Mysql – thesql 5.0.23 vs 5.5 performance benefits and upgrade issues

Initial thought is that you have a process that is connecting to the database for something and then crashing at the client end. It's not exiting gracefully and is leaving the connection open. This is using up the available connections to MySQL and eventually MySQL runs out. The engine isn't so much as hanging but running out of connections.

What you've done by enabling connection pooling will help, but you still have the issue of the number of clients trying to connect at once.

Try running ps axf from the command line while you're seeing this problem to see what is running that could be holding this number of connections open.

Wow, you have some awfully beefy hardware for this problem. There's not much more you can throw at it hardware wise, with the exception of upgrading to maybe Sandy/Ivy Bridge CPUs for 20-50% better performance out of Btree searches, etc.

Please note that my forte is Innodb, so I'm going to

1. Ignore that you're myisam and act as if it won't make a difference.
2. Assume this problem is enough impetus to get you to upgrade. Yes, it is an upgrade.

Innodb can help take great advantage of all that memory by storing these frequently accessed rows in its buffer pool. You can tune it to be as large as you want (say 80% of memory) and fresh reads/writes remain in memory until it needs to push them to disk to make more room for latest accessed data. In memory is an order of magnitude faster than your FusionIOs.

There are many more Innodb features, such as adaptive hashes, auto-inc locking mechanisms, etc. that can be a boon to your environment. You, however, know your data better than I do.

In the innodb world, an good short term solution is to optimize your slave - do you really need every index on your slave that you have on your master? Indexes are a ball and chain on inserts/updates/deletes, EVEN with Fusion IO cards. IOPS are not everything here. Sandy/Ivy bridge procs have much better memory throughput and computing performance - they can make a huge difference of the Westmeres you have now. (Figure 20-50% overall). Remove all indexes you don't need on the slave!

Second, and almost certainly applies to innodb only, is that mk-prefetch can know which updates and before the slave writes them. This allows mk-prefetch to run a read query first, thereby forcing the data to be in memory by the time the the single repl runs the write query. This means the data is in memory and not in fusionIO, a quick order of magnitude performance gain. This makes a HUGE difference, more than one might expect. Lots of companies use this as a permanent solution. Find out more by checking out the Percona Toolkit.

Third, and most importantly, once you've upgraded to Innodb, definitely checkout Tokutek. These guys have some wickedly awesome stuff that exceeds the write/update/delete performance of Innodb by a long shot. They tout improved replication speed as one of the key benefits, and you can see from their benchmarks why Fusions crazy IOPS still won't help you in the case of Btrees. (Note: Not independently verified by me.) They use a drop-in replace of a btree index that, while hideously more complex, ameliorates many of the algorithmic speed limitations of btree indexes.

I am in the process of considering an adoption of Tokutek. If they free up so much write speed, that allows me to add more indexes. Since they compress the data and indexes at such wonderful ratios (25x they quote), you don't even pay a (performance, maintenance) price for increased data. You do pay ($) for their engine though, $2500/year per pre-compressed GB, IIRC. They have discounts if you have the data replicated, but you can even just install Tokutek on your slave and keep your master as-is. Check out the technical details in the MIT Algoritms Open Courseware lecture. Alternatively, they have tons of technical stuff on their blog and regular whitepapers for those who don't have 1:20 to watch the video. I believe this video also gives the Big-O formula for how fast reads are. I have to assume that reads are slower (There's always a tradeoff!), but the formula is too complex for me to gauge how much. They claim it's roughly the same, but I'd rather understand the math (not likely!). You may be in a better situation to discover this than I am.

P.s. I am not affiliated with Tokutek, I have never run their product and they don't even know I'm looking at them.

Update:

I see you have some other questions on this page and thought I'd chip in:

First, slave pre-fetch almost certainly will not work for myisam unless you have an exceptional environment. This is mostly because the prefetching will be locking the very tables you intend to write to, or the slave thread has the table locked that the pre-fetch daemon needs. If your tables are extremely well balanced for replication and different tables are being written to in a round-robin fashion, this may work - but keep in mind this is very theoretical. The book "High Performance Mysql" has more information in the "Replication Problems" section.

Second, presumably your slave holds a load of 1.0-1.5, it may be higher if you have other procs or queries running but a baseline of 1.0. This means you are likely CPU bound, which is likely with your FusionIO on board. As I mentioned earlier, Sandy/Ivy Bridge is going to give a little bit more oomph, but probably not enough to get you through the rougher times with minimal lag. If the load on this slave is mostly write-only (i.e. not many reads), your CPU is almost certainly spending it's time calculating positions for btree insertions/deletions. This should reinforce my point above about removing non-critical indexes - you can always re-add them later. Disabling hyperthreading will not work, more CPU is not your enemy. Once you get above 32GB ram, say 64GB, you need to worry about ram distribution, but even then the symptoms are different.

Finally, and most importantly (don't skip this part;)), I'm assuming you're now running RBR (Row based replication) because you mentioned a non-trivial performance increase when switching too it. However - there may be a way to get even more performance here. Mysql bug 53375 can manifest if you have tables being replicated with no primary key. The slave is basically not smart enough to use anything but a primary key, so the absence of one forces the replication thread to do a full table scan for every update. A fix is simply adding a benign, surrogate autoincrementing primary key. I'd only do this if the table were large (say several 10s of thousands rows or larger). This, of course, comes at the cost of having another index on the table, which brings up the price you pay in CPU. Note there are very few theoretical arguments against this, as InnoDB adds one behind the scenes if you don't. The phantom one, however, is not a useful defense against 53375. Tungsten can overcome this problem too, but you need to be sure when using Tungsten that you have your encoding straight. The last time I played with it, it would die horribly when any non-UTF8 string needed replicating. That's about the time I gave up on it.