Re: metadata copied/data not copied?

[Chris Mason <chris.mason@oracle.com>]

On Tue, 2009-03-17 at 13:08 -0400, CSights wrote:
> Hi everyone,
> 
> > > 	Here is an expanded example which is how I imagined COW would handle
> > > changes to the file's data ("file contents").  One can pretend it is an
> > > attempt to inject malicious code into /bin/sh (e.g. file1 is /bin/sh).
> > >
> > > [METADATA]                     -->  DATA
> > > [file1 perms olduser:oldgroup] -->  file contents
> > >
> > >
> > > # cp file1 file2
> > > [file1 perms olduser:oldgroup "COW"]  \
> > >                                        -->  file contents
> > > [file1 perms olduser:oldgroup "COW"]  /
> > > (A "COW" flag is set in btrfs's (hidden) metadata.)
> > >
> > >
> > > # chown newuser:newgroup file2
> > > [file1 perms olduser:oldgroup "COW"]  \
> > >                                        -->  file contents
> > > [file1 perms newuser:newgroup "COW"]  /
> > > (chown, chmod, others? are not "writes" to file contents, so file
> > > contents don't need to be copied-on-write yet.)
> > >
> > >
> > > # cat newcontent >> file2
> > > [file1 perms olduser:oldgroup] -->  file contents
> > > [file2 perms newuser:newgroup] -->  file contents + newcontent
> > > (File contents are modified.  This is a "write" that triggers COW.  The
> > > file contents are copied and then modified. Metadata for file2 are hooked
> > > up to copied then modified file contents.  "COW" flag is cleared.)
> >
> > It would work, but it is slightly different from how btrfs works.  There
> > are two ways to read COW (copy on write):
> >
> > 1) Before changing something, make a copy of the old data and put it
> > somewhere else.  Then overwrite the original location.
> >
> > 2) Don't ever overwrite the original location, write somewhere new
> > instead.  The old copy stays in the original location.
> >
> > Btrfs does #2.
> 
> 	Does the choice #1 or #2 change whether the extended example works or not?  
> It seems as though either way makes sense for the example given...?
> 

Yes, either way works.  #1 is what lvm snapshotting uses, which avoids
fragmentation of the original, but it doesn't scale well to lots of
snapshots.

> > The bcp command creates a second inode that points to the same data
> > extents as the first inode.  So, modifications to the inodes themselves
> > (such as chown, chmod, touch etc) don't touch the data extents.
> >
> > Modifications to the data extents go through the COW mechanism to make
> > sure we don't overwrite the originals.
> 
> 	To me it sounds like if cp were replaced with bcp, then btrfs would behave as 
> I imagined in my example...

The long term goal is to get cp to use a new system call to cow link
files.

> 	Why is a "bcp" separate from cp needed?  Is it because with cp btrfs 
> doesn't "know" a simple copy is being made, but just gets a stream of data to 
> write to disk?
> 	Is it possible to update cp to do the btrfs ioctl automatically, or must the 
> commands always remain separate because there are situations where it would 
> be a problem for the file contents to be COW?  (It seems to me the fact that 
> the data contents are COW would be transparent to userland apps, so the bcp 
> ioctl could be built in to cp.)
> 
> Looking forward to (a stable) btrfs!
> 	Eager User. :)

;)

-chris