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Re: Growth of .ibd and .ibdata files
Posted by: Rick James
Date: October 08, 2016 12:00PM

Data and indexes are subject to the whims of BTrees. A block split of a block that is not cached may involve multiple 16KB reads and/or writes. So, a .ibd will usually grow by 16KB, the block size. For any non-small tables, space is pre-allocated in megabyte chunks. In your small test, probably the table was cached in the innodb_buffer_pool_size, and you saw none of the BTree I/O.

The undo/rollback logs are mostly aimed at sequential I/O, writing only a small amount of data. The structure is linear (actually circular in iblog*), not BTree. There, I would expect device-sided blocks (512 bytes?) to be written sequentially. In particular, at least one such write occurs on every COMMIT. (This is subject to the fsync setting of innodb_flush_log_at_trx_commit.)

Index changes are 'delayed' in the "Change buffer" that will eventually be flushed. This involves 16KB blocks. So, in your little test, you probably did not see such.

Bottom Line: InnoDB's I/O is much more complex that what you are seeing. And InnoDB is highly tuned to minimize I/O for scaling.

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