Re: Triple parity and beyond

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

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.

quoted

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.