Strange performance numbers

[AlgorithmsAreCool]

I have been looking at DBreeze recently and saw the great insert performance numbers. But I am getting very strange results from a pretty simple benchmark i wrote.

Here is my code

private static void Run(int runCount, string table, DBreezeEngine db)
{
    Console.WriteLine("Testing " + runCount);

    string simulatedRow = $@"C:\This\Is\A\Fake\Directory, {Guid.NewGuid()}, 0.00, 0, 2, 3";

    var sw = Stopwatch.StartNew();

    using (var t = db.GetTransaction())
    {
        for (int i = 0; i < runCount; i++)                
            t.Insert(table, i, simulatedRow);
        t.Commit(); 
    }

    sw.Stop();

    Console.WriteLine("Insert took {0:N2}. Rate = {1:N3}", sw.Elapsed.TotalMilliseconds, runCount / sw.Elapsed.TotalSeconds);
    Console.WriteLine();
}

Starting with an empty directory (using on disk storage), i run the following

(Different tables for each test)

Run(100, "t1", db);
Run(1000, "t2", db);
Run(10000, "t3", db);
Run(100000, "t4", db);
Run(1000000, "t5", db);

and get

Testing 100
Insert took 110.10. Rate = 908.285

Testing 1000
Insert took 80.76. Rate = 12,381.984

Testing 10000
Insert took 113.05. Rate = 88,452.993

Testing 100000
Insert took 405.44. Rate = 246,645.924

Which is great!

But if i run (reusing the same table)

Run(100, "t1", db);
Run(1000, "t1", db);
Run(10000, "t1", db);
Run(100000, "t1", db);

My results are very different. The last test is about 66x slower.

Testing 100
Insert took 133.47. Rate = 749.257

Testing 1000
Insert took 309.03. Rate = 3,235.900

Testing 10000
Insert took 2,686.21. Rate = 3,722.711

Testing 100000
Insert took 26,852.87. Rate = 3,723.996

If find that if i delete the table in between each run the speed goes back to normal.

Is it known that multiple transaction against the same table cause this performance drop or am i seeing something strange?

Tested on Windows 8.1 x64, .Net 4.5.2, AnyCPU

[blaze]

Second time you make an update, that intends physically overwrite existing rows. To make batch update faster you have to switch table to a special mode. This switch concerns mostly big update-batches and mechanical hard drives.
Switch is called tran.Technical_SetTable_OverwriteIsNotAllowed("here table name");
Read more in documentation.

[AlgorithmsAreCool]

Ah i see. I will give that a shot.

[KyuubiYoru]

Why is the Disk access not done with tran.Commit(); ?

[blaze]

Definitely, it is done :)

[KyuubiYoru]

Doing an update with tran.Insert() is causing an disk access and is much slower.
Sure i can do tran.Technical_SetTable_OverwriteIsNotAllowed("here table name"); then an Update is fast like an Insert, but the Database file size is growing very fast.

[blaze]

There are a lot of options and possibilities. Please read documentation, find compromises.