ZFS vs ( VxFS | EXT3 | NTFS )

There is interesting entry at http://blogs.sun.com/Peerapong/entry/solaris_zfs_performance
about performance comparison of ZFS versus VxFS, ext3 and NTFS. While in almost all workloads ZFS is better or at least comparable to those three, in one of the most often met environments, OLTP with noncached, 8kB io (like Oracle on filesystem mounted without buffering - aka directio), VxFS is still almost three times faster then ZFS.

DTrace & Postgresql - io tuning

We have one application based on postgresql database. Recently we encountered some performance problems with uploading bigger amount of data into the database. To find out what is going on I wanted to compare normal utility of the server to the state when we are loading the additional data and see the differences. One of the things to watch was IO. To see what the server was doing at the moment I ran simple dtrace one-liner:

time dtrace -wn 'io:::start {@[args[2]->fi_pathname, args[0]->b_flags & B_READ ? "R" : "W"] = count ();}' -n 'END {trunc(@,3)}'

dtrace: description 'io:::start ' matched 6 probes
dtrace: description 'END' matched 1 probe
dtrace: allowing destructive actions
^C
CPU     ID                    FUNCTION:NAME
3      2                             :END

/export/home/postgres/nowa_baza/base/16399/33743                  R              4816
/export/home/postgres/nowa_baza/pg_xlog/000000010000000200000068  W              5436
/export/home/postgres/nowa_baza/base/16399/40932                  R            138166

real    12m36.483s
user    0m0.601s
sys     0m0.477s

At the moment the server was really idle so such amount of reads from database file was astounding ! I told our DBA about it and he answered "Wait a moment ...". After a minute I ran again the same DTrace one-liner and :

dtrace: description 'io:::start ' matched 6 probes
dtrace: description 'END' matched 1 probe
dtrace: allowing destructive actions
^C
CPU     ID                    FUNCTION:NAME
3      2                             :END

/export/home/postgres/nowa_baza/pg_xlog/000000010000000200000067  W              3005
/export/home/postgres/nowa_baza/base/16399/33743                  R              3812
/export/home/postgres/nowa_baza/pg_xlog/000000010000000200000068  W              4226

The reads from nowa_baza/base/16399/40932 disappeared ! I asked again our DBA what he had done and he answered "Just refreshed database statistics."

You know: just daily use of DTrace :-)