Hi all, I've a system, with an LSI 2308 SAS controller and 5 2.5" HDD attached. Each HDD can do around 100MB/sec read/write. This was tested will all HDDs in parallel, to make sure the controller can sustain them. Single disk has same performance. I was testing RAID 0/5/6 perfomances and I found something I could not clearly understand. The test was done with "dd", I wanted to know the maximum possible performance. Specifically, for reading: dd if=/dev/md127 of=/dev/null bs=4k For writing: dd if=/dev/zero of=/dev/md127 bs=4k conv=fdatasync Note than large block size did not change the results. I guess the page size is quite optimal. I tested each RAID with 4 and 5 HDDs, with chunk size of 512k, 64k and 16k. The "stripe_cache_size" was set to the max 32768. The results were observed with "iostat -k 5", taking care to consider variations and ramp up. The table, with MB/sec, the number are the HDDs the "r" is read, "w" is write: chunk RAID 4r 4w 5r 5w 512k 0 400 400 500 500 512k 5 260 300 360 400 512k 6 55 180 100 290 64k 0 400 400 440 500 64k 5 150 300 160 400 64k 6 100 180 140 290 16k 0 380 400 350 500 16k 5 100 300 130 390 16k 6 80 180 100 290 Now, RAID-0/5 seem to perform as expected, depending on the number of HDDs. Expecially with large chunk size. Write performances are not a problem, even if those are CPU intensive, with parity RAID. RAID-0/5 do not react well with small chunk. RAID-6, on the other hand, seems to have an idea of its own. First of all, it does not seem to respect proportionality. I would think a 4 HDDs RAID-6 should more or less read as fast as 2 HDDs. I can understand some loss, due to the parity skip, but not so much. In fact it improves with smaller chunk. With 5 HDDs, I would expect something better than 100MB/sec. Any idea on this? Am I doing something wrong? Some suggestion on tuning something in order to try to improve RAID-6? Thanks, bye, -- piergiorgio -- 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
