Re: [PATCH 1/2] Delete ref $frotz by moving ref file to "deleted-$frotz~ref".

From: Junio C Hamano <junkio@cox.net>
Date: Oct 17, 2006, 04:41 UTC
Message-ID: <7v4pu3tuye.fsf@assigned-by-dhcp.cox.net>
In-Reply-To: <200610170626.25654.chriscool@tuxfamily.org>

Christian Couder <chriscool@tuxfamily.org> writes:

> For example, when git becomes a major SCM, there may be people working on 
> big projects that want to create a new branch for each new bug and then 
> delete the branch when the code on the bug branch has been integrated into 
> a new release and the bug is closed.

I would say that is a very valid way to work with git, regardless of the size of project. Now, how often would you create such a per-bug branch and delete one, compared to the number of operations that would require ref lookups? Your example actually supports what I've said -- optimizing for deletion at the cost of more expensive lookups is wrong.

> The operations that inspect project history may use a ref cache or something 
> so that a lookup on the disk may not be needed. So only the ref creation or 
> update rate versus delete rate may matter.

Stop and think about what you are saying. What's a ref cache? We do not have such a beast today (unless you equate it with packed-refs file), and you would need to design and implement it, but think about how you make that operate. You would need to invalidate it when you delete a ref using the deleted-ref/ approach; that's not much different from repacking packed-refs file without the ref you just deleted, no?

Of course you can argue that instead of repacking you always stat deleted-ref/ hierarchy; in other words, you can argue that you can make deletion path faster by penalizing the lookup path.

So I do not think using "ref cache" (whatever it is, and however it operates) does not change the situation a bit.

> If there are thousand of refs and a heavy I/O load, rewritting the packed 
> ref file for each deletion means writing on disk something that may not fit 
> in the disk cache. It may be very bad.

If the goal is to optimize for deletion path, then that is true. My point is that we do not want to optimize for deletion path at the expense of more costly lookup path.