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DragonFly kernel List (threaded) for 2011-04
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Re: [GSoC] HAMMER compression and new unionfs

From: Matthew Dillon <dillon@xxxxxxxxxxxxxxxxxxxx>
Date: Fri, 1 Apr 2011 16:43:16 -0700 (PDT)

:hm, seems difficult for me.. To have "whiteout database file" on the top
:of writable directory, would be another implementation? Then how can I
:avoid reading from/writing to the database file?

   The alternatives are expensive.  For example, you could create a
   special <filename>.<magicnumber> file or something like that
   to represent the deleted file.  Any directory entry or stat or open
   or other operation would then have to check whether the magic file
   exists or not when looking up <filename> successfully, and act as
   if <filename> didn't exist if found.

   I'm not sure I'd like a unionfs having to do that but it would be
   fairly easy to implement.

   Using whiteouts is a non-starter, they never worked well even when
   unionfs was being used with UFS because the only way to backup a
   UFS filesystem using whiteouts was to use dump/restore.

					Matthew Dillon 

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