Bob Friesenhahn wrote:
There has been no forward progress on the ZFS read performance issue for
a week now. A 4X reduction in file read performance due to having read
the file before is terrible, and of course the situation is considerably
worse if the file was previously mmapped as well. Many of us have sent
a lot of money to Sun and were not aware that ZFS is sucking the life
out of our expensive Sun hardware.
It is trivially easy to reproduce this problem on multiple machines. For
example, I reproduced it on my Blade 2500 (SPARC) which uses a simple
mirrored rpool. On that system there is a 1.8X read slowdown from the
file being accessed previously.
In order to raise visibility of this issue, I invite others to see if
they can reproduce it in their ZFS pools. The script at
http://www.simplesystems.org/users/bfriesen/zfs-discuss/zfs-cache-test.ksh
Implements a simple test. It requires a fair amount of disk space to
run, but the main requirement is that the disk space consumed be more
than available memory so that file data gets purged from the ARC. The
script needs to run as root since it creates a filesystem and uses
mount/umount. The script does not destroy any data.
There are several adjustments which may be made at the front of the
script. The pool 'rpool' is used by default, but the name of the pool
to test may be supplied via an argument similar to:
# ./zfs-cache-test.ksh Sun_2540
zfs create Sun_2540/zfscachetest
Creating data file set (3000 files of 8192000 bytes) under
/Sun_2540/zfscachetest ...
Done!
zfs unmount Sun_2540/zfscachetest
zfs mount Sun_2540/zfscachetest
I've opened the following bug to track this issue:
6859997 zfs caching performance problem
We need to track down if/when this problem was introduced or if it has
always been there.
-Mark
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