Posted by peter |

Let us talk few more about disks. You might have read my previous post and Matt’s Reply and it looks like there are few more things to clarify and explain.

Before I get to main topic of the article lets comment on IO vs Disk question. If you look at Disk Based databases all data accesses are treated as IOs – it can be “logical” if they are cached or “phyiscal” if they require actual IO done but in the general sense all data accesses are IOs. If you use this terminology when most of the problems would come down to IO – making queries to touch fewer rows (or row portions) or having these “touches” resolved as logical IO rather than physical. There is still locking ,networking etc to deal with but it is minor story.

This is not however as Most of the people understand IO and as not as I typically use these terms. For me IO is IO bound workload – disks are moving and CPU sits idle. With such terminology there is instantly much smaller amount of cases are about IO because we would call cases when too much of logical IO is happening CPU bound. The beauty of this terminlogy (and so why I use it) – it is very easy to see if system is IO bound or CPU bound, while to understand if MySQL goes through more rows than it needs to requires look at the queries and schema.

Ok Let us new get to back to main point of the article.
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