In fact, you really need to avoid every kernel between 3.0 and 3.8. While RHEL has been sticking to the 2.6 kernels (which have their own issues, but not as bad as this), Ubuntu has released various 3.X kernels for 12.04. Why is this an issue? Well, let me give you two pictures.
Here's private benchmark workload running against PostgreSQL 9.3 on Ubuntu 12.04 with kernel 3.2.0. This is the IO utilization graph for the replica database, running a read-only workload:
Sorry for cropping; had to eliminate some proprietary information. The X-axis is time. This graph shows MB/s data transfers -- in this case, reads -- for the main database disk array. As you can see, it goes from 150MB/s to over 300MB/s. If this wasn't an SSD array, this machine would have fallen over.
Then we upgraded it to kernel 3.13.0, and ran the same exact workload as the previous test. Here's the new graph:
Bit of a difference, eh? Now we're between 40 and 60MB/s for the exact same workload: an 80% reduction in IO. We can thank the smart folks in the Linux FS/MM group for hammering down a whole slew of performance issues.
So, check your Postgres servers and make sure you're not running a bad kernel!