SmugMug's Don MacAskill

tl;dr: The MySQL community rocks. Percona, XtraDB, Drizzle, SSD storage, InnoDB IO scalability challenges.

For anyone who lives and dies by MySQL and InnoDB, things are finally starting to heat up and get interesting. I’ve been banging the “MySQL/InnoDB scales poorly” drums for years now, and despite having paid Enterprise licenses, I haven’t been able to get anywhere. I was pretty excited when Sun bought MySQL since their future is intrinsically tied to concurrency, but things have been pretty slow going over there this year.

But the community has finally taken up arms and is fighting the good fight. It’s (finally!) a great time to be a MySQL user because there’s been lots of recent progress. Here’re some of my favorites (and highlights of work left to do):

PERCONA

I can’t sing Percona’s praises enough. They’re probably the most knowledgeable MySQL experts out there (possibly even including Sun). Absolutely the best bang for the buck in terms of MySQL service and support – better than MySQL’s own offering. (If I had to guess why that is, I’d bet that MySQL/Sun don’t want to step on Oracle’s toes by fixing InnoDB – but >99% of what we need is related to InnoDB. Percona has no such tip-toeing limitations.) Let me quickly count the ways they’ve helped me in the last few months:

• They knew of a super obscure configuration setting “back_log“. Have you ever heard of it? I hadn’t. But we started seeing latency on MySQL connections (up to *3 seconds*!) on systems that hadn’t changed recently (exactly 3 seconds sounded awfully suspicious, and sure enough, it was TCP retries). After going through every single kernel, network, and MySQL tuning parameter I know (and I know a lot), I finally called Percona. They dug in, investigated the system, and unearthed ‘back_log’ within an hour or two. Popped that into my configuration and boom, everything was fine again. Whew!
• We have servers that easily exceed InnoDB’s transaction limits. Did you know InnoDB has a concurrent transaction limit of 1024? (Technically, 1024 INSERTs and 1024 UPDATEs. But INSERT … ON DUPLICATE KEY UPDATE manages to chew up one of each). I know all about it – I’ve had bugs open with MySQL Enterprise for more than 2 years on the issue. What’s more, these are low-end systems – 4 cores, 16GB of RAM – and they’re no-where near CPU or IO bound. It took MySQL months to figure out what the problem was (years, really, to figure out all the final details like the different undo logs for INSERT vs UPDATE). Their final answer? It’ll be fixed in MySQL 6. 😦 Note that 5.1 *just* went GA after years and years. On the other hand, it took Percona one weekend to diagnose the problem, and 13 days to have a preliminary patch ready to extend it to 4072 undo slots. Talk about progress! (And yes, we want Percona to release the patch to the world)
• Solving the CPU scaling problems. These have been plaguing us for years (we have had some older four-socket systems for awhile … now with quad-core, it’s even worse), and thanks to Google and Percona, this problem is well on its way to being solved. We’re sponsoring this work and can’t wait to see what happens next.
• XtraDB. This is the biggy. So big it deserves its own heading….

XTRADB

Oracle’s done a terrible job of supporting the community with InnoDB. The conspiracy theorists can all say “I told you so! Oracle bought them to halt MySQL progress” now – history supports them. Which is a shame – Heikki is a great guy and has done amazing work with InnoDB, but the fact remains that it wasn’t moving forward. The InnoDB plugin release was disappointing, to say the least. It addressed none of the CPU or IO scalability issues the community has been crying about for years.

Luckily, Percona finally did what everyone else has been too afraid to do – they forked InnoDB. XtraDB is their storage engine, forked from InnoDB (and then turbocharged!). We’re not running it in production yet, but we are running all of the patches that went into XtraDB and I can tell you they’re great. We’re sponsoring more XtraDB development (and yes, we made sure Percona will be contributing anything they build for us back to the community) with Percona, and I’m sure that’ll continue.

DRIZZLE

I’ve already blogged a bit about Drizzle, but it sure looks like Drizzle + XtraDB might be a match made in heaven. Drizzle can be though of as a MySQL engine re-write with an eye towards web workloads and performance, rather than features. MySQL 4.1, 5.0, and 5.1 added a lot of features that bloated the code without offering anything really useful to web-oriented workloads like ours, so the Drizzle team is ripping all that stuff back out and rethinking the approaches to the things that are being left in. Very exciting.

SSD STORAGE

The advent of “cheap enough” super-fast SSD storage is finally upon us. I’ve got Sun S7410 storage appliances in production and they’re blazingly fast. I have a very thorough review coming, but the short version is that even with NFS latencies, we’re able to do obscene write workloads to these boxes (let alone reads). 10000+ write IOPS to 10TB of mirrored, crazy durable (thanks ZFS!) storage is a dream come true. Once you mix in snapshots, clones, replication, and Analytics – well, it just doesn’t get much better than this.

(Don’t get sticker shock looking at the web pricing – no-one pays anything even remotely like that. Sign up for Startup Essentials if you can, or talk to your Sun sales rep if you can’t, and you can get them much cheaper. I nearly had a heart attack myself until I got “real” pricing. Tell them I sent you – enough Sun people read this blog, it might just help 🙂 ).

STILL NEEDED…

So, all in all, there’s been an awful lot of progress this year, which is great. CPUs are finally scaling under InnoDB, and we finally have storage that isn’t bounded by physical rotation and mechanical arms. Unfortunately, great CPU scaling plus amazing IO capabilities isn’t something InnoDB digests very well. As is common in complicated systems, once you fix one bottleneck, another one elsewhere in the system crops up. This time, it’s IOPS. It was eerie reading Mark Callaghan’s post about this last night – I’d come to the exact same conclusions (from an Operations point of view rather than code-level) just yesterday.

Bottom line: Despite having ample CPU and ample IO, InnoDB isn’t capable of using the IO provided. You can bet we’ll be working with Percona, Google and Sun (read: sitting back and admiring their brilliant work while writing the occasional check and providing production workload information) to look into fixing this.

In the meantime, we’re back to the old standbys: replication and data partitioning. Yes, we’re stacking lots of MySQL instances on each S7410 to maximize both our IOPS and our budget. Fun stuff – more on that later. 🙂

UPDATE: Just occurred to me that there are plenty of *new* readers to my blog who haven’t heard me praise Google and their patches before. Mark Callaghan’s team over at Google definitely deserves a shout-out – they’ve really been a catalyst for much of this work along with Percona.