On Mon, Apr 18, 2016 at 11:20:22AM -0400, Eric Sandeen wrote:
>
>
> On 4/14/16 10:17 PM, Dave Chinner wrote:
> > On Thu, Apr 14, 2016 at 09:57:07AM +0200, Florian Margaine wrote:
> >> This lets userland get the filesystem freezing status, aka whether the
> >> filesystem is frozen or not. This is so that an application can know if
> >> it should freeze the filesystem or if it isn't necessary when taking a
> >> snapshot.
> >
> > freezing nests, so there is no reason for avoiding a freeze when
> > doing a snapshot.
>
> Sadly, no:
>
> # xfs_freeze -f /mnt/test
> # xfs_freeze -f /mnt/test
> xfs_freeze: cannot freeze filesystem at /mnt/test: Device or resource busy
>
> It used to, but it was broken^Wchanged quite some time ago.
Ugh. Block device freeze nesting still works (i.e. freeze_bdev, as
snapshots from DM would use), but I didn't realise (or had
forgetten) that superblock level freeze nesting had been removed...
> > Indeed, if you don't wrap freeze/thaw around a
> > snapshot, then if the fs is thawed while the snapshot is in progress
> > then you are going to get a corrupt snapshot....
>
> Yep.
>
> IMHO what really needs to happen is to fix freeze to allow nesting
> again.
Probably. I quick dig shows nesting was intentionally broken more
than 5 years ago in making the freeze ioctl work on btrfs.
commit 18e9e5104fcd9a973ffe3eed3816c87f2a1b6cd2
Author: Josef Bacik <josef@xxxxxxxxxx>
Date: Tue Mar 23 10:34:56 2010 -0400
Introduce freeze_super and thaw_super for the fsfreeze ioctl
.....
The only new gotcha is multiple calls to the fsfreeze ioctl will return EBUSY if
the fs is already frozen. I thought this was a better solution than adding a
freeze counter to the super_block, but if everybody hates this idea I'm open to
suggestions. Thanks,
....
Not sure many people noticed that at the time....
> A way to query freeze state might be nice, I think, but yeah, it's
> racy, so you can't depend on it - but it might be useful in the "huh,
> IO is failing, what's going on? Oh, it's frozen, ok" scenario...
So maybe we should just add the frozen state to /proc/self/mountinfo
or something similar, then people who think it matters can shoot
themselves in the foot all they want without us needing to care
about it.
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
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