below are the results of my tests with the LSI MegaRAID SCSI 320-2 controller on 3.9-release and 4.0-beta (snapshot pulled on Aug 22, 2006). I tested writing to both a RAID-5 and a RAID-0 logical drive. The RAID-0 drive consists of a single drive, what I'd call a JBOD. While the test program was running, I had iostat running, and I noted the results next to each test. The iostat values moved around a lot, so I took a figure close to the peak value.
Naturally all the async tests were influenced by the buffer cache to a certain extent. I'm a little confused as to why writing sequential blocks with O_SYNC should be so slow... Rob Urban #====================================================== # 3.9 #====================================================== RAID-0 drive: ---------------- write-test: ---------------------- Sync ---------------------- 1 proc: 1.7MB/s iostat: 1.8MB/s 2 procs: 1MB/s and 1MB/s iostat: 2MB/s Async ---------------------- 1 proc: 8.7MB/s iostat: 5MB/s 2 procs: 11MB/s and 5.6MB/s iostat: 4.8MB/s dd if=/dev/zero of=testfile bs=64k count=4096 ---------------------- took 28.26 seconds, bw: 9MB/s iostat reported: 4.6MB/s RAID-5 drive: ---------------- write-test: ---------------------- Sync ---------------------- 1 proc: 1.1MB/s iostat: 1.2MB/s 2 procs: 0.7MB/s and 0.7MB/s iostat: 1.6MB/s Async ---------------------- 1 proc: 10MB/s iostat: 3.2MB/s 2 procs: 5.2MB/s and 5.2MB/s iostat: 5MB/s dd if=/dev/zero of=testfile bs=64k count=4096 ---------------------- took 39.1 seconds, bw: 6.5MB/s iostat reported: 3.3MB/s #====================================================== # 4.0-beta #====================================================== RAID-0 drive: ---------------- write-test: ---------------------- Sync ---------------------- 1 proc: 1.5MB/s iostat: 3MB/s 2 procs: 0.76MB/s and 0.76MB/s iostat: 3.4MB/s Async ---------------------- 1 proc: 15MB/s iostat: 10MB/s 2 procs: 8.2MB/s and 5.8MB/s iostat: 10MB/s dd if=/dev/zero of=testfile bs=64k count=4096 ---------------------- took 21.8 seconds, bw: 11.7MB/s iostat reported: 10MB/s RAID-5 drive: ---------------- write-test: ---------------------- Sync ---------------------- 1 proc: 1.02MB/s iostat: 2.5MB/s 2 procs: 0.62MB/s and 0.62MB/s iostat: 2.9MB/s Async ---------------------- 1 proc: 9MB/s iostat: 6.3MB/s 2 procs: 4.1MB/s and 3.8MB/s iostat: 6.5MB/s dd if=/dev/zero of=testfile bs=64k count=4096 ---------------------- took 35 seconds, bw: 7.3MB/s iostat reported: 7MB/s