On Tue, 23 Jul 2013 19:22:06 +0200 Christoph Anton Mitterer <calestyo@xxxxxxxxxxxx> wrote: > On Tue, 2013-07-23 at 19:12 +0200, Mikael Abrahamsson wrote: > > 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
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