Making sense of INNODB buffer pool stats

The Buffer pool size 393215 This is in pages not bytes.

To see the Buffer Pool size in GB run this:

SELECT FORMAT(BufferPoolPages*PageSize/POWER(1024,3),2) BufferPoolDataGB FROM
(SELECT variable_value BufferPoolPages FROM information_schema.global_status
WHERE variable_name = 'Innodb_buffer_pool_pages_total') A,
(SELECT variable_value PageSize FROM information_schema.global_status
WHERE variable_name = 'Innodb_page_size') B;

Database pages 360515 This is the number of pages with data inside the Buffer Pool

To see the amount of data in the Buffer Pool size in GB run this:

SELECT FORMAT(BufferPoolPages*PageSize/POWER(1024,3),2) BufferPoolDataGB FROM
(SELECT variable_value BufferPoolPages FROM information_schema.global_status
WHERE variable_name = 'Innodb_buffer_pool_pages_data') A,
(SELECT variable_value PageSize FROM information_schema.global_status
WHERE variable_name = 'Innodb_page_size') B;

To see the percentage of the Buffer Pool in use, run this:

SELECT CONCAT(FORMAT(DataPages*100.0/TotalPages,2),' %') BufferPoolDataPercentage FROM
(SELECT variable_value DataPages FROM information_schema.global_status
WHERE variable_name = 'Innodb_buffer_pool_pages_data') A,
(SELECT variable_value TotalPages FROM information_schema.global_status
WHERE variable_name = 'Innodb_buffer_pool_pages_total') B;

Modified db pages 300 This is the number of pages in the Buffer Pool that have to be written back to the database. They are also referred to as dirty pages.

To see the Space Taken Up by Dirty Pages, run this:

SELECT FORMAT(DirtyPages*PageSize/POWER(1024,3),2) BufferPoolDirtyGB FROM
(SELECT variable_value DirtyPages FROM information_schema.global_status
WHERE variable_name = 'Innodb_buffer_pool_pages_dirty') A,
(SELECT variable_value PageSize FROM information_schema.global_status
WHERE variable_name = 'Innodb_page_size') B;

To see the Percentage of Dirty Pages, run this:

SELECT CONCAT(FORMAT(DirtyPages*100.0/TotalPages,2),' %') BufferPoolDirtyPercentage FROM
(SELECT variable_value DirtyPages FROM information_schema.global_status
WHERE variable_name = 'Innodb_buffer_pool_pages_dirty') A,
(SELECT variable_value TotalPages FROM information_schema.global_status
WHERE variable_name = 'Innodb_buffer_pool_pages_total') B;

As for the other things in the display, run this:

SHOW GLOBAL STATUS LIKE 'Innodb_buffer_pool%';

You'll see all the status variables for the Buffer Pool. ou can apply the same queries against whatever you need to examine.

 Buffer pool hit rate is 1000 / 1000

This is the only really meaningful value in the situation that you are in... and that situation is that you are lucky enough to have a buffer pool with a perfect 100% hit rate. Don't over-analyze the rest of it, because there is nothing you need to change, unless the server OS is low on memory, causing swapping.

The young/not young values aren't interesting in a case where there's zero pressure on the buffer pool. InnoDB is using it, it doesn't do anything without it. If the pool is too small, pages get evicted and new pages get read in and the other stats help you understand that... but that is problem you don't appear to have.

Free "unused" space in the pool will never be neglected or left idle by InnoDB if it is needed for any reason at all, so the fact that it's free means only that you have some breathing room to expand into as the size of your working dataset grows.

That's all it means, unless, of course, you recently restarted the server, in which case, it's incomplete.. The server needs to run through a full period of "normal" usage (including full backups) before the stats tell the whole story... whether that's an hour, a day, week, month, or year, depends on your application.

I'll disagree with the assessment that "you are lucky enough to have a buffer pool with a perfect 100% hit rate"

At the top of the output (which is chopped off), is a line something like:

Per second averages calculated from the last 16 seconds

This says to me that no reads happened in the last 16 seconds, thereby (artificially) giving you a perfect '1000/1000' score.

0.00 reads/s, 0.00 creates/s, 37.32 writes/s
Buffer pool hit rate 1000 / 1000, young-making rate 0 / 1000 not 0 / 1000

Meanwhile there were some writes. These were possibly deferred writes for either flushing 'dirty' pages or cleaning up indexes from the 'change buffer'.

Probably there was no activity in the young/hot area in the last 16 seconds either.