[Devel] Re: How much of a mess does OpenVZ make? ; ) Was: What can OpenVZ do?

[Oren Laadan]

Dave Hansen wrote:
> On Fri, 2009-03-13 at 14:01 -0700, Linus Torvalds wrote:
>> On Fri, 13 Mar 2009, Alexey Dobriyan wrote:
>>>> Let's face it, we're not going to _ever_ checkpoint any kind of general 
>>>> case process. Just TCP makes that fundamentally impossible in the general 
>>>> case, and there are lots and lots of other cases too (just something as 
>>>> totally _trivial_ as all the files in the filesystem that don't get rolled 
>>>> back).
>>> What do you mean here? Unlinked files?
>> Or modified files, or anything else. "External state" is a pretty damn 
>> wide net. It's not just TCP sequence numbers and another machine.
> 
> This is precisely the reason that we've focused so hard on containers,
> and *didn't* just jump right into checkpoint/restart; we're trying
> really hard to constrain the _truly_ external things that a process can
> interact with.  
> 
> The approach so far has largely been to make things are external to a
> process at least *internal* to a container.  Network, pid, ipc, and uts
> namespaces, for example.  An ipc/sem.c semaphore may be external to a
> process, so we'll just pick the whole namespace up and checkpoint it
> along with the process.
> 
> In the OpenVZ case, they've at least demonstrated that the filesystem
> can be moved largely with rsync.  Unlinked files need some in-kernel TLC
> (or /proc mangling) but it isn't *that* bad.

And in the Zap we have successfully used a log-based filesystem
(specifically NILFS) to continuously snapshot the file-system atomically
with taking a checkpoint, so it can easily branch off past checkpoints,
including the file system.

And unlinked files can be (inefficiently) handled by saving their full
contents with the checkpoint image - it's not a big toll on many apps
(if you exclude Wine and UML...). At least that's a start.

> 
> We can also make the fs problem much easier by using things like dm or
> btrfs snapshotting of the block device, or restricting to where on a fs
> a container is allowed to write with stuff like r/o bind mounts.

(or NILFS)

So we argue that the FS snapshotting is related, but orthogonal in terms
of implementation to c/r.

Oren.

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