On Fri, 3 Dec 2010, Phil Karn wrote:

Except for the admittedly somewhat useful side effect of scanning the 
disks for bad sectors, all this activity seems rather unnecessary. Is 
there a way to create a RAID-5 (or any other RAID level) array so that 
it will immediately come up without an initial rebuild?

"--assume-clean".

File systems generally don't read disk blocks that they haven't already
written. So even when you build a RAID array from drives with old data,
I can't see how skipping the initial rebuild can cause any real harm.
The first write to any block causes the RAID system to initialize the
parity in that stripe, thus making it possible to regenerate that block
in case of a drive failure.

Some raid implementations won't read/write to all drives, but might 
instead read the block being written to, and the parity block, then write 
the new block and recalculate the parity, thus not read/writing to all 
blocks. If this is the case, if the parity is wrong, it'll still be wrong 
after the operation, thus you don't have any redundancy.

Doing a rebuild when creating the array is something I'd only skip if I 
was doing lab work, never in production. I use raid for redundancy, thus I 
want to make sure everything is ok and it doesn't matter to me if it takes 
half a day.

-- 
Mikael Abrahamsson    email: swmike@swm.pp.se