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SubjectRe: checkpoint/restart ABI
Dave Hansen wrote:
>>> I'm not sure what you mean by "closed files". Either the app has a fd,
>>> it doesn't, or it is in sys_open() somewhere. We have to get the app
>>> into a quiescent state before we can checkpoint, so we basically just
>>> say that we won't checkpoint things that are *in* the kernel.
>>>
>> It's common for an app to write a tmp file, close it, and then open it a
>> bit later expecting to find the content it just wrote. If you
>> checkpoint-kill it in the interim, reboot (clearing out /tmp) and then
>> resume, then it will lose its tmp file. There's no explicit connection
>> between the process and its potential working set of files.
>>
>
> I respectfully disagree. The number one prerequisite for
> checkpoint/restart is isolation. Xen just happens to get this for free.
>

(I don't have my Xen hat on at all for this thread.)

> So, instead of saying that there's no explicit connection between the
> process and its working set, ask yourself how we make a connection.
>
> In this case, we can do it with a filesystem (mount) namespace. Each
> container that we might want to checkpoint must have its writable
> filesystems contained to a private set that are not shared with other
> containers. Things like union mounts would help here, but aren't
> necessarily required. They just make it more efficient.
>

We were dealing with checkpointing random sets of processes, and that
posed all sorts of problems. Filesystem namespace was one, the pid
namespace was another. Doing checkpointing at the container-level
granularity definitely solves a lot of problems.

>>> Is there anything specific you are thinking of that particularly worries
>>> you? I could write pages on the list you have there.
>>>
>> No, that's the problem; it all worries me. It's a big problem space.
>>
>
> It's almost as big of a problem as trying to virtualize entire machines
> and expecting them to run as fast as native. :)
>

No, it's much harder. Hardware is relatively simple and immutable
compared to kernel and process state ;)

> Cool! I didn't know you guys did the IRIX implementation. I'm sure you
> guys got a lot farther than any of us are. Did you guys ever write any
> papers or anything on it? I'd be interested in more information.
>

Yeah, there was a paper, but it looks like the internet has lost it. It
was at
http://www.csu.edu.au/special/conference/apwww95/.papers95/cmaltby/cmaltby.ps
http://www.csu.edu.au/special/conference/apwww95/sept-all.html has
mention of the paper.

J


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