On Thu, 21 Nov 2013 16:57:48 -0600 Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx> wrote: > 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. Could you explain your logic here? Why do you think rebuilding parity will take 3 times as long as rebuilding a copy? Can you measure that sort of difference today? Presumably when we have 20TB drives we will also have more cores and quite possibly dedicated co-processors which will make the CPU load less significant. NeilBrown
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