STATUS variables: the difference between innodb_pages_read and innodb_buffer_pool_reads

Submitted by jay on February 25, 2008 - 1:17pm

I haven't gotten any responses to my question from Friday, so I figured I'd post a more direct question:

The two variables 'innodb_pages_read' and 'innodb_buffer_pool_reads' are different.  'Innodb_pages_read' seems consistently higher than the buffer_pool_reads.  Certainly the buffer pool reads 'pages', but what else would be counted as a page read?

On the flip side, 'innodb_pages_written' and 'innodb_buffer_pool_pages_flushed' seem consistent.  

Anyone have any clues?

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from this link

from this link

Innodb_buffer_pool_reads = "The number of logical reads that InnoDB could not satisfy from buffer pool and had to do a single-page read."

Innodb_pages_read="The number of pages read"

I hope that makes it a bit more clear.

jay's picture

Unfortunately, no.  That

Unfortunately, no.  That appears to be exactly what the manual states as well.  

If it was a matter of logical to physical reads, then innodb_buffer_pool_read_requests would equal innodb_pages_read, but it doesn't.  

Innodb_buffer_pool_read_requests > (much >) Innodb_pages_read > Innodb_buffer_pool_reads

What innodb_pages_read - innodb_buffer_pool_reads means is what I'm after.  

innodb_page_reads are the

innodb_page_reads are the actual number of pages read from the innodb_buffer_pool


jay's picture

Ok, but how does that differ

Ok, but how does that differ from "Innodb_buffer_pool_reads".

BTW, I'm going to sit down and grok the source tomorrow to figure this out.

Innodb_buffer_pool_reads is

Innodb_buffer_pool_reads is how many it had to go to disk for because it couldn't get the data from the innodb buffer pool itself.  Compare that with innodb_buffer_pool_read_requests...

buffer pool read performance = innodb_buffer_pool_reads / innodb_buffer_pool_read_requests.

Each read request can read multiple pages AFAIK.

I did grok the source and that's what I found for innodb_page_reads.  : )


jay's picture

I get that, but what about

I get that, but what about 'innodb_page_reads'?  It seems to equal neither of those two metrics. 

It doesn't equal the other

It doesn't equal the other metrics because I believe multiple pages can be read from the innodb_buffer_pool at one time.

It is simply the actual number of pages that have been read from the innodb_buffer_pool, nothing more.

Innodb_pages_read is

Innodb_pages_read is incremented whenever a file page is read from buffer pool. And when the page is not there in the buffer, InnoDB reads from disk to a new buffer while incrementing Innodb_buffer_pool_reads.

I would like to put it this way. Innodb_pages_read are the pages read from buffer pool and Innodb_buffer_pool_reads are the pages read for the buffer pool.

1. [\buf (Buffering) section]
2. buf0buf.c
3. buf0rea.c


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