[sheepdog] [PATCH v2 1/2] sheep, dog: make recycling VID selectable

[Liu Yuan]

On Tue, Mar 17, 2015 at 04:44:46AM +0900, MORITA Kazutaka wrote:
> At Mon, 16 Mar 2015 21:13:29 +0800,
> Liu Yuan wrote:
> > 
> > How about make 'dog vdi clone --no-share' as the default clone operation? And
> > we can add dog vdi clone --share to keep old behavior as optional. By this
> > manner, --no-share will save us from this kind of subtle problem. And your team
> > problem about vdi exhaustion will be achieved :).
> 
> --no-share option disables thin provisioing.  It shouldn't be a default option,
> IMHO.

Following bug will disable vid recycle for old algorithm.

commit 21549a1bd4981fabcc09d062a647162127fe0637
Author: Hitoshi Mitake <mitake.hitoshi at gmail.com>
Date:   Sun Jun 1 23:23:18 2014 +0900

    sheep: don't recycle VDI ID
    
    Recycling VDI IDs of deleted VDIs is a completely wrong idea. It
    breaks relations between inode objects and data objects. For example,
    it can cause a problem of corrupting cloned VDIs (see related
    issue). This patch forbids the recycling.
    
    Related issue:
    https://bugs.launchpad.net/sheepdog-project/+bug/1317755
    
    Signed-off-by: Hitoshi Mitake <mitake.hitoshi at lab.ntt.co.jp>
    Signed-off-by: Liu Yuan <namei.unix at gmail.com>

This means we don't have vid recycle for old algorithm now because of this
subtle problem. This is why I suggest set --no-share as default, in order to
bring this functionality back.

> > 
> > This manner is not perfect, but it will benefit us:
> > 
> > 1. stable code base since old algorithm is long tested.
> 
> Hitoshi's patch enables the stable algorithm by default.  Isn't it enough?

I'm afraid not.

1. as above mentioned, simply disable new algorithm won't bring us back vid
   recycle. But if we bring it back, seems it will conflict with new algorithm.
2. new algorithm has a bug that need to hack vdi_lookup(), which will degrade it
   a lot. I'm not sure if we can hack vdi_lookup() to meet two algorithm's needs.

> > 2. we won't degrate vdi_lookup
> 
> I'm still not sure which code in vdi_lookup() is a problem.  The problem
> happens even when we disable VID garbage collection?

vdi_lookup() becomes a problem if Hitoshi's patch is enabled after he fixes a
fatal bug of new algorithm.

Thanks,
Yuan