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Re: [Maria-developers] Segmented Key Cache - Syntax section


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.

> Thanks.