linux-ext4 - Re: [RFC] ext3 freeze feature

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Date:	Fri, 25 Jan 2008 10:34:25 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Theodore Tso <tytso@....edu>, Takashi Sato <t-sato@...jp.nec.com>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] ext3 freeze feature

Theodore Tso wrote:
> The other approach would be to say, "oh well, the freeze ioctl is
> inherently dangerous, and root is allowed to himself in the foot, so
> who cares".  :-)

I tend to agree.  Either you need your fs frozen, or not, and if you do,
be prepared for the consequences.

> But it was this concern which is why ext3 never exported freeze
> functionality to userspace, even though other commercial filesystems
> do support this.  It wasn't that it wasn't considered, but the concern
> about whether or not it was sufficiently safe to make available.

What's the safety concern; that the admin will forget to unfreeze?

> And I do agree that we probably should just implement this in
> filesystem independent way, in which case all of the filesystems that
> support this already have super_operations functions
> write_super_lockfs() and unlockfs().

That's what I was thinking; can't the path to freeze_bdev just be
elevated out of dm-ioctl.c to fs/ioctl.c and exposed, such that any
filesystem which implements .write_super_lockfs can be frozen?  This is
essentially what the xfs_freeze userspace does via
xfs_ioctl/XFS_IOC_FREEZE - which, AFAIK, isn't used much now that the
lvm hooks are in place.

I'm also not sure I see the point of the timeout in the original patch;
either you are done snapshotting and ready to unfreeze, or you're not;
1, or 2, or 3 seconds doesn't really matter.  When you're done, you're
done, and you can only unfreeze then.  Shouldn't this be done
programmatically, and not with some pre-determined timeout?

-Eric
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