work well at all.
The work that we file system developers do with benchmarking is to
stress particular areas of our respective filesystems. For example,
with Dave's tests he was testing our ability to scale as the amount
of
metadata gets ridiculously huge. He has exposed real problems that
we
are working on fixing. However these real problems are things that I
imagine 99% of you will never run into, and therefore should not be
what you use to base your decisions on.
So let's try to remember that benchmarks mean next to nothing to real
users, unless watching iozone output happens to be what you use your
computer for. Thanks,
Josef
True fsbench can be used for micro benchmarking but if you read the
paper on that page it also goes over the bechmarking suites that are
supposed to provide real world workloads as well. Copying files isn't
much more complex than a couple micro benchmarks. It really is only
testing read/write performance. If you want to do real performance
testing like you said you need to be running real world workloads. The
database benchmarks in the paper cover some of them but also provides
criticism about the nature of the workloads. The cool thing about the
projects on those pages is that they allow you to capture the same
workload and replay them on different filesystems. You can hope you get
the same workload twice across two runs or you can capture the workload
with tracefs and replay it with replayfs. Now this does introduce some
overhead as these are stackable filesystems so they are going to add an
additional thin vfs like layer to your analysis but if both of the
filesystems have that you can factor that overhead out and get
performance data for each filesystem individually.
Dave
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