how to do filesystem testing?

My first answer would also be bonnie++, a quick search also reveals Iozone3. Both are available as Debian packages. If you do simpler tests (cp, mv, rm), you should consider clearing the cache kept by the kernel with echo 1 > /proc/sys/vm/drop_caches before each test otherwise it might cause differences that are not representative of the filesystem itself.

Here are two more options, I've found them really useful for filesystem benchmarking.

1) Postmark - it's a benchmark which emulates real-world usage of a filesystem accessed by a busy mail server.

Download: http://www.freshports.org/benchmarks/postmark/

When testing, make sure you've compared different options of ext3 logging - they may be quite different in terms of performance (writeback logging gave best result in my Postmark experiments).

2) Filebench - another great benchmark, giving you even more flexibility. If you have a specific reason for considering a move from ext3, you'll probably like filebench - it has many different workloads so that you can only concentrate on the performance bottleneck which interests you (file create/delete operations or sequential read/write vs random ones, to give you a few examples).

You have to google for it as I'm not allowed any more hyperlinks yet - the project itself is hosted on SourceForge, and a really nice quick start guide is found on OpenSolaris.org website.

One final note: be sure to spread the load across as many disks (spindles) as possible for the most accurate results. It also makes sense to allocate a few disks for benchmarking specifically, and newfs them before each run of your benchmark.

Don't just test for speed, also consider reliability. Try to, e.g., power down disks on a busy filesystem and see what's left.

The quality of the repair and recovery tools available is also important, and very hard to test discretely. Block structure may e.g. inhibit tools that try to salvage data in raw mode from a unsalvageable filesystem.

For more hints on testing a filesystem under very harsh beatings you may be interesting what the ZFS guys did: One Two