On 11/21/2013 1:05 AM, John Williams wrote: > On Wed, Nov 20, 2013 at 10:52 PM, Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx> wrote: >> On 11/20/2013 8:46 PM, John Williams wrote: >>> For myself or any machines I managed for work that do not need high >>> IOPS, I would definitely choose triple- or quad-parity over RAID 51 or >>> similar schemes with arrays of 16 - 32 drives. >> >> You must see a week long rebuild as acceptable... > > It would not be a problem if it did take that long, since I would have > extra parity units as backup in case of a failure during a rebuild. > > But of course it would not take that long. Take, for example, a 24 x > 3TB triple-parity array (21+3) that has had two drive failures > (perhaps the rebuild started with one failure, but there was soon > another failure). I would expect the rebuild to take about a day. You're looking at today. We're discussing tomorrow's needs. Today's 6TB 3.5" drives have sustained average throughput of ~175MB/s. Tomorrow's 20TB drives will be lucky to do 300MB/s. As I said previously, at that rate a straight disk-disk copy of a 20TB drive takes 18.6 hours. This is what you get with RAID1/10/51. In the real world, rebuilding a failed drive in a 3P array of say 8 of these disks will likely take at least 3 times as long, 2 days 6 hours minimum, probably more. This may be perfectly acceptable to some, but probably not to all. >>> on a subject Adam Leventhal has already >>> covered in detail in an article "Triple-Parity RAID and Beyond" which >>> seems to match the subject of this thread quite nicely: >>> >>> http://queue.acm.org/detail.cfm?id=1670144 >> >> Mr. Leventhal did not address the overwhelming problem we face, which is >> (multiple) parity array reconstruction time. He assumes the time to >> simply 'populate' one drive at its max throughput is the total >> reconstruction time for the array. > > Since Adam wrote the code for RAID-Z3 for ZFS, I'm sure he is aware of > the time to restore data to failed drives. I do not see any flaw in > his analysis related to the time needed to restore data to failed > drives. He wrote that article in late 2009. It seems pretty clear he wasn't looking 10 years forward to 20TB drives, where the minimum mirror rebuild time will be ~18 hours, and parity rebuild will be much greater. -- Stan -- 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
