On Apr 27, 2007, at 18:07:46, Nigel Cunningham wrote:
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.
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).
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.
Cheers,
Kyle Moffett
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