Have you considered running your script with ZFS pre-fetching disabled
altogether to see if
the results are consistent between runs?
Brad
Brad Diggs
Senior Directory Architect
Virtualization Architect
xVM Technology Lead
Sun Microsystems, Inc.
Phone x52957/+1 972-992-0002
Mail bradley.di...@sun.com
Blog http://TheZoneManager.com
Blog http://BradDiggs.com
On Jul 15, 2009, at 9:59 AM, Bob Friesenhahn wrote:
On Wed, 15 Jul 2009, Ross wrote:
Yes, that makes sense. For the first run, the pool has only just
been mounted, so the ARC will be empty, with plenty of space for
prefetching.
I don't think that this hypothesis is quite correct. If you use
'zpool iostat' to monitor the read rate while reading a large
collection of files with total size far larger than the ARC, you
will see that there is no fall-off in read performance once the ARC
becomes full. The performance problem occurs when there is still
metadata cached for a file but the file data has since been expunged
from the cache. The implication here is that zfs speculates that
the file data will be in the cache if the metadata is cached, and
this results in a cache miss as well as disabling the file read-
ahead algorithm. You would not want to do read-ahead on data that
you already have in a cache.
Recent OpenSolaris seems to take a 2X performance hit rather than
the 4X hit that Solaris 10 takes. This may be due to improvement of
existing algorithm function performance (optimizations) rather than
a related design improvement.
I wonder if there is any tuning that can be done to counteract
this? Is there any way to tell ZFS to bias towards prefetching
rather than preserving data in the ARC? That may provide better
performance for scripts like this, or for random access workloads.
Recent zfs development focus has been on how to keep prefetch from
damaging applications like database where prefetch causes more data
to be read than is needed. Since OpenSolaris now apparently
includes an option setting which blocks file data caching and
prefetch, this seems to open the door for use of more aggressive
prefetch in the normal mode.
In summary, I agree with Richard Elling's hypothesis (which is the
same as my own).
Bob
--
Bob Friesenhahn
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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