orenl at cs.columbia.edu
Wed Aug 20 14:54:36 PDT 2008
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
> 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.
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