From mboxrd@z Thu Jan 1 00:00:00 1970 From: NeilBrown Subject: Re: shown disk sizes Date: Wed, 24 Jul 2013 06:57:23 +1000 Message-ID: <20130724065723.3a0d52b3@notabene.brown> References: <1374071867.25339.14.camel@heisenberg.scientia.net> <20130718094327.7d46949e@notabene.brown> <1374596770.8463.23.camel@heisenberg.scientia.net> <1374600126.8463.24.camel@heisenberg.scientia.net> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=PGP-SHA1; boundary="Sig_/3P0ndPqo9zpBITmdp=sfQ6o"; protocol="application/pgp-signature" Return-path: In-Reply-To: <1374600126.8463.24.camel@heisenberg.scientia.net> Sender: linux-raid-owner@vger.kernel.org To: Christoph Anton Mitterer Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids --Sig_/3P0ndPqo9zpBITmdp=sfQ6o Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: quoted-printable On Tue, 23 Jul 2013 19:22:06 +0200 Christoph Anton Mitterer wrote: > On Tue, 2013-07-23 at 19:12 +0200, Mikael Abrahamsson wrote:=20 > > 2^18=3D262144 > > Nothing odd about it. > Sure... it's not e or pi ;-) >=20 > "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 ha= ve a small drive it will still pick a tiny fraction). So we are reserving a bit of space for future flexibility. NeilBrown --Sig_/3P0ndPqo9zpBITmdp=sfQ6o Content-Type: application/pgp-signature; name=signature.asc Content-Disposition: attachment; filename=signature.asc -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.19 (GNU/Linux) iQIVAwUBUe7uNDnsnt1WYoG5AQIykg/+OWz0OthMSll9mNY9w2qrLqiYh0hNgBGx PDyVdd5yZiCIHaFn2mnEpqfXzZgReiKUb5S2/2TaiyjAJZeUaptdln+e/qVgystx qFhAjnA94Rxo+wGLIYPZ6z+tXC+zAKYeMjvPIDQSQ+Hu5Q9X3MLUjgowlIbV69YI uEefX/bDUw1X5yEaQ6+oKLYP0OzxPBnccSnRwlCC9RNCwWb0QvYKR5WvC+DZH7Dj mF8yCZsghGvO/78xQYbI70wk2Nn1mAivtJEEtYOtgnQJVvRxt1UiZDPTvPYZU0kP 1VxjCDqMqJpFVDvqsDYGTTMV9XhNFWfHRqQWd2qRYe2qTgNbo55SkF1tvTECqhFG 4Urkik9FsRE/Z7H1erhikHRizJYYP9VpB0owIsqozK5ufvR8gcQR6oKlORgwGVgA ibExjI7czqt8/AEUMD5n+VaKet7g6fA2nQ4b3zQk3+8K8EV5Rqn7Duzlt6rvvSSr LWWr9CnCGZ0PNrwSNx8aW4UXQoWki2MRhVI2LxUrfTO35odxjP8M5onSRJWb5Q2I EZkToWXF9aG0kxoH6KpVMDNynYoz709bSXNiYN7zJmhGlmNCRt6EPxb7HULxunSI BgVma2Pnz/moJqAKqiD3XboL2DsfgYEW4Y7H3j2xthTIaQ00JLJ4E58Mb0TdBB5U ldgb53fBgJw= =f+AP -----END PGP SIGNATURE----- --Sig_/3P0ndPqo9zpBITmdp=sfQ6o--