Best MySQL cache settings for 8gb RAM dedicated MySQL server using only InnoDB (5gb database)

This is hard without knowing much about the database itself. There are a few tools you should be aware of;

• mysqltuner.pl Github
• MySQL Tuning Primer Github

About storing the entire database in memory; Any queries that are doing changes on the database will remain open until the write is performed on the disk. The only thing that can avoid the disk to be a bottleneck, is a disk-controller with a write cache.

I would start with the following changes from the defaults:

key_buffer_size = 128M
thread_stack = 128K
thread_cache_size = 8
table_cache = 8192
max_heap_table_size = 256M
query_cache_limit = 4M
query_cache_size = 512M

innodb_buffer_pool_size = 4G 

# This is crucial to avoid checkpointing all the time:
innodb_log_file_size = 512M

# If you have control on who consumes the DB, and you don't use hostnames when you've set up permissions - this can help as well.
skip_name_resolve

Then I'd see how things go, and try different things based on (among other things) the output of the tools mentioned above. I would also make sure to graph trends with a monitoring tool, such as Munin or Cacti, to see what kind of workload I'm actually dealing with. Personally, I have great experience with the MySQL-plugins provided with Munin.

IMHO you should be able to go with

innodb_buffer_pool_size=5G

That would be 62.5% of RAM with a sufficient amount of RAM for the Server OS plus memory for DB Connections

@kvisle recommended using mysqltuner.pl. That script is excellent for judging the amount of RAM to dedicate to join_buffer_size, sort_buffer_size, read_buffer_size, and read_rnd_buffer_size. Those 4 buffers added together are multiplied by max_connections. That answer is added to static buffers (innodb_buffer_pool_size + key_buffer_size). The combined sums are reported. If that combined sum exceeds 80% of RAM, that is when you must lower those buffer sizes. mysqltuner.pl will be very helpful in this regard.

Since all you data is InnoDB, you can make key_buffer_size (key cache buffer for MyISAM indexes) very low (I recommend 64M).

Here is a post I made in the DBA StackExchange to calculate a recommended size of innodb_buffer_pool_size.

UPDATE 2011-10-15 19:55 EDT

If you know you will have 5GB of data, then my first recommendation was OK. However, I forgot to add one thing:

[mysqld]
innodb_buffer_pool_size=5G
innodb_log_file_size=1280M

The log file size must be 25% of the InnoDB Buffer Pool

UPDATE 2011-10-16 13:36 EDT

The 25% rule is based strictly on using two log files. While it is possible to use multiple innodb log files, two usually works best.

Others have expressed using 25%

• http://lists.mysql.com/mysql/203879
• http://teaching-2.cs.uml.edu/MySQLdocs/MySQL_Manual/InnoDB_start.html
• http://www.issociate.de/board/post/395589/innodb_buffer_pool_size._How_to_setup_with_more_than_5G_?.html

However, in all fairness, someone from the original InnoBase Oy company expressed not using the 25% rule because of having larger InnoDB Buffers Pool.

Naturally, the 25% rule cannot work when having huge amounts of RAM. In fact, the largest innodb_log_file_size allowed using only 2 log files is 2047M, since the combined size of the log file must be less than 4G (4096M)

CASE IN POINT : One of my employer's clients has a DB server with 192GB RAM. There is no way to have 48G log files. I simlpy use the max file size for an innodb log file, 2047M. @Kvisle's comment on my answer simply gives a link stating you do not have to limit yourself to two log files. If you have N log files, they cannot total 4G. My 25% rule is just in a perfect world (DB Server with 8GB or less).