Starting with kernel 2.6.32-358, there is a serious performance issue with (I assume) the megaraid_sas driver. I'll describe my specific setup, but I suspect the conditions may be broader than this.<br />
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I've been able to reproduce this on multiple Supermicro motherboards including the X8DT3. I'm booting from a RAID1 array and accessing the rest of the disks on the controller as JBOD. Anything that does block writes (mkfs, dd writing to the /dev) causes the system to become extremely sluggish, but I'm unable to say what exactly is happening to cause the slowness. A large sustained write to the filesystem (dd writing to a file) will also cause degraded performance, but not as bad as a block write. For instance - when doing a block write, an SSH to the server will hang and never succeed. When doing an FS write, an SSH attempt will take 15-20 seconds but eventually succeed.<br />
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If I boot from a single drive connected to the motherboard controller, I can write to the disks on the LSI controller without any performance impact. Simultaneous block writes or FS writes to all connected drives cause no performance issues whatsoever.<br />
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I downgraded to 2.6.32-279 and cannot reproduce the issue under that kernel. The version of megaraid_sas in 279 is 00.00.06.14-rh1 and 358 has 06.504.01.00-rh1. Load and iowait% are generally the same when comparing affected systems and unaffected systems. Looking at iostat, affected systems still appear to be getting normal write performance even though other operations are significantly degraded.
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