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

["Jörn Engel" <joern@lazybastard.org>]

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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