Re: metadata copied/data not copied?

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

On Tue, 2009-03-17 at 11:34 -0400, CSights wrote:
> > This is true, cp makes a full copy of the file.
> >
> > The btrfs utilities include a program called bcp that makes a COW copy
> > of a single file (or directory tree) with a btrfs specific ioctl.
> 
> Okay, what happens if in the original example "cp" is changed to "bcp"?
> 
> # bcp file1 file2
> # chown newuser:newgroup file2
> (Where file1 was owned by olduser:oldgroup.)
> 
> 	Is using "bcp" the same as copying with hardlinks ("cp -l file1 file2")?
> 

bcp makes a copy of all the metadata, creating a new inode.  cp -l
creates a second link to the same inode.  For bcp, the only thing that
is shared between the two files is the actual data extents.  The sharing
is COW, so any changes to either file will not be reflected in the other
file.

> 
> 	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.)
> 
> 	Would this work?  At least in this example it looks like the filesystem can 
> track whether the file contents should be copied or not without hints from 
> userspace.

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.

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.

-chris