[Gluster-devel] [Gluster-users] User-serviceable snapshots design

[Ira Cooper]

Anand, I also have a concern regarding the user-serviceable snapshot feature.

You rightfully call out the lack of scaling caused by maintaining the gfid -> gfid mapping tables, and correctly point out that this will limit the use cases this feature will be applicable to, on the client side.

If in fact gluster generates its gfids randomly, and has always done so, I propose that we can change the algorithm used to determine the mapping, to eliminate the lack of scaling of our solution.

We can create a fixed constant per-snapshot.  (Can be in just the client's memory, or stored on disk, that is an implementation detail here.)  We will call this constant "n".

I propose we just add the constant to the gfid determine the new gfid.  It turns out that this new gfid has the same chance of collision as any random gfid.  (It will take a moment for you to convince yourself of this, but the argument is fairly intuitive.)  If we do this, I'd suggest we do it on the first 32 bits of the gfid, because we can use simple unsigned math, and let it just overflow.  (If we get up to 2^32 snapshots, we can revisit this aspect of the design, but we'll have other issues at that number.)

By using addition this way, we also allow for subtraction to be used for a later purpose.

Note: This design relies on our random gfid generator not turning out a linear range of numbers.  If it has in the past, or will in the future, clearly this design has flaws.  But, I know of no such plans.  As long as the randomness is sufficient, there should be no issue.  (IE: It doesn't turn out linear results.)

Thanks,

-Ira / ira@(redhat.com|samba.org)

PS: +1 to Jeff here.  He's spotting major issues, that should be looked at, above the issue above.

----- Original Message -----
> > Attached is a basic write-up of the user-serviceable snapshot feature
> > design (Avati's). Please take a look and let us know if you have
> > questions of any sort...
> 
> A few.
> 
> The design creates a new type of daemon: snapview-server.
> 
> * Where is it started?  One server (selected how) or all?
> 
> * How do clients find it?  Are we dynamically changing the client
>   side graph to add new protocol/client instances pointing to new
>   snapview-servers, or is snapview-client using RPC directly?  Are
>   the snapview-server ports managed through the glusterd portmapper
>   interface, or patched in some other way?
> 
> * Since a snap volume will refer to multiple bricks, we'll need
>   more brick daemons as well.  How are *those* managed?
> 
> * How does snapview-server manage user credentials for connecting
>   to snap bricks?  What if multiple users try to use the same
>   snapshot at the same time?  How does any of this interact with
>   on-wire or on-disk encryption?
> 
> I'm sure I'll come up with more later.  Also, next time it might
> be nice to use the upstream feature proposal template *as it was
> designed* to make sure that questions like these get addressed
> where the whole community can participate in a timely fashion.
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