Re: Back to the future.

From: Rafael J. Wysocki
Date: Fri Apr 27 2007 - 21:11:38 EST

On Saturday, 28 April 2007 03:03, Kyle Moffett wrote:
> On Apr 27, 2007, at 18:07:46, Nigel Cunningham wrote:
> > Hi.
> >
> > On Fri, 2007-04-27 at 14:44 -0700, Linus Torvalds wrote:
> >> It makes it harder to debug (wouldn't it be *nice* to just ssh in,
> >> and do
> >> gdb -p <snapshotter>
> >
> > Make the machine being suspended a VM and you can already do that.
> >> when something goes wrong?) but we also *depend* on user space for
> >> various things (the same way we depend on kernel threads, and why
> >> it has been such a total disaster to try to freeze the kernel
> >> threads too!). For example, if you want to do graphical stuff,
> >> just using X would be quite nice, wouldn't it?
> >
> > But in doing so you make the contents of the disk inconsistent with
> > the state you've just snapshotted, leading to filesystem
> > corruption. Even if you modify filesystems to do checkpointing
> > (which is what we're really talking about), you still also have the
> > problem that your snapshot has to be stored somewhere before you
> > write it to disk, so you also have to either [snip]
> Actually, it's a lot simpler than that. We can just combine the
> device-mapper snapshot with a VM+kernel snapshot system call and be
> almost done:
> sys_snapshot(dev_t snapblockdev, int __user *snapshotfd);
> When sys_snapshot is run, the kernel does:
> 1) Sequentially freeze mounted filesystems using blockdev freezing.
> If it's an fs that doesn't support freezing then either fail or force-
> remount-ro that fs and downgrade all its filedescriptors to RO.
> Doesn't need extra locking since process which try to do IO either
> succeed before the freeze call returns for that blockdev or sleep on
> the unfreeze of that blockdev. Filesystems are synchronized and made
> clean.
> 2) Iterate over the userspace process list, freezing each process
> and remapping all of its pages copy-on-write. Any device-specific
> pages need to have state saved by that device.

Why do you want to do 2) after 1) and not vice versa?

> 3) All processes (except kernel threads) are now frozen.
> 4) Kernel should save internal state corresponding to current
> userspace state. The kernel also swaps out excess pages to free up
> enough RAM and prepares the snapshot file-descriptor with copies of
> kernel memory and the original (pre-COW) mapped userspace pages.
> 5) Kernel substitutes filesystems for either a device-mapper
> snapshot with snapblockdev as backing storage or union with tmpfs and
> remounts the underlying filesystems as read-only.
> 6) Kernel unfreezes all userspace processes and returns the snapshot
> FD to userspace (where it can be read from).

Okay, but how do we do the error recovery if, for example, the image cannot
be saved?

> Then userspace can do whatever it wants. Any changes to filesystems
> mounted at the time of snapshot will be discarded at shutdown.
> Freshly mounted filesystems won't have the union or COW thing done,
> and so you can write your snapshot to a compressed encrypted file on
> a USB key if you want to, you just have to unmount it before the
> snapshot() syscall and remount it right afterwards.

This seems to be a good idea.

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