Here’s something I was up to during the week. It’s a fairly mundane task, but despite that one which is very important, both to the client I was doing it for, and for most people generally who run this type of configuration.
The databases I was working with were part of a true 24×7 configuration where any downtime whatsoever has the potential to lose data (email marketing where every remote click event is tracked, meaning that you can’t even take planned downtime as you can’t control email receipts opening and clicking emails).
The systems in question run a fairly standard database mirroring configuration, 2 physical servers (principal and mirror partners), mirroring multiple databases in high safety mode with auto failover determined by the quorum of a third witness server. The task in question was to run windows update on the 2 partner servers and then apply SP3 for SQL 2008 to bring it up to build 5500.
The guys who had been running these servers previously told me that normally they just patched the mirror partner before failing over and then patching the new mirror (which was previously the principal). This is the standard methodology of a rolling upgrade within a mirroring configuration, but it missed one important step. I’m incredibly risk averse in all situations, and in this scenario it’s essential to remove the mirror witness before starting this process as if you don’t you have the small potential risk that half way through the upgrade and patching process you might suddenly find your mirror partnership failing over.
In all fairness this is a quite unlikely scenario, as it would require a failure at the principal at the exact point in time that patch process was running. It was also require a theoretical problem with all the servers managing their quorum, as they ought to still deal with such a failure properly, but after many years in the business and particularly after many years within Microsoft support, I’ve had the unfortunate experience of experiencing a wide range of very obscure failures across the SQL Server product set, and a mirroring split brain is one of them.
A split brain can very simply be described as a scenario where both partners believe that they are either the principal or the mirror, therefore invalidating the partnership. If you ever get in this scenario it’s extremely horrible and sometimes (again speaking from experience) you are obliged to do some rather dirty troubleshooting to recover the situation.
Sometimes my experiences at Microsoft support can scare people and skew their view of the product, as all we ever used to deal with in the escalation team was obscure problems or bugs that didn’t normally occur and couldn’t easily be solved. This means that whenever someone asks me about a certain procedure I’ve normally seen it break in a really horrible way! 99.9% of the time in production scenarios this doesn’t happen of course, but the moral of this story is that it makes me very risk averse.
So back to the point in hand, if you want to be risk averse when patching mirror partnerships, the thing to do first is to remove the witness and thereby drop back to high safety WITHOUT auto failover, meaning that if something stupid happens whilst patching, the mirroring won’t try to failover and mess things up further.
To achieve this process in a controlled fashion, here are my step by step instructions (remember if you mirror multiple database you need to run the scripts for each DB)
1. Disable witness
ALTER DATABASE [your database] SET WITNESS OFF go
2. Patch current mirror
3. Reboot current mirror if necessary
4. Failover from principal to newly patching mirror
--RUN on THE COMMANDS ON THE CURRENT PRINCIPAL TO MOVE TO OTHER SERVER ALTER DATABASE [your database] SET PARTNER FAILOVER
5. there will be a short outage whilst the new principal comes online
6. patch the current mirror (original principal)
7. reboot current mirror
8. fail back to original principal server (if required – this is optional)
9. add back the witness
ALTER DATABASE [your databse] SET WITNESS = 'TCP://[your FQDN]:[your port]' go
10. you’re good to go
Good luck with your patching!
I published a blog post with Basefarm last week about tracing select statements in SQL Server without using profiler. It demonstrates how to use SQL Audit (based on extended events) to record any select statements issues on particular tables over a prolonged time period in a scalable lightweight way. Although SQL Audit was designed as it name suggests, to provide an audit framework for the product, I find it can be quite useful in other ways as well. In this example we were helping developers effectively check their code base for unexplained access to a particular object.
You can read the full post here:
There’s nothing particularly special about this photo apart from to say that i find it very evocative of my time in Stockholm. Quite what the Viking boat is doing I was never sure, but it’s just that it’s there that makes me think of Stockholm generally in that there always seemed to be something like this happening in and around the town. Not always Viking boats, although myself and Matilda did ride in another one up at Sigtuna, but just always interesting and varied stuff to see and do. it’s not that London hasn’t got things to see and do, it’s just that it’s at a different pace and a different style. I just love the fact that the water and boats were always so accessible. Sometimes me and Tilda would just jump on a bus or a train and go ride the boats from one place to another, just to hang and chill and see the views. Sweet memories indeed.
A beautiful sunny day near the top of the cable car in Åre, this is a surprisingly good photo considering that it was taken with a mobile phone, a winmobile 6 one at that! Those were the days eh, stylus in hand…..thank god for the HTC sense which made it almost useable, although it did hang quite alot anyway! that said though it still remains one of my favourite phones as I just loved the form factor. It was an HTC Diamond 2.
This is one of my favourite photos of all time, this is my default screen saver on all my machines, if you ever employ me or have me around your office, you’ll notice this on all of my devices. (when I haven’t got SSMS or notepadd++ open that it is)
It’s taken from the south downs way looking north over Amberley on a fine September day when summer was seeming to last forever. I’m always looking for property in this area, but it doesn’t come up very often. Feel free to ping me if you have a big family house for rent in this area!
Over the past few years I’ve published content in all sorts of places across the web. I’ve finally managed to find a few hours to set up my domains properly, play with a bit of wordpress and try to centralise all my content going forward. This site is the result of that work and I hope it’s content will be of use to you. Since it’s also a personal vanity project at the same time it’s got some photos, musing and other general things as well. This is why the first few posts are some nice photos, and also because they’re easier to put up quickly as opposed to writing proper technical content which will follow in the coming weeks and months!
Coming back from Grinda, looking out the back of the boat, and there’s another right behind us.