Re: Triple parity and beyond

On Thu, 21 Nov 2013 16:57:48 -0600 Stan Hoeppner [off-list ref]
wrote:

On 11/21/2013 1:05 AM, John Williams wrote:

quoted

On Wed, Nov 20, 2013 at 10:52 PM, Stan Hoeppner [off-list ref] wrote:

quoted

On 11/20/2013 8:46 PM, John Williams wrote:

quoted

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