Re: user transactions and ENOSPC...

[Daniel J Blueman <daniel.blueman@gmail.com>]

On Fri, Sep 25, 2009 at 10:10 PM, Sage Weil <sage@newdream.net> wrote:
> So, the btrfs user transaction ioctls work like so
>
> =A0ioctl(fd, BTRFS_IOC_TRANS_START);
> =A0/* do many operations: write(), setxattr(), rmdir(), whatever. */
> =A0ioctl(fd, BTRFS_IOC_TRANS_END); =A0 =A0/* or close(fd); */
=2E..
> The second problem is that the application may die between START and
> END. The current ioctls are "safe" in that the transaction handle is
> closed when the struct file is released, so the fs won't get wedged i=
f
> you say segfault. =A0On the other hand, they're "unsafe" in that a pr=
ocess
> that is killed or segfaults will result in an imcomplete transaction
> making it to disk, which leaves the file system in an inconsistent st=
ate
> (from the point of view of the application).

With COW, where a transaction is incomplete due to application exit
without closing the transaction, is there a way to drop the
reference/'deallocate' the new tree nodes, thus moving back to the
prior state? Presumably the new tree nodes would get linked in when
the transaction is closed.
--=20
Daniel J Blueman
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