[Btrfs-devel] Re: Initial Planning document for multiple device support

[Andi Kleen]

Chris Mason <chris.mason at oracle.com> writes:

Just commenting on something that tripped me while reading 
the document.

>If Btrfs were to rely on device mapper or MD for mirroring, it would
>not be able to resolve checksum failures by checking the mirrored
>copy. The lower layers don't know the checksum or granularity of the
>filesystem blocks, and so they are not able to verify the data they
>return.

I cannot imagine it would be that difficult to add a new READ_OTHER_COPY
io operation that would cause MD/LVM/... to return the other copy
in a mirror set. 

Even without btrfs that might be even generally useful for other
applications that do some checking on their files.

e.g. I could well imagine a new system call to trigger this on the
page cache level.

There might be other reasons to reinvent another storage manager
of course. Just that one above doesn't seem to be very convincing.
I admit I haven't thought too deeply about the other issues you
raise in the document.

-Andi