We started with the IOzone test profile with a 2GB read test. The EXT4 perfrormance was the same at 67MB/s between the Linux 2.6.28 and 2.6.30 kernels, but in time for the 2.6.31 kernel the read speed had plummeted. Between the Linux 2.6.30 and 2.6.31 kernels the ASRock NetTop went from reading at 67MB/s from its SATA HDD to 36MB/s. In the Linux 2.6.32 and 2.6.33-rc4 tests the average speed was 38MB/s.
While the read speeds with IOzone took a big hit during the Linux 2.6.31 kernel development cycle, the write speed (again using a 2GB size) has been more or less stable between the 2.6.28 and 2.6.33-rc4 kernels in IOzone. There were actually a few improvements made to the write performance in the 2.6.29 and 2.6.30 kernel releases, but by the 2.6.33-rc4 release, those gains have been wiped out.
Similar to the 2GB size, when quadrupling the read size to 8GB there is a big performance hit beginning with the Linux 2.6.31 kernel. Making the 8GB results worse than the 2GB results is that the read speed has dropped even more with this larger test size in the Linux 2.6.33 cycle.