11g upgrade still good

Still looking good.

CPU on the new box is hovering under the 20% mark having been habitually > 80% on the old kit. But it could be a quite time for the business so we will have to compare the metrics that we capture as a proxy for “business activity”.

Best news is that we were getting a significant number of “ORA-01801: date format too long for internal buffer” errors every day causing the client-server application to crash.

These have been due to cursors being shared when they shouldn’t be, for example due to NLS mismatches.

As hoped (and promised to the business), these have disappeared with the upgrade.

we have 11g lift off

Production database was upgraded to 11g over the weekend: 9i -> 11g, SE -> EE, Windows -> Linux.

This is the first application in this client’s company to upgrade.

Seems to have gone well given that I was not called over the weekend.

As ever though, the only true test is going live and I would be very, very surprised if there were no suprises!

So, in the absence of any ringing hotlines and before I start on something new later this morning, I’m just going to be monitoring active session and also seeing what rises to the top by looking at sql with greater than average buffer_gets (I know roughly what’s normally there so will be interesting to see if anything changes).

Upgrading to 11g

I feel very fortunate that my current client is planning to upgrade a significant production system to Oracle 11g very shortly – you can’t get to know something like this properly until you use it in anger and work through the problems in a time-constrained environment. And I get the impression that there are very few production 11g sites out there. So it will be a good experience and it will look good for me.

Some say that 11g is no more than 10gR3, nevertheless it’s a bold upgrade decision – we’ll be going from 9i Standard Edition to 11g Enterprise Edition and switching from Windows to Linux (with a delayed phase of RAC thrown in).

It’s a bit like going from a Ford Focus to a Ferrari (but I’ll keep my thoughts on the futility of certain aspects of this to the RAC rant).

The upgrade is scheduled for March or Aprial and, so far, things have looked relatively pain free. In terms of general performance, not unexpectedly, Oracle 11g on Linux seems significantly faster than 9i on Windows.

The changes we have had to make so far seem mostly to concern changes in XML behaviour (change that actually mostly happened in 10gR2, but we’ve skipped that generation).
For example:

• In 9i, if you tried to create an XML element with a space in the tag name using XMLELEMENT, the space would be replaced with “_x0020_”. In 11i, the space is preserved as a space. I’m not convinced by this change in behaviour as an element surely cannot have a space in it’s name. Anyway, in those unwise places where spaces have been used, these tags will have to change in both our client and database code.
• There has been a change in behaviour regarding dates within XMLELEMENTs. In 9i, a date without an explicit format would pick up the default NLS_DATE_FORMAT of the session. In 11g, the date format will be YYYY-MM-DD. So, to preserve our 9i behaviour, we need to TO_CHAR our dates in XML so that they pick up the session format.
• When using XMLFOREST in 9i, empty columns would result in no tag being present in the resultant XML. Although deliberate, I think there’s something dodgy about this behaviour anyway. However, it’s changed slightly now we’re in 11g. Columns with not null columns will always produce a tag, even if empty (consider outer joins to tables with not null colums). Empty nullable columns continue to have their tags omitted. I’m not sure how we will address this in our particular situation (empty tags cause us issues if the client entity is not a nullable object, but if the tag is missing apparently the whole enigma is avoided. Somehow!). The solution certainly won’t involve changing not null columns to nullable, but, to limit changes in both client and database, it could involve using XSL to strip out empty tags.

I anticipate further issues to come out of the wash however I expect the majority of these to only reveal themselves post-production upgrade. There are bound to be some statements that degrade significantly but it might not be until the business do their daily thing day-in day-out that the most significant problems for them float to the surface (as opposed to things that are most obvious from automated load testing which I am always doubtful regarding the efficacy thereof).

Fighting off RAC

As mentioned, my current client is upgrading from 9i Standard Edition on Windows to 11g Enterprise Edition on Linux.

The timeframe for this upgrade is quite aggressive. While normal development continues at a blistering pace, automated regression testing began part-time in January for an upgrade originally in March but now re-arranged for April.

Also in January, I was told that it had been decided that we would be going live on a 2-node RAC cluster. Alarm bells rang, sirens went off, I was bewildered as to how such a decision could be made.

This was a decision that management had apparently made in conjunction with the DBAs, with RAC sold as a black box database clustering solution with no question marks over its suitability.

There’s no architecture department/function and in terms of Oracle and databases, it’s a role I have previously filled elsewhere. I immediately went into overdrive, briefing against the wisdom of going to RAC before it was too late.

For us, there are numerous issues that I see with RAC:

• The first is the upgrade timeline. The proposal, in January, was that in March we would be changing hardware, changing OS, changing database version, changing database edition and now also moving to RAC. There are plenty of references out there that suggest RAC alone can take a few months to stabilise and to nail your proper configuration. For a business that was committed to a March (even April now) deadline, this seemed to be to be a big risk.
• Secondly, I am far convinced about the suitability of RAC for our system. It’s one thing for senior management to upgrade the business criticality of a system and give the greenlight for some spending to upgrade the platform. It’s another thing to upgrade your Ford Focus to a Ferrari only to stay stuck in the same commuting traffic. That’s my analogy of our system – lots of bad SQL, lots of statements that cope with 1001 different inputs, far too many lookups in functions called from SQL. At the end of the day there’s no getting away from the fact that our system has something like an average of 8 concurrent requests. Historically, it has been maxed out or close to being maxed out on CPU because the SQL is bad.
• More importantly, I remain very concerned about the suitability of RAC for the profile of our system. I think of RAC as suitable for high-end OLTP systems of for multiple systems with non-OLTP systems restricting themselves to one of the nodes (unless non-conflicting activity can be determined somehow and allocated to relevant nodes). I would probably categorise our single system as a hybrid system with a very small element of OLTP and a large element of reporting. Furthermore, a large proportion of that reporting revolves around the top X% of a single table. The RAC architecture is such that repeated requests for the same blocks will cause significant waits as the instances coordinate and pass the blocks back and forth.
• The bottom line is that RAC is complex and there’s no getting away from the fact complex is bad. Complexity eventually comes back round and bites you. And complexity usually costs more.

For now, my briefing has been a partial success, although really the only point that had any effective leverage was that the the upgrade date was under threat. A 2-node RAC solution is currently on hold for the initial upgrade. The idea being that we upgrade to 11g on the new hardware and then we expand to a 2-node cluster once that initial upgrade phase is stable.

But, and here’s the stickler, the upgrade is going to be to a 1-node RAC “cluster”. I can understand that there is a once-in-a-blue-moon opportunity to go to the new hardware, but I don’t like the smell of a 1-node RAC cluster at all. For a start, there surely has to be some overhead to the RAC installation. But we’re not even testing that. But what’s more is the cost. There is a significant licensing overhead to RAC. It’s an overhead that surely means that going to a 2-node cluster is a formality regardless. I’m just glad that it’s not my name on the cheque. It’s one of those things where I just hope that someone down the line doesn’t say that we’re moving off Oracle because it’s so expensive.

A couple of other folks’ thoughts on issues with RAC:

• Mogens Norgaard’s infamous paper
• Chen Shapira
• Jeff Browning
• Dave Brillhart
• Kevin Closson 1
• Kevin Closson 2

Upgrade from 8i to 10g using Shareplex

Plans to upgrade a production database from Oracle 8i to 10g are gradually coming together.

The current situation is that there is a primary 8i database which replicates to a logical standby database using Quest’s Shareplex.

There will be 10g primary and standby databases on new hardware which will probably use Oracle DataGuard to facilitate the standby although Shareplex is not definitely out of the picture.

Because of the new standalone hardware, there is an opportunity to do a relatively simple “upgrade”, bringing the 10g databases through to production readiness whilst replicating production activity from the 8i live database. This is much less risky than upgrading the existing production databases using something like DBUA.

The replication allows testing to be done with “as production” data (subject to any small delay in replication), with accurate volumes and activity and therefore facilitating a good comparison of the output from the applications.

And significantly, the actual go-live becomes a flick of a “switch” (switching application datasources from the 8i address to the 10g address) and, fingers crossed, there is a very small window of downtime. For this to happen, the application drivers need to be upgraded to 10gR2 compatible versions in advance of the database upgrade, drivers being backward not forward compatible.

Below is a messy picture:

What this is trying to say is that, as mentioned above, the current situation is that a live 8i database replicates to a logical standby 8i database. For the migration, a further node of Shareplex replication will be set to go from the standby 8i database to the primary 10g destination.

This should mean that the live database is isolated from the additional load and increased risks of the additional replication.

In broad terms, the replication from 8i to 10g involves restoring an 8i backup on the new hardware followed by an export and import into the 10g database and then a reconciliation of the replication queues.

New Year’s Resolutions II

At work, I am resolved to push through the upgrade from Oracle 8.1.7 to 10.2.x

The client currently has a live 8.1.7 instance on some pretty old hardware that uses Shareplex to replicate to a standby.

Any planned upgrade is going to have some teething problems and I’m sure every installation and every upgrade is unique.

I am encouraged by one case study experience of Database Specialists whilst being wary of some of the problems that others (can’t find the relevant articles at the moment) have experienced.

In terms of application SQL performance, I think that the key to the initial migration is going to be the use of Stored Outlines to preserve the current perceived dodgy optimizer stability. Once the upgrade has been implemented and is stable, then I think we can start to optimise the SQL and, for example,remove some of the push_subq hints from queries that have no subqueries….