Degrading write performance using MySQL 5.0.24

[Phantom]

Hi All

We have an application that stores versioned data in MySQL. Everytime a
piece of data is retrieved and written to, it is stored in the database with
a new version and all old versions are subsequently deleted. We have a
request rate of 2 million reads per hour and 1.25 million per hour. What I
am seeing is that as the DB grows the performance on the writes degrades
substantially. When I start with a fresh database writes are at 70ms. But
once the database reaches around 10GB the writes are at 200 ms. The DB can
grow upto 35GB. I have tried almost performance related tuning described in
the MySQL documentation page.

What do I need to look at to start addressing this problem or this is how
the performance is going to be ?

Some the parameters I have tuned are :

innodb_buffer_pool_size = 1536 MB.
key_buffer = 512M
table_cache  = 50M
innodb_log_file_size = 512M
innodb_log_buffer_size = 8M
inno_flush_method = O_DIRECT
innodb_flush_log_at_trx_commit = 0

What else can I do ? Please advice.

Thanks
A