On Thu, May 13, 2010 at 11:31 PM, Daniel Bartholomew <dbart@askmonty.org> wrote:

== Segmented Key Cache Syntax ==

New global variable: key_cache_partitions, it sets the number of segments in a key cache. Valid values for this variable are whole

Since we changed the name of this feature to key_cache_partitions (after it was committed), should the name of the variable be changed to key_cache_segments?

Question for the developers: A key_cache_partitions value of '0' means the key cache will not be segmented, it will be a regular (or simple) key cache. A value of '1' means the key cache will be a segmented key cache with a single segment. Is there any benefit to having a "single-segment segmented key cache" compared to a regular "simple key cache" or are they practically the same thing?

Excellent question :-) henrik -- email: henrik.ingo@avoinelama.fi tel: +358-40-5697354 www: www.avoinelama.fi/~hingo book: www.openlife.cc

Hi Daniel, Daniel Bartholomew wrote:

Hello everyone,

Summarizing from my previous email: I'm working on the documentation for the Segmented Key Cache, one of the new features in MariaDB 5.2. My previous email was about the "About" section of the documentation. This email is about the "Syntax" section.

For the syntax section, I see there is one new global variable defined for this feature and a new KEY_CACHES table in the information_schema database. Are there any other user-visible items which should be mentioned?

Here is a first draft of the syntax section:

----------------------------------------------------------------------

== Segmented Key Cache Syntax ==

New global variable: key_cache_partitions, it sets the number of segments in a key cache. Valid values for this variable are whole numbers between 0 and 64. If the number of partitions is set to a number greater than 64 the number of partitions will be truncated to 64 and a warning will be issued.

A value of '0' means the key cache is a regular (i.e. non-segmented) key cache. This is the default.

Other global variables used when working with regular key caches also apply to segmented key caches: key_buffer_size, key_cache_age_threshold, key_cache_block_size, and key_cache_division_limit. See the MySQL manual for descriptions of these variables. http://dev.mysql.com/doc/refman/5.1/en/server-system-variables.html

Statistics about the key cache can be found by looking at the KEY_CACHES table in the INFORMATION_SCHEMA database. Columns in this table are:

* KEY_CACHE_NAME: The name of the key cache * PARTITIONS: total number of segments Set to NULL for regular key caches

* PARTITION_NUMBER: segment number (set to NULL if a simple key cache)

set to NULL for any regular key caches for aggregation statistics for partitioned key caches

* FULL_SIZE: memory for cache buffers/auxiliary structures * BLOCK_SIZE: size of the blocks * USED_BLOCKS: number of currently used blocks * UNUSED_BLOCKS: number of currently unused blocks * DIRTY_BLOCKS: number of currently dirty blocks * READ_REQUESTS: number of read requests * READS: number of actual reads from files into buffers * WRITE_REQUESTS: number of write requests * WRITES: number of actual writes from buffers into files

----------------------------------------------------------------------

If there are any factual errors with the above, let me know. Improvements and suggestions are also welcome.

Question for the developers: A key_cache_partitions value of '0' means the key cache will not be segmented, it will be a regular (or simple) key cache. A value of '1' means the key cache will be a segmented key cache with a single segment. Is there any benefit to having a "single-segment segmented key cache" compared to a regular "simple key cache" or are they practically the same thing?

No they are not the same: a partitioned key cache with 1 partition differ from a regular key cache. Using a partitioned key cache with 1 partition makes sense only for testing purposes.

The key_cache_partitions variable also needs to be documented on the Server System Variables page (http://askmonty.org/wiki/Manual:Server_System_Variables). The entry will look something like this:

---------------------------------------------------------------------- * <code>key_cache_partitions</code> ** '''Description:''' The number of segments in a key cache. ** '''Commandline:''' <code>--key_cache_partitions=#</code> ** '''Scope:''' Global ** '''Dynamic:''' No ** '''Type:''' number ** '''Valid values:''' <code>0-64</code> ** '''Default value:''' <code>0</code> ''(non-segmented)'' * '''Introduced:''' MariaDB 5.2 ----------------------------------------------------------------------

Let me know if there is anything wrong with the above.

My task now is to come up with some examples. I'll probably use the test cases for inspiration unless someone has an awesome segmented key cache example that they've been dying to share with me. :)

Here's a suggestion: Put together an example with 3 different clients: The first client (admin) just reports the key cache statistics. Two other clients use the common default key cache and additionally each of them use another key cache: one client uses a regular key cache while the other uses a partitioned one. Let the clients submit the same queries for similar tables with the same contents for both clients. Regards, Igor.

Thanks.