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Re: Automatic Extension Question
Posted by: Rick James
Date: June 26, 2014 02:39PM

First, I will comment on your comments...

> sum( data_length + index_length ) / 1024 / 1024 / 1024 "Data Base Size in GB",

That is the amount of disk space needed for the data and indexes in all the tables. But it does not include certain overhead.

> sum( data_free )/ 1024 / 1024 / 1024 "Free Space in GB"

That is the "free" space that it taking up disk space in the table space(s). Each Data_free is the free space for the tablespace that the table is in. Now it gets messy...

> I think the query isn't quite right because it assumes a per-table tablespace and it looks like the setup is a shared tablespace.

That is an astute observation. The SUM(data_free) works ONLY if _all_ tables are in separate tablespaces. If _none_ of the tables are in their own tablespaces (none created with innodb_file_per_table=1), then each value of Data_free will be the same, and it will be the free space in ibdata1. It is also possible to have a mixture of per-table and shared; this makes it impossible to do anything like SUM().

> command "show variables;" which shows "innodb_data_file_path ibdata1:10M:autoextend" and "innodb_autoextend_increment 8".
> If the database file started at 10MB and autoextends in increments of 8MB, then how did it come to have 1.2GB of free space?

Use MAX(Data_free) instead of SUM(Data_free) in the case of a single tablespace.

You can also look in the filesystem for ibdata1. It's size will probably equal SUM(Data_length+Index_length) + MAX(Data_free). If it is equal (or close), then you have the right formula.

innodb_file_per_table = 1 is useful if you have multi-megabyte table(s). It is less useful for small tables.

Note: There are other things on the disk that take space, so no SELECT will be 'perfect' for checking against disk space. For example, each table has a .frm file of about 8KB (regardless of file_per_table).

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