[sheepdog-users] Users inputs for new reclaim algorithm, please

[MORITA Kazutaka]

At Mon, 17 Mar 2014 21:43:25 +0800,
Liu Yuan wrote:
> > > 
> > > With new algorithm,
> > > 
> > >  $ dog vdi create image
> > >  $ dog vdi snapshot image -s snap1
> > >  $ dog vdi clone -s snap1 image clone
> > >  $ dog vdi delete clone  <-- this operation will surprise you that it won't
> > >                              release space but instead increase the space.
> > > 
> > > Following is the real case, we can see that deletion of a clone, which uses 316MB
> > > space, will actaully cause 5.2GB more space to be used.

This is obviously strange.  Although the algorithm creates additional
objects to count reference counts, the objects are sparse and should
not waste many spaces at all even in the worst case.

> > > So if you have this usage in mind, you'll expect a catastrophic prolem:
> > >  - frequent cloned instance release and creation will pose much more space
> > >    pressure on you.
> > >  - when space is near low watermark, you are not allowed to delete clones because
> > >    deletion will actually increase the space and end up destroying your cluster.
> > >    You have no choise, either add more nodes nor deny create of new clones and
> > >    never try to delete clones later.

After the algorithm is implemented correctly, this looks like a corner
case since the additional space for the new reclaim algorithm is very
small - it should be only 8 bytes for each object IIUC.  However, if
you still concern about the case, we can preallocate some spaces for
that beforehand to be used for ledger objects.  For example,

 1. Preallocate some small files for each device when sheep starts up.

 2. When the sheepdog cluster becomes disk-full and the user requests
    object deletion, we can rename the preallocated file to a ledger
    object and continue object reclaiming.

In either way, I think this should be a future work.  Sheepdog still
have some bugs in handling a disk-full problem even without object
reclaiming.

> There might be some users need this new algorithm for their specific usage, but
> I'd suggest that:
> 
>  1 make old algorithm as default reclaim one
>  2 modularize the reclaim algorithm and add new algorithm as an option for users
>    in this way, we can improve the new algorithm steps by steps and possibly
>    we can introduce more algorithms to meet varoius needs.

IMHO, modularizing object reclaiming is overkill.  I cannot imagine so
many algorithms for that.  Even if we keep the old algorithm, adding a
sheep command line option to enable this experimental object
reclaiming looks enough.  If we come up with another one, then let's
discuss this topic again.

Thanks,

Kazutaka