This is a followup question to my earlier How can I determine which Innodb table is being written?

Using the accepted answer from the above-referenced question I was able to determine which (two) of a hundred Innodb tables were being constantly updated on my server. Using some trial and error, I was able to determine which specific rows in those tables were being updated.

The rows are essentially counters that are updating and updates are happening about 1/s (I ran a query that counted 54 updates in 60 seconds). The rows are tiny with 4 columns: BIGINT, VARCHAR, LONGTEXT, VARCHAR. The value in the LONGTEXT column of the row in question is just a 10-character string and that is the column that is being updated.

I ran a query that calculated the change in Innodb_data_written over 60 seconds and ran it several times. It ranged from 4.5 to 5.5 MB over 60 seconds. When I turned off the process that is responsible for the 1-character row updates, the change in Innodb_data_written over 60 seconds went down to .03 MB, so it definitely seems to be these tiny row updates that are responsible for the ~ 5MB per minute of Innodb disk IO.

The innodb_buffer_pool_size is ample and the Innodb buffer use percentage is around 30%.

So, the question is, how are a couple of 1/s row updates that are only changing 1 character in a tiny row generating ~ 5MB per minute of Innodb disk IO?

  • Can you convert the text column to VARCHAR? Commented Jan 17, 2015 at 23:00
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  • What does the update query look like, and what are the indexes on that table? Commented Feb 28, 2019 at 9:16

1 Answer 1


Any UPDATE to a row requires the following (sooner or later):

  • Read the 16KB block containing the row (unless cached in the buffer pool)
  • Write the block back to disk (or cache it a long time)
  • The "doublebuffer" is written once per transaction
  • Do one write to the InnoDB log for transactional integrity. (The details here depend on various settings and whether you wrap things in BEGIN..COMMIT.) Since you say the buffer_pool is big enough (apparently for the entire table and any secondary indexes?), the first two items will not tally many IOPs.

But that was not your question.

The actual I/O (pages) related to the buffer_pool is (Innodb_buffer_pool_reads + Innodb_buffer_pool_pages_flushed)

You will probably find this very low because of good caching: Innodb_buffer_pool_reads / Innodb_buffer_pool_read_requests

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