Re: [Jfs-discussion] benchmark results

Christian Kujau
2009-12-28T01:23:39Z

On Sun, 27 Dec 2009 at 17:33, tytso@mit.edu wrote:
> Yes, but given many of the file systems have almost *exactly* the same

"Almost" indeed - but curiously enough some filesystem are *not* the same,
although they should. Again: we have 8GB RAM, I'm copying ~3GB of data, so
why _are_ there differences? (Answer: because filesystems are different).
That's the only point of this test. Also note the disclaimer[0] I added to
the results page a few days ago.

> measurement is 5 times the disk bandwidith as measured by hdparm, it
> makes me suspect that you are doing this:
> /bin/time /bin/cp -r /source/tree /filesystem-under-test
> sync

No, I'm not - see the test script[1] - I'm taking the time for cp/rm/tar
*and* sync. But even if I would only take the time *only* for say "cp",
not the sync part. Still, it would be a valid comparison across
filesystems (the same operation for every filesystem) also a not very
realistic one - because in the real world I *want* to make sure my data is
on the disk. But that's as far as I go in these tests, I'm not even
messing around with disk caches or HBA caches - that's not the scope of
these tests.

> You might notice it if you include the "sync" in the timing, i.e.:
> /bin/time /bin/sh -c "/bin/cp -r /source/tree /filesystem-under-test;/bin/sync"

Yes, that's exactly what the tests do.

> "/bin/cp" returns, then sure, do whatever you want. But if you want
> the tests to have meaning if, for example, you have 2GB of memory and
> you are copying 8GB of data,

For the bonnie++ tests I chose a filesize (16GB) so that disk performance
will matter here. As the generic tests shuffle around much more smaller
data, no disk performance, but filesystem performance is measured (and
compared to other filesystems) - well aware of the fact that caches *Are*
being used. Why would I want to discard caches? My daily usage pattern
(opening webrowsers, terminal windows, spreadcheats deal with much smaller
datasets and I'm happy that Linux is so hungry for cache - yet some
filesystems do not seem to utilize this opportunity as good as others do.
That's the whole point of this particular test. But constantly explaining
my point over and over again I see what I have to do: I shall run the
generic tests again with much bigger datasets, so that disk-performance is
also reflected, as people do seem to care about this (I don't - I can
switch filesystems more easily than disks).

> The bottom line is that it's very hard to do good comparisons that are
> useful in the general case.

And it's difficult to find out what's a "useful comparison" for the
general public :-)

Christian.

[0] http://nerdbynature.de/benchmarks/v40z/2009-12-22/
[1] http://nerdbynature.de/benchmarks/v40z/2009-12-22/env/fs-bench.sh.txt
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