[Sheepdog] [PATCH 3/3] sheep: fix nr_copies in vdi.c

[MORITA Kazutaka]

At Tue, 01 May 2012 21:53:07 +0800,
Liu Yuan wrote:
> 
> >  c) fix the recovery algorithm for it.
> 
> 
> What kind of problem to fix?

In the recovery process, sheep needs to get nr_copies of the objects
which should be stored in local.  But currently there is no means to
get it without reading the vdi object (inode->nr_copies).  I think
there are several approaches we could take.

 1) Read all vdi objects to get the nr_copies for each object

    I thought this approach was expensive if there are many vdis, but
    perhaps, it might not be a problem since there should be much more
    data objects.

 2) Include the nr_copies in the object path

    I considered this approach before.  Currently, the simple store
    stores objects in /store_dir/obj/[epoch]/[object id], but I
    thought of changing it to
    /store_dir/obj/[epoch]/[nr_copies]/[object id].  If we run the
    recovery process for each [nr_copies] directory, we can know the
    nr_copies without reading vdi objects.

    However, this works with only the simple store driver, so it is
    not a good approach for now.

 3) Include the nr_copies in the object ID

    We can use some bits of object ID for specifying the nr_copies.
    This might be an efficient approach, but needs changes of the
    object ID rules, so we need to change qemu client code too.

I thought 1) was an expensive approach before, but if we could take
it, it might be easy to support a different redundancy level in
recovery code.


Thanks,

Kazutaka