On Mon, 03 May 2010 16:12:16 PDT, Valerie Aurora said: > +File copyup > +----------- > + > +Any system call that alters the data or metadata of a file on the > +bottom layer, or creates or changes a hard link to it will trigger a > +copyup of the target file from the lower layer to the topmost layer > + > + - open(O_WRITE | O_RDWR | O_APPEND | O_DIRECT) > + - truncate()/open(O_TRUNC) > + - link() > + - rename() > + - chmod() > + - chown()/lchown() > + - utimes() > + - setxattr()/lsetxattr() I spent some time looking at patch 27 trying to figure it out for myself, but my lack of splice()-fu doomed me. :) A few quick questions: 1) For calls like chmod() that only touch the metadata, does it still trigger a copyup of the data, or just the affected metadata? 2) Is the copyup of data synchronous or async done in the background? The comments in union_copyup_len() about "We raced with someone else" imply this is synchronous - if so. probably a note should be made that an open() may take a little while under some conditions. There's a *lot* of code out there that assumes that open() calls are *really* cheap. I wonder how many programs don't correctly deal with an ENOSPC on open() of an already existing file. (The answers probably don't matter unless somebody ends up invoking a copyup of a gigabyte file, which of course implies one of my users will end up doing exactly that. :)
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