1. 26 10月, 2012 6 次提交
  2. 09 10月, 2012 33 次提交
  3. 04 10月, 2012 1 次提交
    • D
      btrfs: allow setting NOCOW for a zero sized file via ioctl · 7e97b8da
      David Sterba 提交于
      Hi,
      
      the patch si simple, but it has user visible impact and I'm not quite sure how
      to resolve it.
      
      In short, $subj says it, chattr -C supports it and we want to use it.
      
      The conditions that acutally allow to change the NOCOW flag are clear. What if
      I try to set the flag on a file that is not empty? Options:
      
      1) whole ioctl will fail, EINVAL
      2.1) ioctl will succeed, the NOCOW flag will be silently removed, but the file
           will stay COW-ed and checksummed
      2.2) ioctl will succeed, flag will not be removed and a syslog message will
           warn that the COW flag has not been changed
      2.2.1) dtto, no syslog message
      
      Man page of chattr states that
      
       "If it is set on a file which already has data blocks, it is undefined when
       the blocks assigned to the file will be fully stable."
      
      Yes, it's undefined and with current implementation it'll never happen. So from
      this end, the user cannot expect anything. I'm trying to find a reasonable
      behaviour, so that a command like 'chattr -R -aijS +C' to tweak a broad set of
      flags in a deep directory does not fail unnecessarily and does not pollute the
      log.
      
      My personal preference is 2.2.1, but my dev's oppinion is skewed, not counting
      the fact that I know the code and otherwise would look there before consulting
      the documentation.
      
      The patch implements 2.2.1.
      
      david
      
      -------------8<-------------------
      From: David Sterba <dsterba@suse.cz>
      
      It's safe to turn off checksums for a zero sized file.
      
      http://thread.gmane.org/gmane.comp.file-systems.btrfs/18030
      
      "We cannot switch on NODATASUM for a file that already has extents that
      are checksummed. The invariant here is that either all the extents or
      none are checksummed.
      
      Theoretically it's possible to add/remove all checksums from a given
      file, but it's a potentially longtime operation, the file has to be in
      some intermediate state where the checksums partially exist but have to
      be ignored (for the csum->nocsum) until the file is fully converted,
      this brings more special cases to extent handling, it has to survive
      power failure and remain consistent, and probably needs to be restarted
      after next mount."
      Signed-off-by: NDavid Sterba <dsterba@suse.cz>
      7e97b8da