Re: checkpoint/restart ABI

[Oren Laadan <orenl@cs.columbia.edu>]

Dave Hansen wrote:
> On Tue, 2008-08-12 at 09:32 -0700, Jeremy Fitzhardinge wrote:
>> Inter-machine networking stuff is hard because its outside the 
>> checkpointed set, so the checkpoint is observable.  Migration is easier, 
>> in principle, because you might be able to shift the connection endpoint 
>> without bringing it down.  Dealing with networking within your 
>> checkpointed set is just fiddly, particularly remembering and restoring 
>> all the details of things like urgent messages, on-the-fly file 
>> descriptors, packet boundaries, etc.
> 
> All true.  Hard stuff.
> 
> The IBM product works partly by limiting migrations to occurring on a
> single physical ethernet network.  Each container gets its own IP and
> MAC address.  The socket state is checkpointed quite fully and moved
> along with the IP.  
> 
>>> Unlinked files, for instance, are actually available in /proc.  You can
>>> freeze the app, write a helper that opens /proc/1234/fd, then copies its
>>> contents to a linked file (ooooh, with splice!)  Anyway, if we can do it
>>> in userspace, we can surely do it in the kernel.
>> Sure, there's no inherent problem.  But do you imagine including the 
>> file contents within your checkpoint image, or would they be saved 
>> separately?
> 
> Me, personally, I think I'd probably "re-link" the thing, mark it as
> such, ship it across like a normal file, then unlink it after the
> restore.  I don't know what we'd choose when actually implementing it.  

Re-linking works well when the file system supports that - some do not
allow this, in which case you need to silently rename instead of really
un-linking (even with NFS), or copy the entire contents.

Of course, you also need a snapshot of the file system in case it changes
after the checkpoint is taken, or take other measures. We can safely
defer addressing this for later.

> 
>>> 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.
> 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 had to 
>> deal with it by setting a bunch of policy files to tell the 
>> checkpoint/restart system what filename patterns it had to look out 
>> for.  But if you just checkpoint the whole filesystem state along with 
>> the process(es), then perhaps it isn't an issue.
> 
> Right.  We just start with "everybody has their own disk" which is slow
> and crappy and optimize it from there.

Yep.

[SNIP]

Oren.