Re: RAID5 losing initial synchronization on restart when one disk is spare

[Neil Brown <neilb@suse.de>]

On Wednesday June 11, hubskml@free.fr wrote:
> 
> By the way and FYI, with my configuration, all disks on the same 
> controller, internal bitmap, v1 superblock, ... the initial RAID-5 
> synchronization duration is the same whether I'm using the option 
> --force or not.

For this to be a valid test, you need to fill one drive up with
garbage to ensure that a resync is no a no-op.

If you don't use the "--force" option, then the recovery process will
read from N-1 drives and write to 1 drive, all completely sequentially
so it will go at a predictable speed.

When you use "--force" it will read from N drive and check parity.
When it finds an error it will re-write that parity block.
So if the parity blocks happen to be all correct (as probably was the
case in your experiment), it will run nice and fast.  If the parity
blocks happen to all be wrong (as is likely when first creating an
array on drives that weren't an array before) it will be much slower.

NeilBrown