Hi, I have a NAS system with 12 spinning disks that has been running with the 2.6.39.4 kernel. It has a 4 core xeon processor (E31275 @ 3.40GHz), with 8 GB of RAM. The 12 disks in my RAID array are Hitachi 4 TB, 7200 RPM SATA drives. The filesystem is XFS. Recently I have been evaluating RAID performance on newer kernels 3.10 and 4.2. I have observed that with the same settings, I am seeing much slower RAID 5 and 6 sequential write speeds with newer kernels compared to what I was seeing with the 2.6.39.4 kernel. However, the 4.2 kernel has much better read speeds for both sequential and random patterns. I understand that there have been many improvements to RAID 5 and 6 in the 4.1 kernel. I definitely am seeing improvement with reads but not writes. If I observe disk and array throughput with iostat, the individual disk utilization and wMB/s is much lower in the newer kernels. With the older 2.6.39.4 kernel, disk utilization seems to stay above 80% with wMB/s around 74 MB/s, whereas the newer kernel disk utilization seems to vary between 20-70% with wMB/s around 9-38 MB/s. CPU iowait gets up to about 10% much of the time. These Hitachi disks are capable of sustaining around 170 MB/s, which is just about what I see when doing sequential writes to all 12 disks concurrently in a JBOD configuration, i.e no RAID. The iowait for 12 disks of JBOD gets up to about 97% - which makes the system very unresponsive. One other observation is that RAID 0 sequential write speeds in newer kernels are only slightly less than what I was seeing in 2.6.39.4. I am frankly surprised at these results. Perhaps there is some configuration or tunable settings that have changed since the 2.6 kernel that I am unaware of that affect RAID 5, 6 performance. Please comment if you have any ideas which might explain what I am seeing. Thanks, Dalla -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
