On Tue, 23 Jul 2013 19:22:06 +0200 Christoph Anton Mitterer
[off-list ref] wrote:

On Tue, 2013-07-23 at 19:12 +0200, Mikael Abrahamsson wrote: 

quoted

2^18=262144
Nothing odd about it.

Sure... it's not e or pi ;-)

"odd" in the sense of "why such a big offset?" respectively "what does
it try to align to?"

It isn't (all) alignment.  It is mostly spare space.

New feature in 3.3 is that when you reshape (e.g.) a RAID5 to a RAID6 it can
do so without using a "backup file" - which is are a pain to work with and
slow things down a lot.
What it does instead is move the "data_offset" towards the start of the
device.  That way it is never writing onto live data, and so no backup is
needed.

For this to work, we need a buffer at the start of the device.  128M is
plenty big enough and just a tiny fraction of a 1TB drive.  (If you only have
a small drive it will still pick a tiny fraction).

So we are reserving a bit of space for future flexibility.

NeilBrown