linux-kernel - Re: [ANNOUNCE] DualFS: File System with Meta-data and Data Separation

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Date:	Sun, 18 Feb 2007 12:46:18 +0000
From:	Jörn Engel <joern@...ybastard.org>
To:	Sorin Faibish <sfaibish@....com>
Cc:	Bill Davidsen <davidsen@....com>,
	Juan Piernas Canovas <piernas@...ec.um.es>,
	Jan Engelhardt <jengelh@...ux01.gwdg.de>,
	kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [ANNOUNCE] DualFS: File System with Meta-data and Data Separation

Maybe this is a decent approach to deal with the problem.  First some
definitions.  T is the target segment to be cleaned, S is the spare
segment that valid data is written to, O are other segments that contain
indirect blocks I for valid data D in T.

Have two different GC mechanisms to choose between:
1. Regular GC that copies D and I into S.  On average D+I should require
   less space than S can offer.
2. Slow GC only copies D into S.  Indirect blocks get modified in-place
   in O.  This variant requires more seeks due to writing in various O,
   but it guarantees that D always requires less space than S can offer.

Whenever you are running out of spare segments and are in danger of the
deadlock, switch to mechanism 2.  Now your correctness problem is
reduced to a performance problem.

Jörn

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