Red Hat Enterprise Linux 6 introduces a number of performance enhancements in the I/O stack:
Solid state disks (SSDs) are now recognized automatically, and the performance of the I/O scheduler is tuned to take advantage of the high I/Os per second (IOPS) that these devices can perform. For information on tuning SSDs, see the Solid State Disk Deployment Guidelines
chapter in the Red Hat Enterprise Linux 6 Storage Administration Guide
Discard support has been added to the kernel to report unused block ranges to the underlying storage. This helps SSDs with their wear-leveling algorithms. It also helps storage that supports logical block provisioning (a sort of virtual address space for storage) by keeping closer tabs on the actual amount of storage in-use.
The file system barrier implementation was overhauled in Red Hat Enterprise Linux 6.1 to make it more performant.
pdflush has been replaced by per-backing-device flusher threads, which greatly improves system scalability on configurations with large LUN counts.