HAMMER update 06-Feb-2008

Michael Neumann mneumann at ntecs.de
Wed Feb 6 16:02:27 PST 2008

Simon 'corecode' Schubert wrote:
Matthew Dillon wrote:
    * Implement the filesystem as one big huge circular FIFO, pretty much
      laid down linearly on disk, with a B-Tree to locate and access 

    * Random modifications (required for manipulating the B-Tree and 
      records as deleted) will append undo records to this FIFO and 
the only
      write ordering requirement will be that the buffers containing 
      undo records be committed before the buffers containing the 
random       modifications are committed.
This sounds quite like LFS now.  LFS however split the volume in smaller 
blocks which could be "used", "empty" or "open", IIRC.  Their background 
cleaner then could push remaining data from used blocks to a currently 
open one, marking the block "empty" after that, allowing the FS to write 
to the blocks again.
How about "Generational Garbarge Collection"? Assuming that there are 
some files that will never be deleted this could give slighly better

Keep a "copy count" (a copy occurs if the cleaner has to copy data from 
the left end to the right end of the FIFO). If that increases over, say 
3, copy it into the old generation FIFO.

One problem of course is how to dimension each generation, and how many
to use.
I think that's basically how LFS works.



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