Date: Thu, 29 Nov 2007 08:19:58 -0800 (PST)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Jing Xue <jingxue@digizenstudio.com>
cc: Al Boldi <a1426z@gawab.com>, linux-kernel@vger.kernel.org,
       git@vger.kernel.org
Subject: Re: git guidance
In-Reply-To: <20071129105220.v40i22q4gw4cgoso@intranet.digizenstudio.com>
Message-ID: <alpine.LFD.0.9999.0711290810170.8458@woody.linux-foundation.org>
References: <20071129105220.v40i22q4gw4cgoso@intranet.digizenstudio.com>
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On Thu, 29 Nov 2007, Jing Xue wrote:
> 
> By the way, the only SCM I have worked with that tries to mount its
> repository (or a view on top of it) as a file system is ClearCase with
> its dynamic views. And, between the buggy file system implementation,
> the intrusion on workflow, and the lack of scalability, at least in
> the organization I worked for, it turned out to be a horrible,
> horrible, horrible idea.

Doing a read-only mount setup tends to be pretty easy, but it's largely 
pointless except for specialty uses. Ie it's obviously not useful for 
actual *development*, but it can be useful for some other cases.

For example, a read-only revctrl filesystem can be a _very_ useful thing 
for test-farms, where you may have hundreds of clients that run tests on 
possibly different versions at the same time. In situations like that, the 
read-only mount can actually often be done as a user-space NFS server on 
some machine.

The advantage is that you don't need to export close to infinite amounts 
of versions from a "real" filesystem, or make the clients have their own 
copies. And if you do it as a user-space NFS server (or samba, for that 
matter), it's even portable, unlike many other approaches. The read-only 
part also makes 99% of all the complexity go away, and it turns out to be 
a fairly easy exercise to do.

So I don't think the filesystem approach is _wrong_ per se. But yes, doing 
it read-write is almost invariably a big mistake. On operatign systems 
that support a "union mount" approach, it's likely much better to have a 
read-only revctl thing, and then over-mount a regular filesystem on top of 
it.

Trying to make it read-write from the revctl engine standpoint is almost 
certainly totally insane.

				Linus
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