On 04/17/2015 01:04 AM, MARK BRINSMEAD wrote:
But that doesn't mean that the database isn't *trying* to do sequential access. Nor does it mean it doesn't *think* that it actually is doing sequential access.Agreed. But I don't really care what does database think it's doing, I only care about what the database is actually doing. And the answer is that you can never tell.
There are a few more cases like this. Many SAN-attached disk arrays these days are highly virtualized, and offer "cool" services like thin provisioning and snapshot capabilities. When the disk array does relocate-on-write (a common way to support snapshots) it may not take a long time before IO operations that _/should/_ be sequential have become almost entirely random. Again, the database won't know this -- but the disk heads certainly will.
