Michael Roth さんは書きました:
On 07/28/2011 03:03 AM, Andrea Arcangeli wrote:
On Thu, Jul 28, 2011 at 11:53:50AM +0900, Fernando Luis Vázquez Cao
wrote:
On Wed, 2011-07-27 at 17:24 +0200, Andrea Arcangeli wrote:
making
sure no lib is calling any I/O function to be able to defreeze the
filesystems later, making sure the oom killer or a wrong kill -9
$RANDOM isn't killing the agent by mistake while the I/O is blocked
and the copy is going.
Yes with the current API if the agent is killed while the filesystems
are frozen we are screwed.
I have just submitted patches that implement a new API that should make
the virtualization use case more reliable. Basically, I am adding a new
ioctl, FIGETFREEZEFD, which freezes the indicated filesystem and
returns
a file descriptor; as long as that file descriptor is held open, the
filesystem remains open. If the freeze file descriptor is closed (be it
through a explicit call to close(2) or as part of process exit
housekeeping) the associated filesystem is automatically thawed.
- fsfreeze: add ioctl to create a fd for freeze control
http://marc.info/?l=linux-fsdevel&m=131175212512290&w=2
- fsfreeze: add freeze fd ioctls
http://marc.info/?l=linux-fsdevel&m=131175220612341&w=2
This is probably how the API should have been implemented originally
instead of FIFREEZE/FITHAW.
It looks a bit overkill though, I would think it'd be enough to have
the fsfreeze forced at FIGETFREEZEFD, and the only way to thaw by
closing the file without requiring any of the
FS_FREEZE_FD/FS_THAW_FD/FS_ISFROZEN_FD. But I guess you have use cases
One of the crappy things about the current implementation is the
inability to determine whether or not a filesystem is frozen. At least
in the context of guest agent at least, it'd be nice if
guest-fsfreeze-status checked the actual system state rather than some
internal state that may not necessarily reflect reality (if we freeze,
and some other application thaws, we currently still report the state
as frozen).
Also in the context of the guest agent, we are indeed screwed if the
agent gets killed while in a frozen state, and remain screwed even if
it's restarted since we have no way of determining whether or not
we're in a frozen state and thus should disable logging operations.
That is precisely the reason I added the new API.
We could check status by looking for a failure from the freeze
operation, but if you're just interested in getting the state, having
to potentially induce a freeze just to get at the state is really
heavy-handed.
So having an open operation that doesn't force a freeze/thaw/status
operation serves some fairly common use cases I think.
Yep. If you think there is something missing API wise let me know and I
will implement it.
Thanks,
Fernando
