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Spotting the Red Flags (Part 1 of n)Posted on 2:15 pm September 24, 2009 by James Morle
As a consultant I get to see many different systems, managed by different people. A large proportion of these systems are broken in exactly the same ways to others, which makes spotting the problems pretty straightforward! It occurred to me at some point that this is a bit like a crime scene - somebody had murdered the system, and it was my job to find out 'whodunnit', or at least 'whatdunnit'. The 'what' is quite frequently one (or few) actions performed by the administrator, normally with good intention, that result in system performance or availability carnage. I call these actions 'Red Flags', and spotting them can save a lot of time.
A couple of years ago at the Hotsos conference, I did a small impromptu survey. It's not all that often that you have 500 Oracle geeks in a room that you can solicit opinion from, so it seemed like a good chance to cast the net for Red Flags. I seeded the process with a few of my personal favourites and then started writing as the suggestions came in. Here are the 'seed' entries:
- Any non-default setting for optimizer_index_cost_adj
- Carefully configured KEEP and RECYCLE pools
- Multiple block sizes
- Some kind of spin_count configuration
Remember, these are not necessarily the wrong thing to do to your system, but they probably are. That's why they attract my attention in the first instance.
I got a whole load of responses. Of those responses, some had missed the point and just identified something that was broken. This is more subtle than that - these are things that are forensic evidence of system homicide. Some of the decent ones I got back follow:
- /*+ RULE */ hint in abundance (I know, lot's of apps do this, and it sometimes makes sense)
- Numeric data in VARCHAR2 columns
- Indexes and tables on 'separate disks'
- All columns in a table NULLable
Of course, there were more cynical responses too, such as (he he): cluster_database=true
I was going to write some kind of presentation about it, but I think this might work nicely as a blog entry, potentially multiple entries! If anyone has some good suggestions, please let me know and I'll start adding them into part 2, along with any more I think of... 🙂
The amazing truth about the Red Flags, though, is that they are incredibly hard to dislodge. Once optimizer_index_cost_adj is set to anything but default, it is nigh on impossible to convince anyone that the system is probably now tuned on its head. Same with a multiple buffer pool configuration, and same with multiple block sizes.